androidView绘制源码分析（下）

Layout的过程

viewGroup会遍历所有子元素并调用 其layout方法&＃xff0c;layout方法来确定子元素的位置。viewgroup如下&＃xff1a;

protected abstract void onLayout(boolean changed,int l , int t, int r, int b) ;

需要子类自己实现。看下view的layout:

public void layout(int l, int t , int r, int b) {if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT ) !&＃61; 0) {onMeasure(mOldWidthMeasureSpec , mOldHeightMeasureSpec) ;mPrivateFlags3 &&＃61; ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;}int oldL &＃61; mLeft;int oldT &＃61; mTop;int oldB &＃61; mBottom;int oldR &＃61; mRight;boolean changed &＃61; isLayoutModeOptical( mParent) ?setOpticalFrame(l, t , r, b) : setFrame(l, t , r, b);if (changed || ( mPrivateFlags & PFLAG_LAYOUT_REQUIRED) &＃61;&＃61; PFLAG_LAYOUT_REQUIRED ) {onLayout(changed, l, t , r, b);mPrivateFlags &&＃61; ~PFLAG_LAYOUT_REQUIRED;ListenerInfo li &＃61; mListenerInfo;if (li !&＃61; null && li.mOnLayoutChangeListeners !&＃61; null) {ArrayList listenersCopy &＃61;(ArrayList)li. mOnLayoutChangeListeners .clone();int numListeners &＃61; listenersCopy.size() ;for ( int i &＃61; 0 ; i }

在setFrame中确定了view的四个顶点坐标。mleft等。onLayout view也没有具体实现&＃xff0c;要看具体的。以LinearLayout为例&＃xff1a;

protected void onLayout(boolean changed, int l , int t, int r, int b) {if (mOrientation &＃61;&＃61; VERTICAL) {layoutVertical(l, t, r , b);} else {layoutHorizontal(l, t, r , b);}
}

以layoutVertical为例&＃xff1a;

void layoutVertical(int left, int top , int right, int bottom) {final int paddingLeft &＃61; mPaddingLeft ;int childTop ;int childLeft ;// Where right end of child should gofinal int width &＃61; right - left;int childRight &＃61; width - mPaddingRight ;// Space available for childint childSpace &＃61; width - paddingLeft - mPaddingRight ;final int count &＃61; getVirtualChildCount() ;final int majorGravity &＃61; mGravity & Gravity.VERTICAL_GRAVITY_MASK;final int minorGravity &＃61; mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;switch (majorGravity) {case Gravity.BOTTOM:// mTotalLength contains the padding alreadychildTop &＃61; mPaddingTop &＃43; bottom - top - mTotalLength;break;// mTotalLength contains the padding alreadycase Gravity.CENTER_VERTICAL:childTop &＃61; mPaddingTop &＃43; (bottom - top - mTotalLength) / 2;break;case Gravity. TOP:default :childTop &＃61; mPaddingTop;break;}for (int i &＃61; 0; i }

主要看以下代码&＃xff1a;

final int childWidth &＃61; child.getMeasuredWidth() ;
final int childHeight &＃61; child.getMeasuredHeight() ;
setChildFrame(child , childLeft, childTop &＃43; getLocationOffset(child) ,childWidth , childHeight);
childTop &＃43;&＃61; childHeight &＃43; lp. bottomMargin &＃43; getNextLocationOffset(child);

top会逐渐增大&＃xff0c;所以会往下排。setChildFrame仅仅是调用子元素的layout方法。

private void setChildFrame(View child, int left, int top , int width, int height) {       child.layout(left, top, left &＃43; width , top &＃43; height);
}

通过子元素的layout来确定自身。

draw过程

它有以下几步&＃xff1a;

• 绘制背景&＃xff0c;(canvas)

• 绘制自己。(onDraw)

• 绘制children(dispatchDraw)

• 绘制装饰&＃xff08;onDrawScrollBars&＃xff09;

