android View绘制源码分析
最后更新于：2022-04-01 14:26:35
在开发过程中我们经常要进行view的自定义。如果熟练掌握自定义技巧的话就能做出很多控件出来。这篇博客来讲讲view绘制背后发生的那些事。
### 一, view的基础知识
## view的绘制概括
首先先说说view绘制的整体过程。
View绘制的源码分析 ，它的三大流程都是在ViewRootImpl中完成的，从ViewRootImpl中的performTraversals开始，有三个方法performMeasure,performLayout,prformDraw分别对measure,layout,draw三个方法。在onMeasure对所有子元素进行measure过程 ，这时measure就从父容器传递到子元素。子元素重复父元素的过程。layout与draw类似，只是draw通过diapatchDraw来实现 。
measure完成后可以通过getMeasureWidth,getMeasureHeight分别获取View测量后的宽高。在实际情况下几乎所有情况它都等于最终宽高。layout过程决定view的四个顶点的坐标和实际view的宽高，完成之后可以通过getTop,getBottom,getLeft,getRight来拿 到view的四个顶点位置。并通过getWidth()和getHeight()来拿到最终宽高。draw决定了view的显示，只有完成才能显示在屏幕上。
## MeasureSpec
在测量过程中系统会将View的LayoutParams根据容器所施加的规则转换成对应的MeasureSpec，然后再根据这个测量出view。
Measure是一个32位的int,高2位代表SpecMode，低30位代表SpecSize。SpecMode表示测量模式，SpecSize指在某种测量模式下规格的大小。其代码如下：
~~~
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
public static final int EXACTLY = 1 << MODE_SHIFT;
public static final int AT_MOST = 2 << MODE_SHIFT;
public static int makeMeasureSpec(int size, int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK) ;
}
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK) ;
}
~~~
其实MeasureSpec中源码很值得我们学习。他用一个32位的int来表示模式和大小，节省了空间，也更直观。MeasureSpec通过将specMode和specSize打包成一个int来避免过多的对象内存分配。以上是MeasureSpec的打包和解包过程。
specMode有三种状态：UNSPECIFIED,EXACTLY（相当于match_parent和精确值这两种模式）,AT_MOST（wrap_content）。
## LayoutParams
对于一般容器，它的MeasureSpec是由父容器的MeasureSpec和自身的LayoutParams共同决定的。上篇博客[LayoutInflater源码解析](http://blog.csdn.net/u014486880/article/details/50707672) 我们己经介绍了android view的结构，PhoneWindow包了一层DecorView,DecorView里才是title和我们的content view。所以行分析DecorView。
先来看下DecorView的产生源码：
~~~
childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
~~~
再看下getRootMeasureSpec方法：
~~~
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATCH_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.//自定义
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
~~~
这里很清楚，分别分MatchPraent和wrap_content和自定义来计算宽高。再来看下普通的view,在ViewGroup的measureChildWIthMargins中：
~~~
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);
}
~~~
再看下getChildMeasureSpec：
~~~
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 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 (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 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) ;
}
~~~
以上表明，如果父是EXACTLY,parentSize，那么子如果是EXACTLY，
1）具体的值size：那子的MeasureSpec就是EXACTLY,size;
2)MATCH_PARENT:那子的MeasureSpec就是EXACTLY,parentSize;
3)WRAP_CONTENT:那子的MeasureSpec就是AT_MOST,parentSize;
如果父是ATMOST,parentSize，那么子如果是EXACTLY，
1）具体的值size：那子的MeasureSpec就是EXACTLY,size;
2)MATCH_PARENT:那子的MeasureSpec就是AT_MOST,parentSize;
3)WRAP_CONTENT:那子的MeasureSpec就是AT_MOST,parentSize;
总结：对于普通View的MeasureSpec由父容器的MeasureSpec和自身的LayoutParams来共同决定。
## 二，三大过程源码分析
### OnMeasure
* measure。如果是view,measure绘制其自身。如果是VIewGroup，measure绘制自身外，还要绘制其子元素。先看View的measure方法，measure是一个final方法，不能重写：
~~~
if (cacheIndex < 0 |if (cacheIndex < 0 || sIgnoreMeasureCache ) {
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec , heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}| sIgnoreMeasureCache ) {
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec , heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
~~~
调用了onMeasure()，来看下它的源码：
~~~
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth() , widthMeasureSpec) ,
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)) ;
}
~~~
看下getSuggestedMinimumWidth()，它就是获取背景大小和mMinWidth的较大值：
~~~
protected int getSuggestedMinimumWidth () {
return (mBackground == null) ? mMinWidth : max(mMinWidth , mBackground .getMinimumWidth());
}
~~~
那么mMinWidth是什么呢，mMinWidth就是设置的android:minWidth的属性，没设置就等于0。不信，看如下代码：
~~~
case R.styleable.View_minWidth:
mMinWidth = a.getDimensionPixelSize(attr , 0) ;
break;
~~~
getMinimumWidth()表示的是获取背景图大小，它位于Drawable下：
~~~
public int getMinimumHeight() {
final int intrinsicHeight = getIntrinsicHeight() ;
return intrinsicHeight > 0 ? intrinsicHeight : 0 ;
}
~~~
看下getDefaultSize方法：
~~~
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;
}
~~~
它返回了specSize，它是测量后的大小。由上面的分析可知，view的宽高由specSize决定，而如果直接继承View的控件需要重写onMeasure方法并设置wrap_content的自身大小。否则wrap_content就相当 于Match_parent了。一般一重写方法如下：
~~~
