在Android所有常用的原生控件當中,用法最複雜的應該就是ListView了,它專門用於處理那種內容元素很多,手機屏幕無法展示出所有內容的情況。ListView可以使用列表的形式來展示內容,超出屏幕部分的內容只需要通過手指滑動就可以移動到屏幕內了。
另外ListView還有一個非常神奇的功能,我相信大家應該都體驗過,即使在ListView中加載非常非常多的數據,比如達到成百上千條甚至更多,ListView都不會發生OOM或者崩潰,而且隨着我們手指滑動來瀏覽更多數據時,程序所佔用的內存竟然都不會跟着增長。那麼ListView是怎麼實現這麼神奇的功能的呢?當初我就抱着學習的心態花了很長時間把ListView的源碼通讀了一遍,基本瞭解了它的工作原理,在感嘆Google大神能夠寫出如此精妙代碼的同時我也有所敬畏,因爲ListView的代碼量比較大,複雜度也很高,很難用文字表達清楚,於是我就放棄了把它寫成一篇博客的想法。那麼現在回想起來這件事我已經腸子都悔青了,因爲沒過幾個月時間我就把當初梳理清晰的源碼又忘的一乾二淨。於是現在我又重新定下心來再次把ListView的源碼重讀了一遍,那麼這次我一定要把它寫成一篇博客,分享給大家的同時也當成我自己的筆記吧。
首先我們先來看一下ListView的繼承結構,如下圖所示:
可以看到,ListView的繼承結構還是相當複雜的,它是直接繼承自的AbsListView,而AbsListView有兩個子實現類,一個是ListView,另一個就是GridView,因此我們從這一點就可以猜出來,ListView和GridView在工作原理和實現上都是有很多共同點的。然後AbsListView又繼承自AdapterView,AdapterView繼承自ViewGroup,後面就是我們所熟知的了。先把ListView的繼承結構瞭解一下,待會兒有助於我們更加清晰地分析代碼。
Adapter的作用
Adapter相信大家都不會陌生,我們平時使用ListView的時候一定都會用到它。那麼話說回來大家有沒有仔細想過,爲什麼需要Adapter這個東西呢?總感覺正因爲有了Adapter,ListView的使用變得要比其它控件複雜得多。那麼這裏我們就先來學習一下Adapter到底起到了什麼樣的一個作用。
其實說到底,控件就是爲了交互和展示數據用的,只不過ListView更加特殊,它是爲了展示很多很多數據用的,但是ListView只承擔交互和展示工作而已,至於這些數據來自哪裏,ListView是不關心的。因此,我們能設想到的最基本的ListView工作模式就是要有一個ListView控件和一個數據源。
不過如果真的讓ListView和數據源直接打交道的話,那ListView所要做的適配工作就非常繁雜了。因爲數據源這個概念太模糊了,我們只知道它包含了很多數據而已,至於這個數據源到底是什麼樣類型,並沒有嚴格的定義,有可能是數組,也有可能是集合,甚至有可能是數據庫表中查詢出來的遊標。所以說如果ListView真的去爲每一種數據源都進行適配操作的話,一是擴展性會比較差,內置了幾種適配就只有幾種適配,不能動態進行添加。二是超出了它本身應該負責的工作範圍,不再是僅僅承擔交互和展示工作就可以了,這樣ListView就會變得比較臃腫。
那麼顯然Android開發團隊是不會允許這種事情發生的,於是就有了Adapter這樣一個機制的出現。顧名思義,Adapter是適配器的意思,它在ListView和數據源之間起到了一個橋樑的作用,ListView並不會直接和數據源打交道,而是會藉助Adapter這個橋樑來去訪問真正的數據源,與之前不同的是,Adapter的接口都是統一的,因此ListView不用再去擔心任何適配方面的問題。而Adapter又是一個接口(interface),它可以去實現各種各樣的子類,每個子類都能通過自己的邏輯來去完成特定的功能,以及與特定數據源的適配操作,比如說ArrayAdapter可以用於數組和List類型的數據源適配,SimpleCursorAdapter可以用於遊標類型的數據源適配,這樣就非常巧妙地把數據源適配困難的問題解決掉了,並且還擁有相當不錯的擴展性。簡單的原理示意圖如下所示:
當然Adapter的作用不僅僅只有數據源適配這一點,還有一個非常非常重要的方法也需要我們在Adapter當中去重寫,就是getView()方法,這個在下面的文章中還會詳細講到。
RecycleBin機制
那麼在開始分析ListView的源碼之前,還有一個東西是我們提前需要了解的,就是RecycleBin機制,這個機制也是ListView能夠實現成百上千條數據都不會OOM最重要的一個原因。其實RecycleBin的代碼並不多,只有300行左右,它是寫在AbsListView中的一個內部類,所以所有繼承自AbsListView的子類,也就是ListView和GridView,都可以使用這個機制。那我們來看一下RecycleBin中的主要代碼,如下所示:
- /**
- * The RecycleBin facilitates reuse of views across layouts. The RecycleBin
- * has two levels of storage: ActiveViews and ScrapViews. ActiveViews are
- * those views which were onscreen at the start of a layout. By
- * construction, they are displaying current information. At the end of
- * layout, all views in ActiveViews are demoted to ScrapViews. ScrapViews
- * are old views that could potentially be used by the adapter to avoid
- * allocating views unnecessarily.
- *
- * @see android.widget.AbsListView#setRecyclerListener(android.widget.AbsListView.RecyclerListener)
- * @see android.widget.AbsListView.RecyclerListener
- */
- class RecycleBin {
- private RecyclerListener mRecyclerListener;
- /**
- * The position of the first view stored in mActiveViews.
- */
- private int mFirstActivePosition;
- /**
- * Views that were on screen at the start of layout. This array is
- * populated at the start of layout, and at the end of layout all view
- * in mActiveViews are moved to mScrapViews. Views in mActiveViews
- * represent a contiguous range of Views, with position of the first
- * view store in mFirstActivePosition.
- */
- private View[] mActiveViews = new View[0];
- /**
- * Unsorted views that can be used by the adapter as a convert view.
- */
- private ArrayList<View>[] mScrapViews;
- private int mViewTypeCount;
- private ArrayList<View> mCurrentScrap;
- /**
- * Fill ActiveViews with all of the children of the AbsListView.
- *
- * @param childCount
- * The minimum number of views mActiveViews should hold
- * @param firstActivePosition
- * The position of the first view that will be stored in
- * mActiveViews
- */
- void fillActiveViews(int childCount, int firstActivePosition) {
- if (mActiveViews.length < childCount) {
- mActiveViews = new View[childCount];
- }
- mFirstActivePosition = firstActivePosition;
- final View[] activeViews = mActiveViews;
- for (int i = 0; i < childCount; i++) {
- View child = getChildAt(i);
- AbsListView.LayoutParams lp = (AbsListView.LayoutParams) child.getLayoutParams();
- // Don't put header or footer views into the scrap heap
- if (lp != null && lp.viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
- // Note: We do place AdapterView.ITEM_VIEW_TYPE_IGNORE in
- // active views.
- // However, we will NOT place them into scrap views.
- activeViews[i] = child;
- }
- }
- }
- /**
- * Get the view corresponding to the specified position. The view will
- * be removed from mActiveViews if it is found.
- *
- * @param position
- * The position to look up in mActiveViews
- * @return The view if it is found, null otherwise
- */
- View getActiveView(int position) {
- int index = position - mFirstActivePosition;
- final View[] activeViews = mActiveViews;
- if (index >= 0 && index < activeViews.length) {
- final View match = activeViews[index];
- activeViews[index] = null;
- return match;
- }
- return null;
- }
- /**
- * Put a view into the ScapViews list. These views are unordered.
- *
- * @param scrap
- * The view to add
- */
- void addScrapView(View scrap) {
- AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();
- if (lp == null) {
- return;
- }
- // Don't put header or footer views or views that should be ignored
- // into the scrap heap
- int viewType = lp.viewType;
- if (!shouldRecycleViewType(viewType)) {
- if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
- removeDetachedView(scrap, false);
- }
- return;
- }
- if (mViewTypeCount == 1) {
- dispatchFinishTemporaryDetach(scrap);
- mCurrentScrap.add(scrap);
- } else {
- dispatchFinishTemporaryDetach(scrap);
- mScrapViews[viewType].add(scrap);
- }
- if (mRecyclerListener != null) {
- mRecyclerListener.onMovedToScrapHeap(scrap);
- }
- }
- /**
- * @return A view from the ScrapViews collection. These are unordered.
