前言
成爲一名優秀的Android開發,需要一份完備的知識體系,在這裏,讓我們一起成長爲自己所想的那樣~。
上一篇中我們講到了Android的觸摸事件傳遞機制,除此之外,關於Android View的繪製流程這一塊也是View相關的核心知識點。我們都知道,PhoneWindow是Android系統中最基本的窗口系統,每個Activity會創建一個。同時,PhoneWindow也是Activity和View系統交互的接口。DecorView本質上是一個FrameLayout,是Activity中所有View的祖先。
一、開始:DecorView被加載到Window中
從Activity的startActivity開始,最終調用到ActivityThread的handleLaunchActivity方法來創建Activity,相關核心代碼如下:
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
…
// 創建Activity,會調用Activity的onCreate方法
// 從而完成DecorView的創建
Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
Bundle oldState = r.state;
handleResumeActivity(r.tolen, false, r.isForward, !r.activity…mFinished && !r.startsNotResumed);
}
}
final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward, boolean reallyResume) {
unscheduleGcIdler();
mSomeActivitiesChanged = true;
// 調用Activity的onResume方法
ActivityClientRecord r = performResumeActivity(token, clearHide);
if (r != null) {
final Activity a = r.activity;
…
if (r.window == null &&& !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
// 得到DecorView
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
// 得到了WindowManager,WindowManager是一個接口
// 並且繼承了接口ViewManager
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (a.mVisibleFromClient) {
a.mWindowAdded = true;
// WindowManager的實現類是WindowManagerImpl,
// 所以實際調用的是WindowManagerImpl的addView方法
wm.addView(decor, l);
}
}
}
}
public final class WindowManagerImpl implements WindowManager {
private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
…
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mDisplay, mParentWindow);
}
…
}
在瞭解View繪製的整體流程之前,我們必須先了解下ViewRoot和DecorView的概念。ViewRoot對應於ViewRootImpl類,它是連接WindowManager和DecorView的紐帶,View的三大流程均是通過ViewRoot來完成的。在ActivityThread中,當Activity對象被創建完畢後,會將DecorView添加到Window中,同時會創建ViewRootImpl對象,並將ViewRootImpl對象和DecorView建立關聯,相關源碼如下所示:
// WindowManagerGlobal的addView方法
public void addView(View view, ViewGroup.LayoutParams params, Display display, Window parentWindow) {
…
ViewRootImpl root;
View pannelParentView = null;
synchronized (mLock) {
…
// 創建ViewRootImpl實例
root = new ViewRootImpl(view…getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
}
try {
// 把DecorView加載到Window中
root.setView(view, wparams, panelParentView);
} catch (RuntimeException e) {
synchronized (mLock) {
final int index = findViewLocked(view, false);
if (index >= 0) {
removeViewLocked(index, true);
}
}
throw e;
}
}
二、瞭解繪製的整體流程
繪製會從根視圖ViewRoot的performTraversals()方法開始,從上到下遍歷整個視圖樹,每個View控件負責繪製自己,而ViewGroup還需要負責通知自己的子View進行繪製操作。performTraversals()的核心代碼如下。
private void performTraversals() {
…
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
…
//執行測量流程
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
…
//執行佈局流程
performLayout(lp, desiredWindowWidth, desiredWindowHeight);
…
//執行繪製流程
performDraw();
}
performTraversals的大致工作流程圖如下所示:
注意:
- preformLayout和performDraw的傳遞流程和performMeasure是類似的,唯一不同的是,performDraw的傳遞過程是在draw方法中通過dispatchDraw來實現的,不過這並沒有本質區別。
- 獲取content:
ViewGroup content = (ViewGroup)findViewById(android.R.id.content);
- 獲取設置的View:
content.getChildAt(0);
三、理解MeasureSpec
1.MeasureSpec源碼解析
MeasureSpec表示的是一個32位的整形值,它的高2位表示測量模式SpecMode,低30位表示某種測量模式下的規格大小SpecSize。MeasureSpec是View類的一個靜態內部類,用來說明應該如何測量這個View。MeasureSpec的核心代碼如下。
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0X3 << MODE_SHIFT;
// 不指定測量模式, 父視圖沒有限制子視圖的大小,子視圖可以是想要
// 的任何尺寸,通常用於系統內部,應用開發中很少用到。
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
// 精確測量模式,視圖寬高指定爲match_parent或具體數值時生效,
// 表示父視圖已經決定了子視圖的精確大小,這種模式下View的測量
// 值就是SpecSize的值。
public static final int EXACTLY = 1 << MODE_SHIFT;
// 最大值測量模式,當視圖的寬高指定爲wrap_content時生效,此時
// 子視圖的尺寸可以是不超過父視圖允許的最大尺寸的任何尺寸。
public static final int AT_MOST = 2 << MODE_SHIFT;
// 根據指定的大小和模式創建一個MeasureSpec
public static int makeMeasureSpec(int size, int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
// 微調某個MeasureSpec的大小
static int adjust(int measureSpec, int delta) {
final int mode = getMode(measureSpec);
if (mode == UNSPECIFIED) {
// No need to adjust size for UNSPECIFIED mode.