看下view的draw源码&＃xff1a;

public void draw(Canvas canvas) {final int privateFlags &＃61; mPrivateFlags;final boolean dirtyOpaque &＃61; (privateFlags & PFLAG_DIRTY_MASK ) &＃61;&＃61; PFLAG_DIRTY_OPAQUE &&(mAttachInfo &＃61;&＃61; null || !mAttachInfo .mIgnoreDirtyState );mPrivateFlags &＃61; (privateFlags & ~ PFLAG_DIRTY_MASK ) | PFLAG_DRAWN;/** Draw traversal performs several drawing steps which must be executed* in the appropriate order:**      1. Draw the background*      2. If necessary, save the canvas&＃39; layers to prepare for fading*      3. Draw view&＃39;s content*      4. Draw children*      5. If necessary, draw the fading edges and restore layers*      6. Draw decorations (scrollbars for instance)*/// Step 1, draw the background, if neededint saveCount;if (!dirtyOpaque) {drawBackground(canvas);}// skip step 2 & 5 if possible (common case)final int viewFlags &＃61; mViewFlags;boolean horizontalEdges &＃61; (viewFlags & FADING_EDGE_HORIZONTAL ) !&＃61; 0;boolean verticalEdges &＃61; (viewFlags & FADING_EDGE_VERTICAL ) !&＃61; 0;if (!verticalEdges && !horizontalEdges) {// Step 3, draw the contentif (!dirtyOpaque) onDraw(canvas) ;// Step 4, draw the childrendispatchDraw(canvas) ;// Step 6, draw decorations (scrollbars)onDrawScrollBars(canvas) ;if ( mOverlay !&＃61; null && !mOverlay.isEmpty()) {mOverlay .getOverlayView().dispatchDraw(canvas) ;}// we&＃39;re done...return;}

viewgroup中的dispatchDraw用于遍历子view并调用子view的draw方法。这样就一层层的传下去。到此源码分析就结束了。在绘制view的时候经常会在activity中获得view的宽高&＃xff0c;因为activity的生命周期和view不同步&＃xff0c;在oncreate中无法获取到view的宽高&＃xff0c;接下来讲讲activity中如何获取view。

三、view宽高确定

• onWindowFocusChanged:view己经初始化完毕&＃xff0c;宽高己经准备好。当Activity得到焦点和失去焦点均会被调用&＃xff0c;所以它会调用多次。

• 通过view.post&＃xff0c;将一个runnable投弟到消息队列尾部&＃xff0c;等待looper调用时&＃xff0c;view己经初始化好。

protected void onStart(){
super.onStart();
view.post(new Runnable(){
public void run(){
int width &＃61; view.getMeasuredWidth();
int height &＃61; new .getMeasuredHeight();
}
})
}

• ViewTreeObserver: 

  使用ViewTreeObserver众多回调接口来完成&＃xff0c;如OnGlobalLayoutListener&＃xff0c;当view树状态发生改变时或内部view可见性发生改变时会回调。

ViewObserver obserber &＃61; view.getViewObserver ();
obserber.addOnGlobalLayoutListener(new OnGlobalLayoutListener(){
public void onGlobalLayout(){
obserber.removeOnGlobalLayoutListener(this);
int width &＃61; view.getMeasuredWidth();
int height &＃61; new .getMeasuredHeight();
}
})

• 通过view进行measure来得到view的宽高。

int width &＃61; MeasureSpec.makeMeasureSpec(100,Measure.EXACTLY);//确定值
int height&＃61; MeasureSpec.makeMeasureSpec(100,Measure.EXACTLY);//确定值
view.measure(width,height);
对于wrap_content:
int width &＃61; MeasureSpec.makeMeasureSpec((1<<30)-1,Measure.AT_MOST);//wrap_content
int height&＃61; MeasureSpec.makeMeasureSpec((1<<30)-1,Measure.AT_MOST);//wrap_content
view.measure(width,height);

四、自定义view中注意事项

自定义View需要注意的事项&＃xff1a;

• 如果是继承view或者viewGroup&＃xff0c;让view支持wrap_content。

• 如果有必要&＃xff0c;让view支持padding。

• View中如果有动画或者线程&＃xff0c;要在onDetachedFromWindow中及时停止。当view的Activity退出或者当前view被remove时&＃xff0c;调用它。