- */
- View getScrapView(int position) {
- ArrayList<View> scrapViews;
- if (mViewTypeCount == 1) {
- scrapViews = mCurrentScrap;
- int size = scrapViews.size();
- if (size > 0) {
- return scrapViews.remove(size - 1);
- } else {
- return null;
- }
- } else {
- int whichScrap = mAdapter.getItemViewType(position);
- if (whichScrap >= 0 && whichScrap < mScrapViews.length) {
- scrapViews = mScrapViews[whichScrap];
- int size = scrapViews.size();
- if (size > 0) {
- return scrapViews.remove(size - 1);
- }
- }
- }
- return null;
- }
- public void setViewTypeCount(int viewTypeCount) {
- if (viewTypeCount < 1) {
- throw new IllegalArgumentException("Can't have a viewTypeCount < 1");
- }
- // noinspection unchecked
- ArrayList<View>[] scrapViews = new ArrayList[viewTypeCount];
- for (int i = 0; i < viewTypeCount; i++) {
- scrapViews[i] = new ArrayList<View>();
- }
- mViewTypeCount = viewTypeCount;
- mCurrentScrap = scrapViews[0];
- mScrapViews = scrapViews;
- }
- }
這裏的RecycleBin代碼並不全,我只是把最主要的幾個方法提了出來。那麼我們先來對這幾個方法進行簡單解讀,這對後面分析ListView的工作原理將會有很大的幫助。
- fillActiveViews() 這個方法接收兩個參數,第一個參數表示要存儲的view的數量,第二個參數表示ListView中第一個可見元素的position值。RecycleBin當中使用mActiveViews這個數組來存儲View,調用這個方法後就會根據傳入的參數來將ListView中的指定元素存儲到mActiveViews數組當中。
- getActiveView() 這個方法和fillActiveViews()是對應的,用於從mActiveViews數組當中獲取數據。該方法接收一個position參數,表示元素在ListView當中的位置,方法內部會自動將position值轉換成mActiveViews數組對應的下標值。需要注意的是,mActiveViews當中所存儲的View,一旦被獲取了之後就會從mActiveViews當中移除,下次獲取同樣位置的View將會返回null,也就是說mActiveViews不能被重複利用。
- addScrapView() 用於將一個廢棄的View進行緩存,該方法接收一個View參數,當有某個View確定要廢棄掉的時候(比如滾動出了屏幕),就應該調用這個方法來對View進行緩存,RecycleBin當中使用mScrapViews和mCurrentScrap這兩個List來存儲廢棄View。
- getScrapView 用於從廢棄緩存中取出一個View,這些廢棄緩存中的View是沒有順序可言的,因此getScrapView()方法中的算法也非常簡單,就是直接從mCurrentScrap當中獲取尾部的一個scrap view進行返回。
- setViewTypeCount() 我們都知道Adapter當中可以重寫一個getViewTypeCount()來表示ListView中有幾種類型的數據項,而setViewTypeCount()方法的作用就是爲每種類型的數據項都單獨啓用一個RecycleBin緩存機制。實際上,getViewTypeCount()方法通常情況下使用的並不是很多,所以我們只要知道RecycleBin當中有這樣一個功能就行了。
瞭解了RecycleBin中的主要方法以及它們的用處之後,下面就可以開始來分析ListView的工作原理了,這裏我將還是按照以前分析源碼的方式來進行,即跟着主線執行流程來逐步閱讀並點到即止,不然的話要是把ListView所有的代碼都貼出來,那麼本篇文章將會很長很長了。
第一次Layout
不管怎麼說,ListView即使再特殊最終還是繼承自View的,因此它的執行流程還將會按照View的規則來執行,對於這方面不太熟悉的朋友可以參考我之前寫的 Android視圖繪製流程完全解析,帶你一步步深入瞭解View(二) 。
View的執行流程無非就分爲三步,onMeasure()用於測量View的大小,onLayout()用於確定View的佈局,onDraw()用於將View繪製到界面上。而在ListView當中,onMeasure()並沒有什麼特殊的地方,因爲它終歸是一個View,佔用的空間最多並且通常也就是整個屏幕。onDraw()在ListView當中也沒有什麼意義,因爲ListView本身並不負責繪製,而是由ListView當中的子元素來進行繪製的。那麼ListView大部分的神奇功能其實都是在onLayout()方法中進行的了,因此我們本篇文章也是主要分析的這個方法裏的內容。
如果你到ListView源碼中去找一找,你會發現ListView中是沒有onLayout()這個方法的,這是因爲這個方法是在ListView的父類AbsListView中實現的,代碼如下所示:
- /**
- * Subclasses should NOT override this method but {@link #layoutChildren()}
- * instead.
- */
- @Override
- protected void onLayout(boolean changed, int l, int t, int r, int b) {
- super.onLayout(changed, l, t, r, b);
- mInLayout = true;
- if (changed) {
- int childCount = getChildCount();
- for (int i = 0; i < childCount; i++) {
- getChildAt(i).forceLayout();
- }
- mRecycler.markChildrenDirty();
- }
- layoutChildren();
- mInLayout = false;
- }
- @Override
- protected void layoutChildren() {
- final boolean blockLayoutRequests = mBlockLayoutRequests;
- if (!blockLayoutRequests) {
- mBlockLayoutRequests = true;
- } else {
- return;
- }
- try {
- super.layoutChildren();
- invalidate();
- if (mAdapter == null) {
- resetList();
- invokeOnItemScrollListener();
- return;
- }
- int childrenTop = mListPadding.top;
- int childrenBottom = getBottom() - getTop() - mListPadding.bottom;
- int childCount = getChildCount();
- int index = 0;
- int delta = 0;
- View sel;
- View oldSel = null;
- View oldFirst = null;
- View newSel = null;
- View focusLayoutRestoreView = null;
- // Remember stuff we will need down below
- switch (mLayoutMode) {
- case LAYOUT_SET_SELECTION:
- index = mNextSelectedPosition - mFirstPosition;
- if (index >= 0 && index < childCount) {
- newSel = getChildAt(index);
- }
- break;
- case LAYOUT_FORCE_TOP:
- case LAYOUT_FORCE_BOTTOM:
- case LAYOUT_SPECIFIC:
- case LAYOUT_SYNC:
- break;
- case LAYOUT_MOVE_SELECTION:
- default:
- // Remember the previously selected view
- index = mSelectedPosition - mFirstPosition;
- if (index >= 0 && index < childCount) {
- oldSel = getChildAt(index);
- }
- // Remember the previous first child
- oldFirst = getChildAt(0);
- if (mNextSelectedPosition >= 0) {
- delta = mNextSelectedPosition - mSelectedPosition;
- }
- // Caution: newSel might be null
- newSel = getChildAt(index + delta);
- }
- boolean dataChanged = mDataChanged;
- if (dataChanged) {
- handleDataChanged();
- }
- // Handle the empty set by removing all views that are visible
- // and calling it a day
- if (mItemCount == 0) {
- resetList();
- invokeOnItemScrollListener();
- return;
- } else if (mItemCount != mAdapter.getCount()) {
- throw new IllegalStateException("The content of the adapter has changed but "
- + "ListView did not receive a notification. Make sure the content of "
- + "your adapter is not modified from a background thread, but only "
- + "from the UI thread. [in ListView(" + getId() + ", " + getClass()
- + ") with Adapter(" + mAdapter.getClass() + ")]");
- }
- setSelectedPositionInt(mNextSelectedPosition);
- // Pull all children into the RecycleBin.
