首先要明白invalidate()方法是做什麼的？
View#invalidate()：
/**
* Invalidate the whole view. If the view is visible,
* {@link #onDraw(android.graphics.Canvas)} will be called at some point in
* the future.
* <p>
* This must be called from a UI thread. To call from a non-UI thread, call
* {@link #postInvalidate()}.
*/
public void invalidate() {
invalidate(true);
}
英文註釋的大致意思是：如果View是可見的，使整個View視圖無效，然後在未來的某個時間點View的onDraw(android.graphics.Canvas)方法將被調用。
注意： 該方法必須在UI線程，也就是主線程中才能調用。如果要在非UI線程中調用，可以調用View#postInvalidate() 方法，這也是invalidate()和postInvalidate()的區別之一；
也就是說，invalidate()方法是用來刷新重繪當前的View的，如果當前View的佈局尺寸、位置沒有變化，僅僅是繪製內容變化了，那麼我們就可以調用invalidate()方法。
跟蹤源碼進入 View#invalidate(boolean invalidateCache)：
@UnsupportedAppUsage
public void invalidate(boolean invalidateCache) {
invalidateInternal(0, 0, mRight - mLeft, mBottom - mTop, invalidateCache, true);
}
// View#invalidate(boolean invalidateCache)方法內部，又調用了下面的方法
void invalidateInternal(int l, int t, int r, int b, boolean invalidateCache,
boolean fullInvalidate) {
// 這裏挑重點代碼查看,以免迷失在代碼的海洋中,與源碼流程分析無關的代碼,暫時不關注
......省略代碼
// 這裏判斷該子View是否可見或者是否處於動畫中,如果子View不可見或者沒有處於動畫中,則不讓該子View失效,即該子View不會被重繪
if (skipInvalidate()) {
return;
}
......省略代碼
// 根據View的標記位來判斷該子View是否需要重繪,假如View沒有任何變化,那麼就不需要重繪
if ((mPrivateFlags & (PFLAG_DRAWN | PFLAG_HAS_BOUNDS)) == (PFLAG_DRAWN | PFLAG_HAS_BOUNDS)
|| (invalidateCache && (mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == PFLAG_DRAWING_CACHE_VALID)
|| (mPrivateFlags & PFLAG_INVALIDATED) != PFLAG_INVALIDATED
|| (fullInvalidate && isOpaque() != mLastIsOpaque)) {
if (fullInvalidate) {
mLastIsOpaque = isOpaque();
mPrivateFlags &= ~PFLAG_DRAWN;
}
// 設置PFLAG_DIRTY標記位
mPrivateFlags |= PFLAG_DIRTY;
if (invalidateCache) {
mPrivateFlags |= PFLAG_INVALIDATED;
mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;
}
// Propagate the damage rectangle to the parent view.
final AttachInfo ai = mAttachInfo;
final ViewParent p = mParent;
if (p != null && ai != null && l < r && t < b) {
final Rect damage = ai.mTmpInvalRect;
damage.set(l, t, r, b);
// 子View的ViewParent就是它的父View即ViewGroup
// 調用了父類ViewParent的invalidateChild()方法
p.invalidateChild(this, damage);
}
// Damage the entire projection receiver, if necessary.