return make MeasureSpec(0, UNSPECIFIED);
}
int size = getSize(measureSpec) + delta;
if (size < 0) {
size = 0;
}
return makeMeasureSpec(size, mode);
}
}
MeasureSpec通過將SpecMode和SpecSize打包成一個int值來避免過多的對象內存分配,爲了方便操作,其提供了打包和解包的方法,打包方法爲上述源碼中的makeMeasureSpec,解包方法源碼如下:
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
2.DecorView的MeasureSpec的創建過程:
//desiredWindowWidth和desiredWindowHeight是屏幕的尺寸
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
private static int getRootMeaureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATRCH_PARENT:
// Window can’t resize. Force root view to be windowSize.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break
default:
// Window wants to be an exact size. Force root view to be that size.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
3.子元素的MeasureSpec的創建過程
// ViewGroup的measureChildWithMargins方法
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
// 子元素的MeasureSpec的創建與父容器的MeasureSpec和子元素本身
// 的LayoutParams有關,此外還和View的margin及padding有關
final int childWidthMeasureSpec = getChildMeasureSpec(
parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin + widthUsed,
lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(
parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin + heightUsed,
lp.height);
child…measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
public static int getChildMeasureSpec(int spec, int padding, int childDimesion) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
// padding是指父容器中已佔用的空間大小,因此子元素可用的
// 大小爲父容器的尺寸減去padding
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (sepcMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimesion == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can’t be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size… so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can’t be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size… let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size… find out how big it should be
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size…
// find out how big it should be
resultSize = 0;
resultMode == MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
普通View的MeasureSpec的創建規則如下:
注意:UNSPECIFIED模式主要用於系統內部多次Measure的情形,一般不需關注。
結論:對於DecorView而言,它的MeasureSpec由窗口尺寸和其自身的LayoutParams共同決定;對於普通的View,它的MeasureSpec由父視圖的MeasureSpec和其自身的LayoutParams共同決定。
四、View繪製流程之Measure
1.Measure的基本流程
由前面的分析可知,頁面的測量流程是從performMeasure方法開始的,相關的核心代碼流程如下。
private void perormMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
…
// 具體的測量操作分發給ViewGroup
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
…
}
// 在ViewGroup中的measureChildren()方法中遍歷測量ViewGroup中所有的View
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
// 當View的可見性處於GONE狀態時,不對其進行測量
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
// 測量某個指定的View
protected void measureChild(View child, int parentWidthMeasureSpec, int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
// 根據父容器的MeasureSpec和子View的LayoutParams等信息計算
// 子View的MeasureSpec
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
// View的measure方法
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
…
// ViewGroup沒有定義測量的具體過程,因爲ViewGroup是一個
// 抽象類,其測量過程的onMeasure方法需要各個子類去實現
onMeasure(widthMeasureSpec, heightMeasureSpec);
…
}
// 不同的ViewGroup子類有不同的佈局特性,這導致它們的測量細節各不相同,如果需要自定義測量過程,則子類可以重寫這個方法
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// setMeasureDimension方法用於設置View的測量寬高
setMeasureDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
// 如果View沒有重寫onMeasure方法,則會默認調用getDefaultSize來獲得View的寬高
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = sepcSize;
break;
}
return result;
}
2.對getSuggestMinimumWidth的分析
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinmumWidth());
}
protected int getSuggestedMinimumHeight() {
return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());
}
public int getMinimumWidth() {
final int intrinsicWidth = getIntrinsicWidth();
return intrinsicWidth > 0 ? intrinsicWidth : 0;
}
如果View沒有設置背景,那麼返回android:minWidth這個屬性所指定的值,這個值可以爲0;如果View設置了背景,則返回android:minWidth和背景的最小寬度這兩者中的最大值。
3.自定義View時手動處理wrap_content時的情形
直接繼承View的控件需要重寫onMeasure方法並設置wrap_content時的自身大小,否則在佈局中使用wrap_content就相當於使用match_parent。解決方式如下:
protected void onMeasure(int widthMeasureSpec,
int height MeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
int widtuhSpecSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
// 在wrap_content的情況下指定內部寬/高(mWidth和mHeight)
int heightSpecSize = MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(mWidth, mHeight);
} else if (widthSpecMode == MeasureSpec.AT_MOST) {
setMeasureDimension(mWidth, heightSpecSize);
} else if (heightSpecMode == MeasureSpec.AT_MOST) {
setMeasureDimension(widthSpecSize, mHeight);
}
}
4.LinearLayout的onMeasure方法實現解析
protected void onMeasure(int widthMeasureSpec, int hegithMeasureSpec) {
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
// measureVertical核心源碼
// See how tall everyone is. Also remember max width.