- // These views will be reused if possible
- final int firstPosition = mFirstPosition;
- final RecycleBin recycleBin = mRecycler;
- // reset the focus restoration
- View focusLayoutRestoreDirectChild = null;
- // Don't put header or footer views into the Recycler. Those are
- // already cached in mHeaderViews;
- if (dataChanged) {
- for (int i = 0; i < childCount; i++) {
- recycleBin.addScrapView(getChildAt(i));
- if (ViewDebug.TRACE_RECYCLER) {
- ViewDebug.trace(getChildAt(i),
- ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP, index, i);
- }
- }
- } else {
- recycleBin.fillActiveViews(childCount, firstPosition);
- }
- // take focus back to us temporarily to avoid the eventual
- // call to clear focus when removing the focused child below
- // from messing things up when ViewRoot assigns focus back
- // to someone else
- final View focusedChild = getFocusedChild();
- if (focusedChild != null) {
- // TODO: in some cases focusedChild.getParent() == null
- // we can remember the focused view to restore after relayout if the
- // data hasn't changed, or if the focused position is a header or footer
- if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)) {
- focusLayoutRestoreDirectChild = focusedChild;
- // remember the specific view that had focus
- focusLayoutRestoreView = findFocus();
- if (focusLayoutRestoreView != null) {
- // tell it we are going to mess with it
- focusLayoutRestoreView.onStartTemporaryDetach();
- }
- }
- requestFocus();
- }
- // Clear out old views
- detachAllViewsFromParent();
- switch (mLayoutMode) {
- case LAYOUT_SET_SELECTION:
- if (newSel != null) {
- sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
- } else {
- sel = fillFromMiddle(childrenTop, childrenBottom);
- }
- break;
- case LAYOUT_SYNC:
- sel = fillSpecific(mSyncPosition, mSpecificTop);
- break;
- case LAYOUT_FORCE_BOTTOM:
- sel = fillUp(mItemCount - 1, childrenBottom);
- adjustViewsUpOrDown();
- break;
- case LAYOUT_FORCE_TOP:
- mFirstPosition = 0;
- sel = fillFromTop(childrenTop);
- adjustViewsUpOrDown();
- break;
- case LAYOUT_SPECIFIC:
- sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);
- break;
- case LAYOUT_MOVE_SELECTION:
- sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
- break;
- default:
- if (childCount == 0) {
- if (!mStackFromBottom) {
- final int position = lookForSelectablePosition(0, true);
- setSelectedPositionInt(position);
- sel = fillFromTop(childrenTop);
- } else {
- final int position = lookForSelectablePosition(mItemCount - 1, false);
- setSelectedPositionInt(position);
- sel = fillUp(mItemCount - 1, childrenBottom);
- }
- } else {
- if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
- sel = fillSpecific(mSelectedPosition,
- oldSel == null ? childrenTop : oldSel.getTop());
- } else if (mFirstPosition < mItemCount) {
- sel = fillSpecific(mFirstPosition,
- oldFirst == null ? childrenTop : oldFirst.getTop());
- } else {
- sel = fillSpecific(0, childrenTop);
- }
- }
- break;
- }
- // Flush any cached views that did not get reused above
- recycleBin.scrapActiveViews();
- if (sel != null) {
- // the current selected item should get focus if items
- // are focusable
- if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {
- final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&
- focusLayoutRestoreView.requestFocus()) || sel.requestFocus();
- if (!focusWasTaken) {
- // selected item didn't take focus, fine, but still want
- // to make sure something else outside of the selected view
- // has focus
- final View focused = getFocusedChild();
- if (focused != null) {
- focused.clearFocus();
- }
- positionSelector(sel);
- } else {
- sel.setSelected(false);
- mSelectorRect.setEmpty();
- }
- } else {
- positionSelector(sel);
- }
- mSelectedTop = sel.getTop();
- } else {
- if (mTouchMode > TOUCH_MODE_DOWN && mTouchMode < TOUCH_MODE_SCROLL) {
- View child = getChildAt(mMotionPosition - mFirstPosition);
- if (child != null) positionSelector(child);
- } else {
- mSelectedTop = 0;
- mSelectorRect.setEmpty();
- }
- // even if there is not selected position, we may need to restore
- // focus (i.e. something focusable in touch mode)
- if (hasFocus() && focusLayoutRestoreView != null) {
- focusLayoutRestoreView.requestFocus();
- }
- }
- // tell focus view we are done mucking with it, if it is still in
- // our view hierarchy.
- if (focusLayoutRestoreView != null
- && focusLayoutRestoreView.getWindowToken() != null) {
- focusLayoutRestoreView.onFinishTemporaryDetach();
- }
- mLayoutMode = LAYOUT_NORMAL;
- mDataChanged = false;
- mNeedSync = false;
- setNextSelectedPositionInt(mSelectedPosition);
- updateScrollIndicators();
- if (mItemCount > 0) {
- checkSelectionChanged();
- }
- invokeOnItemScrollListener();
- } finally {
- if (!blockLayoutRequests) {
- mBlockLayoutRequests = false;
- }
- }
- }
這段代碼比較長,我們挑重點的看。首先可以確定的是,ListView當中目前還沒有任何子View,數據都還是由Adapter管理的,並沒有展示到界面上,因此第19行getChildCount()方法得到的值肯定是0。接着在第81行會根據dataChanged這個布爾型的值來判斷執行邏輯,dataChanged只有在數據源發生改變的情況下才會變成true,其它情況都是false,因此這裏會進入到第90行的執行邏輯,調用RecycleBin的fillActiveViews()方法。按理來說,調用fillActiveViews()方法是爲了將ListView的子View進行緩存的,可是目前ListView中還沒有任何的子View,因此這一行暫時還起不了任何作用。
接下來在第114行會根據mLayoutMode的值來決定佈局模式,默認情況下都是普通模式LAYOUT_NORMAL,因此會進入到第140行的default語句當中。而下面又會緊接着進行兩次if判斷,childCount目前是等於0的,並且默認的佈局順序是從上往下,因此會進入到第145行的fillFromTop()方法,我們跟進去瞧一瞧:
- /**
- * Fills the list from top to bottom, starting with mFirstPosition
- *
- * @param nextTop The location where the top of the first item should be
- * drawn
- *
- * @return The view that is currently selected
- */
- private View fillFromTop(int nextTop) {
- mFirstPosition = Math.min(mFirstPosition, mSelectedPosition);
- mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);
- if (mFirstPosition < 0) {
- mFirstPosition = 0;
- }
- return fillDown(mFirstPosition, nextTop);
- }
- /**
- * Fills the list from pos down to the end of the list view.
- *
- * @param pos The first position to put in the list
- *
- * @param nextTop The location where the top of the item associated with pos
- * should be drawn
- *
- * @return The view that is currently selected, if it happens to be in the
- * range that we draw.
- */
- private View fillDown(int pos, int nextTop) {
- View selectedView = null;
- int end = (getBottom() - getTop()) - mListPadding.bottom;
- while (nextTop < end && pos < mItemCount) {
- // is this the selected item?
- boolean selected = pos == mSelectedPosition;
- View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);
- nextTop = child.getBottom() + mDividerHeight;
- if (selected) {
- selectedView = child;
- }
- pos++;
- }
- return selectedView;
- }
可以看到,這裏使用了一個while循環來執行重複邏輯,一開始nextTop的值是第一個子元素頂部距離整個ListView頂部的像素值,pos則是剛剛傳入的mFirstPosition的值,而end是ListView底部減去頂部所得的像素值,mItemCount則是Adapter中的元素數量。因此一開始的情況下nextTop必定是小於end值的,並且pos也是小於mItemCount值的。那麼每執行一次while循環,pos的值都會加1,並且nextTop也會增加,當nextTop大於等於end時,也就是子元素已經超出當前屏幕了,或者pos大於等於mItemCount時,也就是所有Adapter中的元素都被遍歷結束了,就會跳出while循環。
那麼while循環當中又做了什麼事情呢?值得讓人留意的就是第18行調用的makeAndAddView()方法,進入到這個方法當中,代碼如下所示:
- /**
- * Obtain the view and add it to our list of children. The view can be made
- * fresh, converted from an unused view, or used as is if it was in the
- * recycle bin.
- *
- * @param position Logical position in the list
- * @param y Top or bottom edge of the view to add
- * @param flow If flow is true, align top edge to y. If false, align bottom
- * edge to y.
- * @param childrenLeft Left edge where children should be positioned
- * @param selected Is this position selected?
- * @return View that was added
- */
- private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
- boolean selected) {
- View child;
- if (!mDataChanged) {
- // Try to use an exsiting view for this position
- child = mRecycler.getActiveView(position);
- if (child != null) {
- // Found it -- we're using an existing child
- // This just needs to be positioned
- setupChild(child, position, y, flow, childrenLeft, selected, true);
- return child;
- }
- }
- // Make a new view for this position, or convert an unused view if possible
- child = obtainView(position, mIsScrap);
- // This needs to be positioned and measured
- setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
- return child;
- }
- /**
- * Get a view and have it show the data associated with the specified
- * position. This is called when we have already discovered that the view is
- * not available for reuse in the recycle bin. The only choices left are
- * converting an old view or making a new one.