......省略代碼
}
}
該方法中，首先判斷該子View是否可見或者是否處於動畫中，如果子View不可見或者沒有處於動畫中，則不讓該子View失效，即該子View不會被重繪。然後根據View的標記位來判斷該子View是否需要重繪，假如View沒有任何變化，那麼就不需要重繪。最後調用了父類ViewParent的invalidateChild()方法，子View的ViewParent就是它的父View即ViewGroup。
跟進ViewGroup#invalidateChild(View child, final Rect dirty) 方法：
public final void invalidateChild(View child, final Rect dirty) {
......省略代碼
// parent爲當前的ViewGroup
ViewParent parent = this;
if (attachInfo != null) {
final boolean drawAnimation = (child.mPrivateFlags & PFLAG_DRAW_ANIMATION) != 0;
Matrix childMatrix = child.getMatrix();
final boolean isOpaque = child.isOpaque() && !drawAnimation &&
child.getAnimation() == null && childMatrix.isIdentity();
int opaqueFlag = isOpaque ? PFLAG_DIRTY_OPAQUE : PFLAG_DIRTY;
if (child.mLayerType != LAYER_TYPE_NONE) {
mPrivateFlags |= PFLAG_INVALIDATED;
mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;
}
final int[] location = attachInfo.mInvalidateChildLocation;
location[CHILD_LEFT_INDEX] = child.mLeft;
location[CHILD_TOP_INDEX] = child.mTop;
......省略代碼
do {
View view = null;
if (parent instanceof View) {
view = (View) parent;
}
if (drawAnimation) {
if (view != null) {
view.mPrivateFlags |= PFLAG_DRAW_ANIMATION;
} else if (parent instanceof ViewRootImpl) {
((ViewRootImpl) parent).mIsAnimating = true;
}
}
// If the parent is dirty opaque or not dirty, mark it dirty with the opaque
// flag coming from the child that initiated the invalidate
if (view != null) {
if ((view.mViewFlags & FADING_EDGE_MASK) != 0 &&
view.getSolidColor() == 0) {
opaqueFlag = PFLAG_DIRTY;
}
if ((view.mPrivateFlags & PFLAG_DIRTY_MASK) != PFLAG_DIRTY) {
// 標誌位的設置
view.mPrivateFlags = (view.mPrivateFlags & ~PFLAG_DIRTY_MASK) | opaqueFlag;
}
}
// 調用parent的invalidateChildInParent,即調用ViewGroup的invalidateChildInParent
parent = parent.invalidateChildInParent(location, dirty);
if (view != null) {
// Account for transform on current parent
Matrix m = view.getMatrix();
if (!m.isIdentity()) {
RectF boundingRect = attachInfo.mTmpTransformRect;
boundingRect.set(dirty);
m.mapRect(boundingRect);
dirty.set((int) Math.floor(boundingRect.left),
(int) Math.floor(boundingRect.top),
(int) Math.ceil(boundingRect.right),
(int) Math.ceil(boundingRect.bottom));
}
}
} while (parent != null);
}
}
ViewGroup#invalidateChild(View child, final Rect dirty) 方法內部，不斷的do while循環，直到循環到最外層view的invalidateChildInParent方法。
內層的parent是調用的ViewGroup的invalidateChildInParent方法。
最外層的View，即DecorView，也就是調用DecorView的ViewParent#invalidateChildInParent方法；
那麼，DecorView的ViewParent是什麼呢？上面的圖，給出了答案，是ViewRootImpl。下面我們從源碼中探尋究竟爲何是ViewRootImpl：
在Activity中，當調用 setContentView() 方法後，經過installDecor() -> generateLayout(mDecor) -> mLayoutInflater.inflate(layoutResID, mContentParent)等方法調用後，Activity的佈局文件就已成功添加到了DecorView的mContentParent中，此時，DecorView還未添加到Window中。
Activity調用onResume方法，然後調用makeVisible方法後，DecorView才被添加到Window中。
這裏簡要分析一下Activity的啓動過程，不做過多分析，簡單寫一下，順序調用ActivityThread的handleLauncheActivity，handleResumeActivity方法，handleResumeActivity方法中首先調用performResumeActivity方法，performResumeActivity方法中調用Activity中的performResume方法，之後調用Activity的onResume方法，最後調用Activity的makeVisible方法，makeVisible方法中會把當前的頂層DecoView通過WindowManager的addView方法添加到WindowManager中，而WindowManager的實現類WindowManagerImpl中調用的是WindowManagerGlobal的addView方法。
如下： 看註釋的關係代碼，這裏實例化了一個ViewRootImpl，調用了setView方法，傳入的view參數就是DecorView；
WindowManagerGlobal#addView：
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
......省略代碼
final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
if (parentWindow != null) {
parentWindow.adjustLayoutParamsForSubWindow(wparams);
} else {
// If there's no parent, then hardware acceleration for this view is
// set from the application's hardware acceleration setting.
final Context context = view.getContext();
if (context != null
&& (context.getApplicationInfo().flags
& ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {
wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;
}
}
ViewRootImpl root;
View panelParentView = null;
synchronized (mLock) {
// Start watching for system property changes.
