繪制流程

• ViewRootImpl.performTraversals 起點
• Measure 決定view大小
• Layout 決定view的位置
• Draw 繪制

performTraversals

依次執行measure,layout,draw

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");try {mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);} finally {Trace.traceEnd(Trace.TRACE_TAG_VIEW);}} private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,int desiredWindowHeight) {...host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());...} private void performDraw() {...} private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,boolean scalingRequired, Rect dirty) {...}

measure

父容器傳遞measureSpec至子容器。子容器根據measureSpec，并結合自身的layoutParams，測量出大小。父容器最后根據子容器的測量值，再測量出自身規格.

• 元素visibility為GONE的不參與measure
• 當viewGroup的子類的屬性有margin或padding時，子類會執行viewGroup中的measureChildWithMargins，最終測量的大小會specSize - padding。
• View中的源碼

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));} //獲取的是android:minHeight屬性的值或者View背景圖片的大小 protected int getSuggestedMinimumHeight() {return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());} //獲取默認的大小,size為自身大小，measureSpec為父元素傳下來的，可見一個view的大小是由父元素的measureSpec和view的layoutParams共通決定 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 = specSize;break;}return result;}

• 其它view的子類有各自的實現方式，不是直接拿measurespec和layoutParams進行測量
• ViewGroup中沒有實現onMeasure,都由ViewGroup中的子類進行實現，如FrameLayout等，ViewGroup的子類在onMeasure過程中，遞歸所有子view，執行measureChildWithMargins、measure、onMeasure。測量完所有子View后，再通過用view中的setMeasuredDimension進行測量。

measureChildWithMargins

• 如果view有固定值，則view的measureSpec的mode為EXACTLY，size為該固定值。
• 在父元素為EXACTLY的情況下。子元素若為match_parent,則子元素的measureSpec，mode則為EXACTLY,size為父元素size。子元素若為wrap_content,則子元素的measureSpec，mode為AT_MOST,size為Math.max(0, specSize - padding)
• 在父元素為AT_MOST的情況下。子元素無論layoutParams是match_parent還是wrap_content,子元素的measureSpec，mode都為AT_MOST,size都為Math.max(0, specSize - padding)
• 在父元素為UNSPECIFIED時，子元素無論是match還是wrap，子元素的measureSpec,mode為UNSPECIFIED，size為View.sUseZeroUnspecifiedMeasureSpec ? 0 : size；

protected void measureChildWithMargins(View child,int parentWidthMeasureSpec, int widthUsed,int parentHeightMeasureSpec, int heightUsed) {final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();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 childDimension) {int specMode = MeasureSpec.getMode(spec);int specSize = MeasureSpec.getSize(spec);int size = Math.max(0, specSize - padding);int resultSize = 0;int resultMode = 0;switch (specMode) {// Parent has imposed an exact size on uscase 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 (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 has imposed a maximum size on uscase MeasureSpec.AT_MOST:if (childDimension >= 0) {// Child wants a specific size... so be itresultSize = 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 becase MeasureSpec.UNSPECIFIED:if (childDimension >= 0) {// Child wants a specific size... let him have itresultSize = childDimension;resultMode = MeasureSpec.EXACTLY;} else if (childDimension == LayoutParams.MATCH_PARENT) {// Child wants to be our size... find out how big it should// beresultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;resultMode = MeasureSpec.UNSPECIFIED;} else if (childDimension == LayoutParams.WRAP_CONTENT) {// Child wants to determine its own size.... find out how// big it should beresultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;resultMode = MeasureSpec.UNSPECIFIED;}break;}//noinspection ResourceTypereturn MeasureSpec.makeMeasureSpec(resultSize, resultMode);}

layout

• 最先執行的是viewGroup中的layout,判斷當前有沒有正在執行的layoutTransition,沒有就調用view中的layout，否則，就等transition執行完后，繼續調用。
• view的layout中，有兩種設置view位置的方法，一種是setOpticalFrame，另外一種是setFrame。調用哪個，取決于父對象是不是viewGroup，且是否可見，若全是，則調用setOpticalFrame,getOpticalInsets則是獲取drawable的insets。

private boolean setOpticalFrame(int left, int top, int right, int bottom) {Insets parentInsets = mParent instanceof View ?((View) mParent).getOpticalInsets() : Insets.NONE;Insets childInsets = getOpticalInsets();return setFrame(left + parentInsets.left - childInsets.left,top + parentInsets.top - childInsets.top,right + parentInsets.left + childInsets.right,bottom + parentInsets.top + childInsets.bottom);}

• setFrame結束后，還會執行OnLayoutChangeListener.onLayoutChange函數，若需要監聽，layout的變化，可addOnLayoutChangeListener。
• layout的四個參數，默認是建立在measure的width和height基礎上的，當也可以重寫。并不依賴measure。如果重寫ltrb，則getMeasuredWidth和getWidth取得的值不一致。

draw

ViewGroup中并沒有重寫draw，則執行view中draw

如果有設置背景，則繪制背景 Draw the background

保存canvas層 If necessary, save the canvas' layers to prepare for fading

繪制自身內容 Draw view's content

如果有子元素則繪制子元素 Draw children

繪制效果 If necessary, draw the fading edges and restore layers

繪制裝飾品(scrollbars) Draw decorations (scrollbars for instance)

• canvas對象由ViewRootImpl的drawSoftware建立，傳遞至view中。
• 每當調用invalite時，都會將之前view區域設為dirty

private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,boolean scalingRequired, Rect dirty) {...canvas = mSurface.lockCanvas(dirty);...} public Canvas lockCanvas(Rect inOutDirty)throws Surface.OutOfResourcesException, IllegalArgumentException {synchronized (mLock) {checkNotReleasedLocked();if (mLockedObject != 0) {// Ideally, nativeLockCanvas() would throw in this situation and prevent the// double-lock, but that won't happen if mNativeObject was updated. We can't// abandon the old mLockedObject because it might still be in use, so instead// we just refuse to re-lock the Surface.throw new IllegalArgumentException("Surface was already locked");}mLockedObject = nativeLockCanvas(mNativeObject, mCanvas, inOutDirty);return mCanvas;}}

surfaceView

measureSpec

• 一個measureSpec包含了父容器傳遞給子容器的布局要求。
• 包含了mode與size的組合int值，高兩位為mode，后面是size。

public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,@MeasureSpecMode int mode) {if (sUseBrokenMakeMeasureSpec) {return size + mode;} else {return (size & ~MODE_MASK) | (mode & MODE_MASK);}}

• 三種模式
• • UPSPECIFIED : 父容器對于子容器沒有任何限制,子容器想要多大就多大
• • EXACTLY: 父容器已經為子容器設置了尺寸,子容器應當服從這些邊界,不論子容器想要多大的空間。
• • AT_MOST：子容器可以是聲明大小內的任意大小

轉載于:https://www.cnblogs.com/qinhe/p/6370858.html

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