- *
- * @param position
- * The position to display
- * @param isScrap
- * Array of at least 1 boolean, the first entry will become true
- * if the returned view was taken from the scrap heap, false if
- * otherwise.
- *
- * @return A view displaying the data associated with the specified position
- */
- View obtainView(int position, boolean[] isScrap) {
- isScrap[0] = false;
- View scrapView;
- scrapView = mRecycler.getScrapView(position);
- View child;
- if (scrapView != null) {
- child = mAdapter.getView(position, scrapView, this);
- if (child != scrapView) {
- mRecycler.addScrapView(scrapView);
- if (mCacheColorHint != 0) {
- child.setDrawingCacheBackgroundColor(mCacheColorHint);
- }
- } else {
- isScrap[0] = true;
- dispatchFinishTemporaryDetach(child);
- }
- } else {
- child = mAdapter.getView(position, null, this);
- if (mCacheColorHint != 0) {
- child.setDrawingCacheBackgroundColor(mCacheColorHint);
- }
- }
- return child;
- }
那麼我們平時寫ListView的Adapter時,getView()方法通常會怎麼寫呢?這裏我舉個簡單的例子:
- @Override
- public View getView(int position, View convertView, ViewGroup parent) {
- Fruit fruit = getItem(position);
- View view;
- if (convertView == null) {
- view = LayoutInflater.from(getContext()).inflate(resourceId, null);
- } else {
- view = convertView;
- }
- ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
- TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);
- fruitImage.setImageResource(fruit.getImageId());
- fruitName.setText(fruit.getName());
- return view;
- }
那麼這個View也會作爲obtainView()的結果進行返回,並最終傳入到setupChild()方法當中。其實也就是說,第一次layout過程當中,所有的子View都是調用LayoutInflater的inflate()方法加載出來的,這樣就會相對比較耗時,但是不用擔心,後面就不會再有這種情況了,那麼我們繼續往下看:
- /**
- * Add a view as a child and make sure it is measured (if necessary) and
- * positioned properly.
- *
- * @param child The view to add
- * @param position The position of this child
- * @param y The y position relative to which this view will be positioned
- * @param flowDown If true, align top edge to y. If false, align bottom
- * edge to y.
- * @param childrenLeft Left edge where children should be positioned
- * @param selected Is this position selected?
- * @param recycled Has this view been pulled from the recycle bin? If so it
- * does not need to be remeasured.
- */
- private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
- boolean selected, boolean recycled) {
- final boolean isSelected = selected && shouldShowSelector();
- final boolean updateChildSelected = isSelected != child.isSelected();
- final int mode = mTouchMode;
- final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&
- mMotionPosition == position;
- final boolean updateChildPressed = isPressed != child.isPressed();
- final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();
- // Respect layout params that are already in the view. Otherwise make some up...
- // noinspection unchecked
- AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
- if (p == null) {
- p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
- ViewGroup.LayoutParams.WRAP_CONTENT, 0);
- }
- p.viewType = mAdapter.getItemViewType(position);
- if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&
- p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
- attachViewToParent(child, flowDown ? -1 : 0, p);
- } else {
- p.forceAdd = false;
- if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
- p.recycledHeaderFooter = true;
- }
- addViewInLayout(child, flowDown ? -1 : 0, p, true);
- }
- if (updateChildSelected) {
- child.setSelected(isSelected);
- }
- if (updateChildPressed) {
- child.setPressed(isPressed);
- }
- if (needToMeasure) {
- int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
- mListPadding.left + mListPadding.right, p.width);
- int lpHeight = p.height;
- int childHeightSpec;
- if (lpHeight > 0) {
- childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
- } else {
- childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
- }
- child.measure(childWidthSpec, childHeightSpec);
- } else {
- cleanupLayoutState(child);
- }
- final int w = child.getMeasuredWidth();
- final int h = child.getMeasuredHeight();
- final int childTop = flowDown ? y : y - h;
- if (needToMeasure) {
- final int childRight = childrenLeft + w;
- final int childBottom = childTop + h;
- child.layout(childrenLeft, childTop, childRight, childBottom);
- } else {
- child.offsetLeftAndRight(childrenLeft - child.getLeft());
- child.offsetTopAndBottom(childTop - child.getTop());
- }
- if (mCachingStarted && !child.isDrawingCacheEnabled()) {
- child.setDrawingCacheEnabled(true);
- }
- }
那麼到此爲止,第一次Layout過程結束。
第二次Layout
雖然我在源碼中並沒有找出具體的原因,但如果你自己做一下實驗的話就會發現,即使是一個再簡單的View,在展示到界面上之前都會經歷至少兩次onMeasure()和兩次onLayout()的過程。其實這只是一個很小的細節,平時對我們影響並不大,因爲不管是onMeasure()或者onLayout()幾次,反正都是執行的相同的邏輯,我們並不需要進行過多關心。但是在ListView中情況就不一樣了,因爲這就意味着layoutChildren()過程會執行兩次,而這個過程當中涉及到向ListView中添加子元素,如果相同的邏輯執行兩遍的話,那麼ListView中就會存在一份重複的數據了。因此ListView在layoutChildren()過程當中做了第二次Layout的邏輯處理,非常巧妙地解決了這個問題,下面我們就來分析一下第二次Layout的過程。
其實第二次Layout和第一次Layout的基本流程是差不多的,那麼我們還是從layoutChildren()方法開始看起:
- @Override
- protected void layoutChildren() {
- final boolean blockLayoutRequests = mBlockLayoutRequests;
- if (!blockLayoutRequests) {
- mBlockLayoutRequests = true;
- } else {
- return;
- }
- try {
- super.layoutChildren();
- invalidate();
- if (mAdapter == null) {
- resetList();
- invokeOnItemScrollListener();
- return;
- }
- int childrenTop = mListPadding.top;
- int childrenBottom = getBottom() - getTop() - mListPadding.bottom;
- int childCount = getChildCount();
- int index = 0;
- int delta = 0;
- View sel;
- View oldSel = null;
- View oldFirst = null;
- View newSel = null;
- View focusLayoutRestoreView = null;
- // Remember stuff we will need down below
- switch (mLayoutMode) {
- case LAYOUT_SET_SELECTION:
- index = mNextSelectedPosition - mFirstPosition;
- if (index >= 0 && index < childCount) {
- newSel = getChildAt(index);
- }
- break;
- case LAYOUT_FORCE_TOP:
- case LAYOUT_FORCE_BOTTOM:
- case LAYOUT_SPECIFIC:
- case LAYOUT_SYNC:
- break;
- case LAYOUT_MOVE_SELECTION:
- default:
- // Remember the previously selected view
- index = mSelectedPosition - mFirstPosition;
- if (index >= 0 && index < childCount) {
- oldSel = getChildAt(index);
- }
- // Remember the previous first child
- oldFirst = getChildAt(0);
- if (mNextSelectedPosition >= 0) {
- delta = mNextSelectedPosition - mSelectedPosition;
- }
- // Caution: newSel might be null
- newSel = getChildAt(index + delta);
- }
- boolean dataChanged = mDataChanged;
- if (dataChanged) {
- handleDataChanged();
- }
- // Handle the empty set by removing all views that are visible
- // and calling it a day
- if (mItemCount == 0) {
- resetList();
- invokeOnItemScrollListener();
- return;
- } else if (mItemCount != mAdapter.getCount()) {
- throw new IllegalStateException("The content of the adapter has changed but "
- + "ListView did not receive a notification. Make sure the content of "
- + "your adapter is not modified from a background thread, but only "
- + "from the UI thread. [in ListView(" + getId() + ", " + getClass()
- + ") with Adapter(" + mAdapter.getClass() + ")]");
- }
- setSelectedPositionInt(mNextSelectedPosition);
- // Pull all children into the RecycleBin.