......省略代碼
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
}
// do this last because it fires off messages to start doing things
try {
// view即爲addView傳入的DecorView
root.setView(view, wparams, panelParentView);
} catch (RuntimeException e) {
// BadTokenException or InvalidDisplayException, clean up.
synchronized (mLock) {
final int index = findViewLocked(view, false);
if (index >= 0) {
removeViewLocked(index, true);
}
}
throw e;
}
}
ViewRootImpl#setView(View view, WindowManager.LayoutParams attrs, View panelParentView)：
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
synchronized (this) {
if (mView == null) {
mView = view;
......省略代碼
// Schedule the first layout -before- adding to the window
// manager, to make sure we do the relayout before receiving
// any other events from the system.
requestLayout();
// 設置DecorView的viewParent爲ViewRootImpl
view.assignParent(this);
mAddedTouchMode = (res & WindowManagerGlobal.ADD_FLAG_IN_TOUCH_MODE) != 0;
mAppVisible = (res & WindowManagerGlobal.ADD_FLAG_APP_VISIBLE) != 0;
if (mAccessibilityManager.isEnabled()) {
mAccessibilityInteractionConnectionManager.ensureConnection();
}
if (view.getImportantForAccessibility() == View.IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
view.setImportantForAccessibility(View.IMPORTANT_FOR_ACCESSIBILITY_YES);
}
// Set up the input pipeline.
......省略代碼
}
}
ViewRootImpl#setView()方法，其中的參數view即爲DecorView，在該方法裏面調用了view.assigenParent(this)，把ViewRootImpl設置爲DecorView的ViewParent。
插入了一段小插曲，下面回頭跟蹤查看
ViewGroup#invalidateChildInParent(final int[] location, final Rect dirty)：
public ViewParent invalidateChildInParent(final int[] location, final Rect dirty) {
if ((mPrivateFlags & (PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID)) != 0) {
// either DRAWN, or DRAWING_CACHE_VALID
if ((mGroupFlags & (FLAG_OPTIMIZE_INVALIDATE | FLAG_ANIMATION_DONE))
!= FLAG_OPTIMIZE_INVALIDATE) {
// 調用offset方法,把當前dirty區域的座標轉化爲父容器中的座標
dirty.offset(location[CHILD_LEFT_INDEX] - mScrollX,
location[CHILD_TOP_INDEX] - mScrollY);
if ((mGroupFlags & FLAG_CLIP_CHILDREN) == 0) {
// 調用union方法,把子dirty區域與父容器的區域求並集,換句話說,dirty區域變成父容器區域
dirty.union(0, 0, mRight - mLeft, mBottom - mTop);
}
final int left = mLeft;
final int top = mTop;
if ((mGroupFlags & FLAG_CLIP_CHILDREN) == FLAG_CLIP_CHILDREN) {
if (!dirty.intersect(0, 0, mRight - left, mBottom - top)) {
dirty.setEmpty();
}
}
location[CHILD_LEFT_INDEX] = left;
location[CHILD_TOP_INDEX] = top;
} else {
if ((mGroupFlags & FLAG_CLIP_CHILDREN) == FLAG_CLIP_CHILDREN) {
dirty.set(0, 0, mRight - mLeft, mBottom - mTop);
} else {
// in case the dirty rect extends outside the bounds of this container
dirty.union(0, 0, mRight - mLeft, mBottom - mTop);
}
location[CHILD_LEFT_INDEX] = mLeft;