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
…
// Determine how big this child would like to be. If this or
// previous children have given a weight, then we allow it to
// use all available space (and we will shrink things later
// if need)
measureChildBeforeLayout(
child, i, widthMeasureSpec, 0, heightMeasureSpec,
totalWeight == 0 ? mTotalLength : 0);
if (oldHeight != Integer.MIN_VALUE) {
lp.height = oldHeight;
}
final int childHeight = child.getMeasuredHeight();
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child));
}
系統會遍歷子元素並對每個子元素執行measureChildBeforeLayout方法,這個方法內部會調用子元素的measure方法,這樣各個子元素就開始依次進入measure過程,並且系統會通過mTotalLength這個變量來存儲LinearLayout在豎直方向的初步高度。每測量一個子元素,mTotalLength就會增加,增加的部分主要包括了子元素的高度以及子元素在豎直方向上的margin等。
// LinearLayout測量自己大小的核心源碼
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
int heightSize = mTotalLength;
// Check against our minimum height
heightSize = Math.max(heightSize, getSuggestedMinimumHeight());
// Reconcile our calculated size with the heightMeasureSpec
int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
heightSize = heightSizeAndState & MEASURED_SIZE_MASK;
…
setMeasuredDimension(resolveSizeAndSize(maxWidth, widthMeasureSpec, childState),
heightSizeAndState);
public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
// 高度不能超過父容器的剩餘空間
if (specSize < size) {
result = specSize | MEASURED_STATE_TOO_SMALL;
} else {
result = size;
}
break;
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result | (childMeasuredState & MEASURED_STATE_MASK);
}
5.在Activity中獲取某個View的寬高
由於View的measure過程和Activity的生命週期方法不是同步執行的,如果View還沒有測量完畢,那麼獲得的寬/高就是0。所以在onCreate、onStart、onResume中均無法正確得到某個View的寬高信息。解決方式如下:
- Activity/View#onWindowFocusChanged
// 此時View已經初始化完畢
// 當Activity的窗口得到焦點和失去焦點時均會被調用一次
// 如果頻繁地進行onResume和onPause,那麼onWindowFocusChanged也會被頻繁地調用
public void onWindowFocusChanged(boolean hasFocus) {
super.onWindowFocusChanged(hasFocus);
if (hasFocus) {
int width = view.getMeasureWidth();
int height = view.getMeasuredHeight();
}
}
- view.post(runnable)
// 通過post可以將一個runnable投遞到消息隊列的尾部,// 然後等待Looper調用次runnable的時候,View也已經初
// 始化好了
protected void onStart() {
super.onStart();
view.post(new Runnable() {
@Override
public void run() {
int width = view.getMeasuredWidth();
int height = view.getMeasuredHeight();
}
});
}
- ViewTreeObserver
// 當View樹的狀態發生改變或者View樹內部的View的可見// 性發生改變時,onGlobalLayout方法將被回調
protected void onStart() {
super.onStart();
ViewTreeObserver observer = view.getViewTreeObserver();
observer.addOnGlobalLayoutListener(new OnGlobalLayoutListener() {
@SuppressWarnings(“deprecation”)
@Override
public void onGlobalLayout() {
view.getViewTreeObserver().removeGlobalOnLayoutListener(this);
int width = view.getMeasuredWidth();
int height = view.getMeasuredHeight();
}
});
}
- View.measure(int widthMeasureSpec, int heightMeasureSpec)
五、View的繪製流程之Layout
1.Layout的基本流程
// ViewRootImpl.java
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) {
…
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
…
}
// View.java
public void layout(int l, int t, int r, int b) {
…
// 通過setFrame方法來設定View的四個頂點的位置,即View在父容器中的位置
boolean changed = isLayoutModeOptical(mParent) ?
set OpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
…
onLayout(changed, l, t, r, b);
…
}
// 空方法,子類如果是ViewGroup類型,則重寫這個方法,實現ViewGroup
// 中所有View控件佈局流程
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
2.LinearLayout的onLayout方法實現解析
protected void onlayout(boolean changed, int l, int t, int r, int b) {
if (mOrientation == VERTICAL) {
layoutVertical(l, t, r, b);
} else {
layoutHorizontal(l,)
}
}
// layoutVertical核心源碼
void layoutVertical(int left, int top, int right, int bottom) {
…
final int count = getVirtualChildCount();
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {
final int childWidth = child.getMeasureWidth();
final int childHeight = child.getMeasuredHeight();
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams();
…
if (hasDividerBeforeChildAt(i)) {
childTop += mDividerHeight;
}
childTop += lp.topMargin;
// 爲子元素確定對應的位置
setChildFrame(child, childLeft, childTop + getLocationOffset(child), childWidth, childHeight);
// childTop會逐漸增大,意味着後面的子元素會被
// 放置在靠下的位置
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child,i)
}
}
}
private void setChildFrame(View child, int left, int top, int width, int height) {
child.layout(left, top, left + width, top + height);
}
注意:在View的默認實現中,View的測量寬/高和最終寬/高是相等的,只不過測量寬/高形成於View的measure過程,而最終寬/高形成於View的layout過程,即兩者的賦值時機不同,測量寬/高的賦值時機稍微早一些。在一些特殊的情況下則兩者不相等:
- 重寫View的layout方法,使最終寬度總是比測量寬/高大100px
public void layout(int l, int t, int r, int b) {
super.layout(l, t, r + 100, b + 100);
}
- View需要多長measure才能確定自己的測量寬/高,在前幾次測量的過程中,其得出的測量寬/高有可能和最終寬/高不一致,但最終來說,測量寬/高還是和最終寬/高相同
六、View的繪製流程之Draw
1.Draw的基本流程
private void performDraw() {
…
draw(fullRefrawNeeded);
…
}
private void draw(boolean fullRedrawNeeded) {
…
if (!drawSoftware(surface, mAttachInfo, xOffest, yOffset,
scalingRequired, dirty)) {
return;
}
…
}
private boolean drawSoftware(Surface surface, AttachInfo attachInfo,
int xoff, int yoff, boolean scallingRequired, Rect dirty) {
…
mView.draw(canvas);
…
}
// 繪製基本上可以分爲六個步驟
public void draw(Canvas canvas) {
…
// 步驟一:繪製View的背景
drawBackground(canvas);
…
// 步驟二:如果需要的話,保持canvas的圖層,爲fading做準備
saveCount = canvas.getSaveCount();
…
canvas.saveLayer(left, top, right, top + length, null, flags);
…
// 步驟三:繪製View的內容
onDraw(canvas);
…
// 步驟四:繪製View的子View
dispatchDraw(canvas);
…
// 步驟五:如果需要的話,繪製View的fading邊緣並恢復圖層
canvas.drawRect(left, top, right, top + length, p);
…
canvas.restoreToCount(saveCount);
…
// 步驟六:繪製View的裝飾(例如滾動條等等)
onDrawForeground(canvas)
}
2.setWillNotDraw的作用
// 如果一個View不需要繪製任何內容,那麼設置這個標記位爲true以後,
// 系統會進行相應的優化。
public void setWillNotDraw(boolean willNotDraw) {
setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);
}
- 默認情況下,View沒有啓用這個優化標記位,但是ViewGroup會默認啓用這個優化標記位。
- 當我們的自定義控件繼承於ViewGroup並且本身不具備繪製功能時,就可以開啓這個標記位從而便於系統進行後續的優化。
- 當明確知道一個ViewGroup需要通過onDraw來繪製內容時,我們需要顯示地關閉WILL_NOT_DRAW這個標記位。
七、總結
View的繪製流程和事件分發機制都是Android開發中的核心知識點,也是自定義View高手的內功心法。對於一名優秀的Android開發來說,主流三方源碼分析和Android核心源碼分析可以說是必修課,下一篇,將會帶領大家更進一步深入Android。
參考鏈接:
1、Android開發藝術探索
2、Android進階之光
3、Android高級進階
讚賞
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About me
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Email: [email protected]
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Blog: https://jsonchao.github.io/
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