- // These views will be reused if possible
- final int firstPosition = mFirstPosition;
- final RecycleBin recycleBin = mRecycler;
- // reset the focus restoration
- View focusLayoutRestoreDirectChild = null;
- // Don't put header or footer views into the Recycler. Those are
- // already cached in mHeaderViews;
- if (dataChanged) {
- for (int i = 0; i < childCount; i++) {
- recycleBin.addScrapView(getChildAt(i));
- if (ViewDebug.TRACE_RECYCLER) {
- ViewDebug.trace(getChildAt(i),
- ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP, index, i);
- }
- }
- } else {
- recycleBin.fillActiveViews(childCount, firstPosition);
- }
- // take focus back to us temporarily to avoid the eventual
- // call to clear focus when removing the focused child below
- // from messing things up when ViewRoot assigns focus back
- // to someone else
- final View focusedChild = getFocusedChild();
- if (focusedChild != null) {
- // TODO: in some cases focusedChild.getParent() == null
- // we can remember the focused view to restore after relayout if the
- // data hasn't changed, or if the focused position is a header or footer
- if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)) {
- focusLayoutRestoreDirectChild = focusedChild;
- // remember the specific view that had focus
- focusLayoutRestoreView = findFocus();
- if (focusLayoutRestoreView != null) {
- // tell it we are going to mess with it
- focusLayoutRestoreView.onStartTemporaryDetach();
- }
- }
- requestFocus();
- }
- // Clear out old views
- detachAllViewsFromParent();
- switch (mLayoutMode) {
- case LAYOUT_SET_SELECTION:
- if (newSel != null) {
- sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
- } else {
- sel = fillFromMiddle(childrenTop, childrenBottom);
- }
- break;
- case LAYOUT_SYNC:
- sel = fillSpecific(mSyncPosition, mSpecificTop);
- break;
- case LAYOUT_FORCE_BOTTOM:
- sel = fillUp(mItemCount - 1, childrenBottom);
- adjustViewsUpOrDown();
- break;
- case LAYOUT_FORCE_TOP:
- mFirstPosition = 0;
- sel = fillFromTop(childrenTop);
- adjustViewsUpOrDown();
- break;
- case LAYOUT_SPECIFIC:
- sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);
- break;
- case LAYOUT_MOVE_SELECTION:
- sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
- break;
- default:
- if (childCount == 0) {
- if (!mStackFromBottom) {
- final int position = lookForSelectablePosition(0, true);
- setSelectedPositionInt(position);
- sel = fillFromTop(childrenTop);
- } else {
- final int position = lookForSelectablePosition(mItemCount - 1, false);
- setSelectedPositionInt(position);
- sel = fillUp(mItemCount - 1, childrenBottom);
- }
- } else {
- if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
- sel = fillSpecific(mSelectedPosition,
- oldSel == null ? childrenTop : oldSel.getTop());
- } else if (mFirstPosition < mItemCount) {
- sel = fillSpecific(mFirstPosition,
- oldFirst == null ? childrenTop : oldFirst.getTop());
- } else {
- sel = fillSpecific(0, childrenTop);
- }
- }
- break;
- }
- // Flush any cached views that did not get reused above
- recycleBin.scrapActiveViews();
- if (sel != null) {
- // the current selected item should get focus if items
- // are focusable
- if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {
- final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&
- focusLayoutRestoreView.requestFocus()) || sel.requestFocus();
- if (!focusWasTaken) {
- // selected item didn't take focus, fine, but still want
- // to make sure something else outside of the selected view
- // has focus
- final View focused = getFocusedChild();
- if (focused != null) {
- focused.clearFocus();
- }
- positionSelector(sel);
- } else {
- sel.setSelected(false);
- mSelectorRect.setEmpty();
- }
- } else {
- positionSelector(sel);
- }
- mSelectedTop = sel.getTop();
- } else {
- if (mTouchMode > TOUCH_MODE_DOWN && mTouchMode < TOUCH_MODE_SCROLL) {
- View child = getChildAt(mMotionPosition - mFirstPosition);
- if (child != null) positionSelector(child);
- } else {
- mSelectedTop = 0;
- mSelectorRect.setEmpty();
- }
- // even if there is not selected position, we may need to restore
- // focus (i.e. something focusable in touch mode)
- if (hasFocus() && focusLayoutRestoreView != null) {
- focusLayoutRestoreView.requestFocus();
- }
- }
- // tell focus view we are done mucking with it, if it is still in
- // our view hierarchy.
- if (focusLayoutRestoreView != null
- && focusLayoutRestoreView.getWindowToken() != null) {
- focusLayoutRestoreView.onFinishTemporaryDetach();
- }
- mLayoutMode = LAYOUT_NORMAL;
- mDataChanged = false;
- mNeedSync = false;
- setNextSelectedPositionInt(mSelectedPosition);
- updateScrollIndicators();
- if (mItemCount > 0) {
- checkSelectionChanged();
- }
- invokeOnItemScrollListener();
- } finally {
- if (!blockLayoutRequests) {
- mBlockLayoutRequests = false;
- }
- }
- }
同樣還是在第19行,調用getChildCount()方法來獲取子View的數量,只不過現在得到的值不會再是0了,而是ListView中一屏可以顯示的子View數量,因爲我們剛剛在第一次Layout過程當中向ListView添加了這麼多的子View。下面在第90行調用了RecycleBin的fillActiveViews()方法,這次效果可就不一樣了,因爲目前ListView中已經有子View了,這樣所有的子View都會被緩存到RecycleBin的mActiveViews數組當中,後面將會用到它們。
接下來將會是非常非常重要的一個操作,在第113行調用了detachAllViewsFromParent()方法。這個方法會將所有ListView當中的子View全部清除掉,從而保證第二次Layout過程不會產生一份重複的數據。那有的朋友可能會問了,這樣把已經加載好的View又清除掉,待會還要再重新加載一遍,這不是嚴重影響效率嗎?不用擔心,還記得我們剛剛調用了RecycleBin的fillActiveViews()方法來緩存子View嗎,待會兒將會直接使用這些緩存好的View來進行加載,而並不會重新執行一遍inflate過程,因此效率方面並不會有什麼明顯的影響。
那麼我們接着看,在第141行的判斷邏輯當中,由於不再等於0了,因此會進入到else語句當中。而else語句中又有三個邏輯判斷,第一個邏輯判斷不成立,因爲默認情況下我們沒有選中任何子元素,mSelectedPosition應該等於-1。第二個邏輯判斷通常是成立的,因爲mFirstPosition的值一開始是等於0的,只要adapter中的數據大於0條件就成立。那麼進入到fillSpecific()方法當中,代碼如下所示:
- /**
- * Put a specific item at a specific location on the screen and then build
- * up and down from there.
- *
- * @param position The reference view to use as the starting point
- * @param top Pixel offset from the top of this view to the top of the
- * reference view.
- *
- * @return The selected view, or null if the selected view is outside the
- * visible area.
- */
- private View fillSpecific(int position, int top) {
- boolean tempIsSelected = position == mSelectedPosition;
- View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected);
- // Possibly changed again in fillUp if we add rows above this one.
- mFirstPosition = position;
- View above;
- View below;
- final int dividerHeight = mDividerHeight;
- if (!mStackFromBottom) {
- above = fillUp(position - 1, temp.getTop() - dividerHeight);
- // This will correct for the top of the first view not touching the top of the list
- adjustViewsUpOrDown();
- below = fillDown(position + 1, temp.getBottom() + dividerHeight);
- int childCount = getChildCount();
- if (childCount > 0) {
- correctTooHigh(childCount);
- }
- } else {
- below = fillDown(position + 1, temp.getBottom() + dividerHeight);
- // This will correct for the bottom of the last view not touching the bottom of the list
- adjustViewsUpOrDown();
- above = fillUp(position - 1, temp.getTop() - dividerHeight);
- int childCount = getChildCount();
- if (childCount > 0) {
- correctTooLow(childCount);
- }
- }
- if (tempIsSelected) {
- return temp;
- } else if (above != null) {
- return above;
- } else {
- return below;
- }
- }
fillSpecific()這算是一個新方法了,不過其實它和fillUp()、fillDown()方法功能也是差不多的,主要的區別在於,fillSpecific()方法會優先將指定位置的子View先加載到屏幕上,然後再加載該子View往上以及往下的其它子View。那麼由於這裏我們傳入的position就是第一個子View的位置,於是fillSpecific()方法的作用就基本上和fillDown()方法是差不多的了,這裏我們就不去關注太多它的細節,而是將精力放在makeAndAddView()方法上面。再次回到makeAndAddView()方法,代碼如下所示:
- /**
- * Obtain the view and add it to our list of children. The view can be made
- * fresh, converted from an unused view, or used as is if it was in the
- * recycle bin.