location[CHILD_TOP_INDEX] = mTop;
mPrivateFlags &= ~PFLAG_DRAWN;
}
mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;
if (mLayerType != LAYER_TYPE_NONE) {
mPrivateFlags |= PFLAG_INVALIDATED;
}
// 返回當前View的父容器,以便進行下一次循環
return mParent;
}
return null;
}
調用offset方法，把當前dirty區域的座標轉化爲父容器中的座標。接着調用union方法，把子dirty區域與父容器的區域求並集，換句話說，dirty區域變成父容器區域。最後返回當前視圖的父容器，以便進行下一次循環。
由於ViewGroup#invalidateChild() 方法裏面的do while循環完最終會調用最外層 ViewRootImpl 裏面的 invaludateChild 方法。
ViewRootImpl#invalidateChild(View child, Rect dirty)：
@Override
public void invalidateChild(View child, Rect dirty) {
// 內部調用了invalidateChildInParent方法
invalidateChildInParent(null, dirty);
}
ViewRootImpl#invalidateChildInParent(int[] location, Rect dirty)：
@Override
public ViewParent invalidateChildInParent(int[] location, Rect dirty) {
checkThread();
if (DEBUG_DRAW) Log.v(mTag, "Invalidate child: " + dirty);
if (dirty == null) {
invalidate();
return null;
} else if (dirty.isEmpty() && !mIsAnimating) {
return null;
}
if (mCurScrollY != 0 || mTranslator != null) {
mTempRect.set(dirty);
dirty = mTempRect;
if (mCurScrollY != 0) {
dirty.offset(0, -mCurScrollY);
}
if (mTranslator != null) {
mTranslator.translateRectInAppWindowToScreen(dirty);
}
if (mAttachInfo.mScalingRequired) {
dirty.inset(-1, -1);
}
}
// 最後調用 invalidateRectOnScreen 方法
invalidateRectOnScreen(dirty);
return null;
}
該方法主要還是dirty區域的計算。然後調用
ViewRootImpl#invalidateRectOnScreen(Rect dirty)：
private void invalidateRectOnScreen(Rect dirty) {
final Rect localDirty = mDirty;
// Add the new dirty rect to the current one
localDirty.union(dirty.left, dirty.top, dirty.right, dirty.bottom);
// Intersect with the bounds of the window to skip
// updates that lie outside of the visible region
final float appScale = mAttachInfo.mApplicationScale;
final boolean intersected = localDirty.intersect(0, 0,
(int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
if (!intersected) {
localDirty.setEmpty();
}
if (!mWillDrawSoon && (intersected || mIsAnimating)) {
// 重點走到這裏
scheduleTraversals();
}
}
該方法主要是刷新屏幕上面的一個Rect區域，而Rect區域就是調用invalidate方法的那個View大小。然後調用了ViewRootImpl的scheduleTraversals() 方法。
ViewRootImpl#scheduleTraversals()：
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
scheduleTraversals() 方法裏面發送了一個消息，將執行TraversalRunnable 任務方法。
ViewRootImpl#TraversalRunnable：
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
TraversalRunnable 任務方法中又調用了doTraversal() 方法。
ViewRootImpl#doTraversal() ：
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
// 走到這裏,這裏也是很多博客開始分析View的繪製流程時,選擇的切入入口
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
performTraversals() 這個方法非常重要，方法非常多，簡單講我們需要關注以下幾個方法：
ViewRootImpl#performTraversals() ：
private void performTraversals() {
// cache mView since it is used so much below...