- *
- * @param position Logical position in the list
- * @param y Top or bottom edge of the view to add
- * @param flow If flow is true, align top edge to y. If false, align bottom
- * edge to y.
- * @param childrenLeft Left edge where children should be positioned
- * @param selected Is this position selected?
- * @return View that was added
- */
- private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
- boolean selected) {
- View child;
- if (!mDataChanged) {
- // Try to use an exsiting view for this position
- child = mRecycler.getActiveView(position);
- if (child != null) {
- // Found it -- we're using an existing child
- // This just needs to be positioned
- setupChild(child, position, y, flow, childrenLeft, selected, true);
- return child;
- }
- }
- // Make a new view for this position, or convert an unused view if possible
- child = obtainView(position, mIsScrap);
- // This needs to be positioned and measured
- setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
- return child;
- }
仍然還是在第19行嘗試從RecycleBin當中獲取Active View,然而這次就一定可以獲取到了,因爲前面我們調用了RecycleBin的fillActiveViews()方法來緩存子View。那麼既然如此,就不會再進入到第28行的obtainView()方法,而是會直接進入setupChild()方法當中,這樣也省去了很多時間,因爲如果在obtainView()方法中又要去infalte佈局的話,那麼ListView的初始加載效率就大大降低了。
注意在第23行,setupChild()方法的最後一個參數傳入的是true,這個參數表明當前的View是之前被回收過的,那麼我們再次回到setupChild()方法當中:
- /**
- * Add a view as a child and make sure it is measured (if necessary) and
- * positioned properly.
- *
- * @param child The view to add
- * @param position The position of this child
- * @param y The y position relative to which this view will be positioned
- * @param flowDown If true, align top edge to y. If false, align bottom
- * edge to y.
- * @param childrenLeft Left edge where children should be positioned
- * @param selected Is this position selected?
- * @param recycled Has this view been pulled from the recycle bin? If so it
- * does not need to be remeasured.
- */
- private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
- boolean selected, boolean recycled) {
- final boolean isSelected = selected && shouldShowSelector();
- final boolean updateChildSelected = isSelected != child.isSelected();
- final int mode = mTouchMode;
- final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&
- mMotionPosition == position;
- final boolean updateChildPressed = isPressed != child.isPressed();
- final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();
- // Respect layout params that are already in the view. Otherwise make some up...
- // noinspection unchecked
- AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
- if (p == null) {
- p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
- ViewGroup.LayoutParams.WRAP_CONTENT, 0);
- }
- p.viewType = mAdapter.getItemViewType(position);
- if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&
- p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
- attachViewToParent(child, flowDown ? -1 : 0, p);
- } else {
- p.forceAdd = false;
- if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
- p.recycledHeaderFooter = true;
- }
- addViewInLayout(child, flowDown ? -1 : 0, p, true);
- }
- if (updateChildSelected) {
- child.setSelected(isSelected);
- }
- if (updateChildPressed) {
- child.setPressed(isPressed);
- }
- if (needToMeasure) {
- int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
- mListPadding.left + mListPadding.right, p.width);
- int lpHeight = p.height;
- int childHeightSpec;
- if (lpHeight > 0) {
- childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
- } else {
- childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
- }
- child.measure(childWidthSpec, childHeightSpec);
- } else {
- cleanupLayoutState(child);
- }
- final int w = child.getMeasuredWidth();
- final int h = child.getMeasuredHeight();
- final int childTop = flowDown ? y : y - h;
- if (needToMeasure) {
- final int childRight = childrenLeft + w;
- final int childBottom = childTop + h;
- child.layout(childrenLeft, childTop, childRight, childBottom);
- } else {
- child.offsetLeftAndRight(childrenLeft - child.getLeft());
- child.offsetTopAndBottom(childTop - child.getTop());
- }
- if (mCachingStarted && !child.isDrawingCacheEnabled()) {
- child.setDrawingCacheEnabled(true);
- }
- }
經歷了這樣一個detach又attach的過程,ListView中所有的子View又都可以正常顯示出來了,那麼第二次Layout過程結束。
滑動加載更多數據
經歷了兩次Layout過程,雖說我們已經可以在ListView中看到內容了,然而關於ListView最神奇的部分我們卻還沒有接觸到,因爲目前ListView中只是加載並顯示了第一屏的數據而已。比如說我們的Adapter當中有1000條數據,但是第一屏只顯示了10條,ListView中也只有10個子View而已,那麼剩下的990是怎樣工作並顯示到界面上的呢?這就要看一下ListView滑動部分的源碼了,因爲我們是通過手指滑動來顯示更多數據的。
由於滑動部分的機制是屬於通用型的,即ListView和GridView都會使用同樣的機制,因此這部分代碼就肯定是寫在AbsListView當中的了。那麼監聽觸控事件是在onTouchEvent()方法當中進行的,我們就來看一下AbsListView中的這個方法:
- @Override
- public boolean onTouchEvent(MotionEvent ev) {
- if (!isEnabled()) {
- // A disabled view that is clickable still consumes the touch
- // events, it just doesn't respond to them.
- return isClickable() || isLongClickable();
- }
- final int action = ev.getAction();
- View v;
- int deltaY;
- if (mVelocityTracker == null) {
- mVelocityTracker = VelocityTracker.obtain();
- }
- mVelocityTracker.addMovement(ev);
- switch (action & MotionEvent.ACTION_MASK) {
- case MotionEvent.ACTION_DOWN: {
- mActivePointerId = ev.getPointerId(0);
- final int x = (int) ev.getX();
- final int y = (int) ev.getY();
- int motionPosition = pointToPosition(x, y);
- if (!mDataChanged) {
- if ((mTouchMode != TOUCH_MODE_FLING) && (motionPosition >= 0)
- && (getAdapter().isEnabled(motionPosition))) {
- // User clicked on an actual view (and was not stopping a
- // fling). It might be a
- // click or a scroll. Assume it is a click until proven
- // otherwise
- mTouchMode = TOUCH_MODE_DOWN;
- // FIXME Debounce
- if (mPendingCheckForTap == null) {
- mPendingCheckForTap = new CheckForTap();
- }
- postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
- } else {
- if (ev.getEdgeFlags() != 0 && motionPosition < 0) {
- // If we couldn't find a view to click on, but the down
- // event was touching
- // the edge, we will bail out and try again. This allows
- // the edge correcting
- // code in ViewRoot to try to find a nearby view to
- // select
- return false;
- }
- if (mTouchMode == TOUCH_MODE_FLING) {
- // Stopped a fling. It is a scroll.
- createScrollingCache();
- mTouchMode = TOUCH_MODE_SCROLL;
- mMotionCorrection = 0;
- motionPosition = findMotionRow(y);
- reportScrollStateChange(OnScrollListener.SCROLL_STATE_TOUCH_SCROLL);
- }
- }
- }
- if (motionPosition >= 0) {
- // Remember where the motion event started
- v = getChildAt(motionPosition - mFirstPosition);
- mMotionViewOriginalTop = v.getTop();
- }
- mMotionX = x;
- mMotionY = y;
- mMotionPosition = motionPosition;
- mLastY = Integer.MIN_VALUE;
- break;
- }
- case MotionEvent.ACTION_MOVE: {
- final int pointerIndex = ev.findPointerIndex(mActivePointerId);
- final int y = (int) ev.getY(pointerIndex);
- deltaY = y - mMotionY;
- switch (mTouchMode) {
- case TOUCH_MODE_DOWN:
- case TOUCH_MODE_TAP:
- case TOUCH_MODE_DONE_WAITING:
- // Check if we have moved far enough that it looks more like a
- // scroll than a tap
- startScrollIfNeeded(deltaY);
- break;
- case TOUCH_MODE_SCROLL:
- if (PROFILE_SCROLLING) {
- if (!mScrollProfilingStarted) {
- Debug.startMethodTracing("AbsListViewScroll");
- mScrollProfilingStarted = true;
- }
- }
- if (y != mLastY) {
- deltaY -= mMotionCorrection;
- int incrementalDeltaY = mLastY != Integer.MIN_VALUE ? y - mLastY : deltaY;
- // No need to do all this work if we're not going to move
- // anyway
- boolean atEdge = false;
- if (incrementalDeltaY != 0) {
- atEdge = trackMotionScroll(deltaY, incrementalDeltaY);
- }
- // Check to see if we have bumped into the scroll limit
- if (atEdge && getChildCount() > 0) {
- // Treat this like we're starting a new scroll from the
- // current
- // position. This will let the user start scrolling back
- // into
- // content immediately rather than needing to scroll
- // back to the
- // point where they hit the limit first.
- int motionPosition = findMotionRow(y);
- if (motionPosition >= 0) {
- final View motionView = getChildAt(motionPosition - mFirstPosition);
- mMotionViewOriginalTop = motionView.getTop();
- }
- mMotionY = y;
- mMotionPosition = motionPosition;
- invalidate();
- }
- mLastY = y;
- }
- break;
- }
- break;
- }
- case MotionEvent.ACTION_UP: {
- switch (mTouchMode) {
- case TOUCH_MODE_DOWN:
- case TOUCH_MODE_TAP:
- case TOUCH_MODE_DONE_WAITING:
- final int motionPosition = mMotionPosition;
- final View child = getChildAt(motionPosition - mFirstPosition);
- if (child != null && !child.hasFocusable()) {
- if (mTouchMode != TOUCH_MODE_DOWN) {
- child.setPressed(false);
- }
- if (mPerformClick == null) {
- mPerformClick = new PerformClick();
- }
- final AbsListView.PerformClick performClick = mPerformClick;
- performClick.mChild = child;
- performClick.mClickMotionPosition = motionPosition;
- performClick.rememberWindowAttachCount();
- mResurrectToPosition = motionPosition;
- if (mTouchMode == TOUCH_MODE_DOWN || mTouchMode == TOUCH_MODE_TAP) {
- final Handler handler = getHandler();
- if (handler != null) {
- handler.removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ? mPendingCheckForTap
- : mPendingCheckForLongPress);
- }
- mLayoutMode = LAYOUT_NORMAL;
- if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {
- mTouchMode = TOUCH_MODE_TAP;
- setSelectedPositionInt(mMotionPosition);
- layoutChildren();
- child.setPressed(true);
- positionSelector(child);
- setPressed(true);
- if (mSelector != null) {
- Drawable d = mSelector.getCurrent();
- if (d != null && d instanceof TransitionDrawable) {
- ((TransitionDrawable) d).resetTransition();
- }
- }
- postDelayed(new Runnable() {
- public void run() {
- child.setPressed(false);
- setPressed(false);
- if (!mDataChanged) {
- post(performClick);
- }
- mTouchMode = TOUCH_MODE_REST;
- }
- }, ViewConfiguration.getPressedStateDuration());
- } else {
- mTouchMode = TOUCH_MODE_REST;
- }
- return true;
- } else if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {
- post(performClick);
- }
- }
- mTouchMode = TOUCH_MODE_REST;
- break;
- case TOUCH_MODE_SCROLL:
- final int childCount = getChildCount();
- if (childCount > 0) {
- if (mFirstPosition == 0
- && getChildAt(0).getTop() >= mListPadding.top
- && mFirstPosition + childCount < mItemCount
- && getChildAt(childCount - 1).getBottom() <= getHeight()
- - mListPadding.bottom) {
- mTouchMode = TOUCH_MODE_REST;
- reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
- } else {
- final VelocityTracker velocityTracker = mVelocityTracker;
- velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
- final int initialVelocity = (int) velocityTracker
- .getYVelocity(mActivePointerId);
- if (Math.abs(initialVelocity) > mMinimumVelocity) {
- if (mFlingRunnable == null) {
- mFlingRunnable = new FlingRunnable();
- }
- reportScrollStateChange(OnScrollListener.SCROLL_STATE_FLING);
- mFlingRunnable.start(-initialVelocity);
- } else {
- mTouchMode = TOUCH_MODE_REST;
- reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
- }
- }
- } else {
- mTouchMode = TOUCH_MODE_REST;
- reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
- }
- break;
- }
- setPressed(false);
- // Need to redraw since we probably aren't drawing the selector
- // anymore
- invalidate();
- final Handler handler = getHandler();
- if (handler != null) {
- handler.removeCallbacks(mPendingCheckForLongPress);
- }
- if (mVelocityTracker != null) {
- mVelocityTracker.recycle();
- mVelocityTracker = null;
- }
- mActivePointerId = INVALID_POINTER;
- if (PROFILE_SCROLLING) {
- if (mScrollProfilingStarted) {
- Debug.stopMethodTracing();
- mScrollProfilingStarted = false;
- }
- }
- break;
- }
- case MotionEvent.ACTION_CANCEL: {
- mTouchMode = TOUCH_MODE_REST;
- setPressed(false);
- View motionView = this.getChildAt(mMotionPosition - mFirstPosition);
- if (motionView != null) {
- motionView.setPressed(false);
- }
- clearScrollingCache();
- final Handler handler = getHandler();
- if (handler != null) {
- handler.removeCallbacks(mPendingCheckForLongPress);
- }
- if (mVelocityTracker != null) {
- mVelocityTracker.recycle();
- mVelocityTracker = null;
- }
- mActivePointerId = INVALID_POINTER;
- break;
- }
- case MotionEvent.ACTION_POINTER_UP: {
- onSecondaryPointerUp(ev);
- final int x = mMotionX;
- final int y = mMotionY;
- final int motionPosition = pointToPosition(x, y);
- if (motionPosition >= 0) {
- // Remember where the motion event started
- v = getChildAt(motionPosition - mFirstPosition);
- mMotionViewOriginalTop = v.getTop();
- mMotionPosition = motionPosition;
- }
- mLastY = y;
- break;
- }
- }
- return true;
- }
這個方法中的代碼就非常多了,因爲它所處理的邏輯也非常多,要監聽各種各樣的觸屏事件。但是我們目前所關心的就只有手指在屏幕上滑動這一個事件而已,對應的是ACTION_MOVE這個動作,那麼我們就只看這部分代碼就可以了。
可以看到,ACTION_MOVE這個case裏面又嵌套了一個switch語句,是根據當前的TouchMode來選擇的。那這裏我可以直接告訴大家,當手指在屏幕上滑動時,TouchMode是等於TOUCH_MODE_SCROLL這個值的,至於爲什麼那又要牽扯到另外的好幾個方法,這裏限於篇幅原因就不再展開講解了,喜歡尋根究底的朋友們可以自己去源碼裏找一找原因。
這樣的話,代碼就應該會走到第78行的這個case裏面去了,在這個case當中並沒有什麼太多需要注意的東西,唯一一點非常重要的就是第92行調用的trackMotionScroll()方法,相當於我們手指只要在屏幕上稍微有一點點移動,這個方法就會被調用,而如果是正常在屏幕上滑動的話,那麼這個方法就會被調用很多次。那麼我們進入到這個方法中瞧一瞧,代碼如下所示:
- boolean trackMotionScroll(int deltaY, int incrementalDeltaY) {
- final int childCount = getChildCount();
- if (childCount == 0) {
- return true;
- }
- final int firstTop = getChildAt(0).getTop();
- final int lastBottom = getChildAt(childCount - 1).getBottom();
- final Rect listPadding = mListPadding;
- final int spaceAbove = listPadding.top - firstTop;
- final int end = getHeight() - listPadding.bottom;
- final int spaceBelow = lastBottom - end;
- final int height = getHeight() - getPaddingBottom() - getPaddingTop();
- if (deltaY < 0) {
- deltaY = Math.max(-(height - 1), deltaY);
- } else {
- deltaY = Math.min(height - 1, deltaY);
- }
- if (incrementalDeltaY < 0) {
- incrementalDeltaY = Math.max(-(height - 1), incrementalDeltaY);
- } else {
- incrementalDeltaY = Math.min(height - 1, incrementalDeltaY);
- }
- final int firstPosition = mFirstPosition;
- if (firstPosition == 0 && firstTop >= listPadding.top && deltaY >= 0) {
- // Don't need to move views down if the top of the first position
- // is already visible
- return true;
- }
- if (firstPosition + childCount == mItemCount && lastBottom <= end && deltaY <= 0) {
- // Don't need to move views up if the bottom of the last position
- // is already visible
- return true;
- }
- final boolean down = incrementalDeltaY < 0;
- final boolean inTouchMode = isInTouchMode();
- if (inTouchMode) {
- hideSelector();
- }
- final int headerViewsCount = getHeaderViewsCount();
- final int footerViewsStart = mItemCount - getFooterViewsCount();
- int start = 0;
- int count = 0;
- if (down) {
- final int top = listPadding.top - incrementalDeltaY;