final View host = mView;
mIsInTraversal = true;
......省略代碼
if (mFirst || windowShouldResize || insetsChanged ||
viewVisibilityChanged || params != null || mForceNextWindowRelayout) {
mForceNextWindowRelayout = false;
if (!mStopped || mReportNextDraw) {
boolean focusChangedDueToTouchMode = ensureTouchModeLocally(
(relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);
if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()
|| mHeight != host.getMeasuredHeight() || contentInsetsChanged ||
updatedConfiguration) {
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
// Ask host how big it wants to be
// 關注方法 1 performMeasure 測量方法
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
// Implementation of weights from WindowManager.LayoutParams
// We just grow the dimensions as needed and re-measure if
// needs be
int width = host.getMeasuredWidth();
int height = host.getMeasuredHeight();
boolean measureAgain = false;
......省略代碼
if (measureAgain) {
// 關注方法 1 performMeasure 測量方法
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
}
layoutRequested = true;
}
}
} else {
maybeHandleWindowMove(frame);
}
final boolean didLayout = layoutRequested && (!mStopped || mReportNextDraw);
boolean triggerGlobalLayoutListener = didLayout
|| mAttachInfo.mRecomputeGlobalAttributes;
if (didLayout) {
// 關注方法 2 performLayout 位置擺放方法
performLayout(lp, mWidth, mHeight);
......省略代碼
}
......省略代碼
boolean cancelDraw = mAttachInfo.mTreeObserver.dispatchOnPreDraw() || !isViewVisible;
if (!cancelDraw) {
if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
for (int i = 0; i < mPendingTransitions.size(); ++i) {
mPendingTransitions.get(i).startChangingAnimations();
}
mPendingTransitions.clear();
}
// 關注方法 3 performDraw 繪製方法
performDraw();
} else {
......省略代碼
}
mIsInTraversal = false;
}
ViewRootImpl#performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) ：
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
if (mView == null) {
return;
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
// 調用View的measure方法,measure方法內部又會調用View的onMeasure方法對View進行測量
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
該方法內部調用了View的measure方法，measure方法內部又會調用View的onMeasure方法對View進行測量。
ViewRootImpl#performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight)：
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
mLayoutRequested = false;
mScrollMayChange = true;
mInLayout = true;
final View host = mView;
if (host == null) {
return;
}
try {
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
mInLayout = false;
int numViewsRequestingLayout = mLayoutRequesters.size();
if (numViewsRequestingLayout > 0) {
......省略代碼
if (validLayoutRequesters != null) {
......省略代碼
measureHierarchy(host, lp, mView.getContext().getResources(),
desiredWindowWidth, desiredWindowHeight);
mInLayout = true;
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
......省略代碼
}
}
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
mInLayout = false;
}
該方法內部調用了View的layout方法，layout方法內部又會調用View的onLayout方法對View進行佈局擺放。
ViewRootImpl#performDraw() ：
private void performDraw() {
......省略代碼
try {
boolean canUseAsync = draw(fullRedrawNeeded);
if (usingAsyncReport && !canUseAsync) {
mAttachInfo.mThreadedRenderer.setFrameCompleteCallback(null);
usingAsyncReport = false;
}
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
該方法中調用了ViewRootImpl#draw()方法，ViewRootImpl#draw()方法又調用了ViewRootImpl#drawSoftware()方法，ViewRootImpl#drawSoftware()方法中，當前的View調用了View#draw() 方法，即mView.draw(canvas)對當前的View進行繪製;
ViewRootImpl#performTraversals() 方法依次可能會調用了performMeasure，performLayout，performDraw。這三個和我們常用的onMeasure，onLayout，onDraw方法就很像，因爲這三個方法最終也會調用我們常用的onXXX方法。但在這裏這三個方法不一定都會調用，當我們調用invalidate的時候，也就是說我們只想調用繪製我們的View的方法，這個時候只會調用到performDraw方法；當我們的view如果位置發生改變了，則也會調用到performLayout方法；如果大小也改變了，則也會調用perforMeasure方法。這三個方法就會回調View裏面的mesure，layout，draw方法，measure內部會回調onMeasure，layout內部會回調onLayout，draw內部會回調onDraw。
至此，View#invalidate()方法的源碼流程分析完畢；