- for (int i = 0; i < childCount; i++) {
- final View child = getChildAt(i);
- if (child.getBottom() >= top) {
- break;
- } else {
- count++;
- int position = firstPosition + i;
- if (position >= headerViewsCount && position < footerViewsStart) {
- mRecycler.addScrapView(child);
- }
- }
- }
- } else {
- final int bottom = getHeight() - listPadding.bottom - incrementalDeltaY;
- for (int i = childCount - 1; i >= 0; i--) {
- final View child = getChildAt(i);
- if (child.getTop() <= bottom) {
- break;
- } else {
- start = i;
- count++;
- int position = firstPosition + i;
- if (position >= headerViewsCount && position < footerViewsStart) {
- mRecycler.addScrapView(child);
- }
- }
- }
- }
- mMotionViewNewTop = mMotionViewOriginalTop + deltaY;
- mBlockLayoutRequests = true;
- if (count > 0) {
- detachViewsFromParent(start, count);
- }
- offsetChildrenTopAndBottom(incrementalDeltaY);
- if (down) {
- mFirstPosition += count;
- }
- invalidate();
- final int absIncrementalDeltaY = Math.abs(incrementalDeltaY);
- if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {
- fillGap(down);
- }
- if (!inTouchMode && mSelectedPosition != INVALID_POSITION) {
- final int childIndex = mSelectedPosition - mFirstPosition;
- if (childIndex >= 0 && childIndex < getChildCount()) {
- positionSelector(getChildAt(childIndex));
- }
- }
- mBlockLayoutRequests = false;
- invokeOnItemScrollListener();
- awakenScrollBars();
- return false;
- }
這個方法接收兩個參數,deltaY表示從手指按下時的位置到當前手指位置的距離,incrementalDeltaY則表示據上次觸發event事件手指在Y方向上位置的改變量,那麼其實我們就可以通過incrementalDeltaY的正負值情況來判斷用戶是向上還是向下滑動的了。如第34行代碼所示,如果incrementalDeltaY小於0,說明是向下滑動,否則就是向上滑動。
下面將會進行一個邊界值檢測的過程,可以看到,從第43行開始,當ListView向下滑動的時候,就會進入一個for循環當中,從上往下依次獲取子View,第47行當中,如果該子View的bottom值已經小於top值了,就說明這個子View已經移出屏幕了,所以會調用RecycleBin的addScrapView()方法將這個View加入到廢棄緩存當中,並將count計數器加1,計數器用於記錄有多少個子View被移出了屏幕。那麼如果是ListView向上滑動的話,其實過程是基本相同的,只不過變成了從下往上依次獲取子View,然後判斷該子View的top值是不是大於bottom值了,如果大於的話說明子View已經移出了屏幕,同樣把它加入到廢棄緩存中,並將計數器加1。
接下來在第76行,會根據當前計數器的值來進行一個detach操作,它的作用就是把所有移出屏幕的子View全部detach掉,在ListView的概念當中,所有看不到的View就沒有必要爲它進行保存,因爲屏幕外還有成百上千條數據等着顯示呢,一個好的回收策略才能保證ListView的高性能和高效率。緊接着在第78行調用了offsetChildrenTopAndBottom()方法,並將incrementalDeltaY作爲參數傳入,這個方法的作用是讓ListView中所有的子View都按照傳入的參數值進行相應的偏移,這樣就實現了隨着手指的拖動,ListView的內容也會隨着滾動的效果。
然後在第84行會進行判斷,如果ListView中最後一個View的底部已經移入了屏幕,或者ListView中第一個View的頂部移入了屏幕,就會調用fillGap()方法,那麼因此我們就可以猜出fillGap()方法是用來加載屏幕外數據的,進入到這個方法中瞧一瞧,如下所示:
- /**
- * Fills the gap left open by a touch-scroll. During a touch scroll,
- * children that remain on screen are shifted and the other ones are
- * discarded. The role of this method is to fill the gap thus created by
- * performing a partial layout in the empty space.
- *
- * @param down
- * true if the scroll is going down, false if it is going up
- */
- abstract void fillGap(boolean down);
- void fillGap(boolean down) {
- final int count = getChildCount();
- if (down) {
- final int startOffset = count > 0 ? getChildAt(count - 1).getBottom() + mDividerHeight :
- getListPaddingTop();
- fillDown(mFirstPosition + count, startOffset);
- correctTooHigh(getChildCount());
- } else {
- final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight :
- getHeight() - getListPaddingBottom();
- fillUp(mFirstPosition - 1, startOffset);
- correctTooLow(getChildCount());
- }
- }
- /**
- * Obtain the view and add it to our list of children. The view can be made
- * fresh, converted from an unused view, or used as is if it was in the
- * recycle bin.
- *
- * @param position Logical position in the list
- * @param y Top or bottom edge of the view to add
- * @param flow If flow is true, align top edge to y. If false, align bottom
- * edge to y.
- * @param childrenLeft Left edge where children should be positioned
- * @param selected Is this position selected?
- * @return View that was added
- */
- private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
- boolean selected) {
- View child;
- if (!mDataChanged) {
- // Try to use an exsiting view for this position
- child = mRecycler.getActiveView(position);
- if (child != null) {
- // Found it -- we're using an existing child
- // This just needs to be positioned
- setupChild(child, position, y, flow, childrenLeft, selected, true);
- return child;
- }
- }
- // Make a new view for this position, or convert an unused view if possible
- child = obtainView(position, mIsScrap);
- // This needs to be positioned and measured
- setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
- return child;
- }
既然getActiveView()方法返回的值是null,那麼就還是會走到第28行的obtainView()方法當中,代碼如下所示:
- /**
- * Get a view and have it show the data associated with the specified
- * position. This is called when we have already discovered that the view is
- * not available for reuse in the recycle bin. The only choices left are
- * converting an old view or making a new one.
- *
- * @param position
- * The position to display
- * @param isScrap
- * Array of at least 1 boolean, the first entry will become true
- * if the returned view was taken from the scrap heap, false if
- * otherwise.
- *
- * @return A view displaying the data associated with the specified position
- */
- View obtainView(int position, boolean[] isScrap) {
- isScrap[0] = false;
- View scrapView;
- scrapView = mRecycler.getScrapView(position);
- View child;
- if (scrapView != null) {
- child = mAdapter.getView(position, scrapView, this);
- if (child != scrapView) {
- mRecycler.addScrapView(scrapView);
- if (mCacheColorHint != 0) {
- child.setDrawingCacheBackgroundColor(mCacheColorHint);
- }
- } else {
- isScrap[0] = true;
- dispatchFinishTemporaryDetach(child);
- }
- } else {
- child = mAdapter.getView(position, null, this);
- if (mCacheColorHint != 0) {
- child.setDrawingCacheBackgroundColor(mCacheColorHint);
- }
- }
- return child;
- }
那麼另外還有一點是需要大家留意的,這裏獲取到了一個scrapView,然後我們在第22行將它作爲第二個參數傳入到了Adapter的getView()方法當中。那麼第二個參數是什麼意思呢?我們再次看一下一個簡單的getView()方法示例:
- @Override
- public View getView(int position, View convertView, ViewGroup parent) {
- Fruit fruit = getItem(position);
- View view;
- if (convertView == null) {
- view = LayoutInflater.from(getContext()).inflate(resourceId, null);
- } else {
- view = convertView;
- }
- ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
- TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);
- fruitImage.setImageResource(fruit.getImageId());
- fruitName.setText(fruit.getName());
- return view;
- }
之後的代碼又都是我們熟悉的流程了,從緩存中拿到子View之後再調用setupChild()方法將它重新attach到ListView當中,因爲緩存中的View也是之前從ListView中detach掉的,這部分代碼就不再重複進行分析了。
爲了方便大家理解,這裏我再附上一張圖解說明:
那麼到目前爲止,我們就把ListView的整個工作流程代碼基本分析結束了,文章比較長,希望大家可以理解清楚,下篇文章中會講解我們平時使用ListView時遇到的問題,敬請期待。
轉載請註明出處:http://blog.csdn.net/guolin_blog/article/details/44996879