protected void onMeasure( int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);
if (widthSpecMode == MeasureSpec. AT_MOST
&& heightSpecMode == MeasureSpec. AT_MOST) {
setMeasuredDimension(200, 200);
} else if (widthSpecMode == MeasureSpec. AT_MOST) {
setMeasuredDimension(200, heightSpecSize);
} else if (heightSpecMode == MeasureSpec. AT_MOST) {
setMeasuredDimension(widthSpecSize, 200);
}
}
~~~
上面的200是指定的一个默认宽高。
2.ViewGroup的measure过程，它没有重写onMeasure，它会调用measureChildren如下：
~~~
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] ;
if ((child. mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec , heightMeasureSpec);
}
}
}
~~~
分别绘制child，进入measureChild：
~~~
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams() ;
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec ,
mPaddingLeft + mPaddingRight, lp.width) ;
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec ,
mPaddingTop + mPaddingBottom, lp.height) ;
child.measure(childWidthMeasureSpec , childHeightMeasureSpec);
}
~~~
获取LayoutParams，通过getChildMeasureSpec来创建子无素的MeasureSpec，调用child.measure，因为ViewGroup有不同的特性，所以无法实现统一的onMeasure。
## Layout的过程
viewGroup会遍历所有子元素并调用 其layout方法，layout方法来确定子元素的位置。viewgroup如下：
~~~
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 ) != 0) {
onMeasure(mOldWidthMeasureSpec , mOldHeightMeasureSpec) ;
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
boolean changed = isLayoutModeOptical( mParent) ?
setOpticalFrame(l, t , r, b) : setFrame(l, t , r, b);
if (changed || ( mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED ) {
onLayout(changed, l, t , r, b);
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList<OnLayoutChangeListener> listenersCopy =
(ArrayList<OnLayoutChangeListener>)li. mOnLayoutChangeListeners .clone();
int numListeners = listenersCopy.size() ;
for ( int i = 0 ; i < numListeners; ++i) {
listenersCopy.get(i).onLayoutChange( this, l, t, r , b, oldL, oldT , oldR, oldB);
}
}
}
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}
~~~
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，要看具体的。以LinearLayout为例：
~~~
protected void onLayout(boolean changed, int l , int t, int r, int b) {
if (mOrientation == VERTICAL) {
layoutVertical(l, t, r , b);
} else {
layoutHorizontal(l, t, r , b);
}
}
~~~
以layoutVertical为例：
~~~
void layoutVertical(int left, int top , int right, int bottom) {
final int paddingLeft = mPaddingLeft ;
int childTop ;
int childLeft ;
// Where right end of child should go
final int width = right - left;
int childRight = width - mPaddingRight ;
// Space available for child
int childSpace = width - paddingLeft - mPaddingRight ;
final int count = getVirtualChildCount() ;
final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
switch (majorGravity) {
case Gravity.BOTTOM:
// mTotalLength contains the padding already
childTop = mPaddingTop + bottom - top - mTotalLength;
break;
// mTotalLength contains the padding already
case Gravity.CENTER_VERTICAL:
childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
break;
case Gravity. TOP:
default :
childTop = mPaddingTop;
break;
}
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.getMeasuredWidth() ;
final int childHeight = child.getMeasuredHeight();
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams() ;
int gravity = lp. gravity;
if (gravity < 0) {
gravity = minorGravity;
}
final int layoutDirection = getLayoutDirection() ;
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection) ;
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity. CENTER_HORIZONTAL :
childLeft = paddingLeft + ((childSpace - childWidth) / 2)
+ lp. leftMargin - lp.rightMargin ;
break;
case Gravity.RIGHT:
childLeft = childRight - childWidth - lp. rightMargin;
break;
case Gravity.LEFT:
default:
childLeft = paddingLeft + lp. leftMargin;
break;
}
if (hasDividerBeforeChildAt(i)) {
childTop += mDividerHeight;
}
childTop += lp.topMargin;
setChildFrame(child , childLeft, childTop + getLocationOffset(child) ,
childWidth, childHeight);
childTop += childHeight + lp. bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child , i);
}
}
}
~~~
主要看以下代码：
~~~
final int childWidth = child.getMeasuredWidth() ;
final int childHeight = child.getMeasuredHeight() ;
setChildFrame(child , childLeft, childTop + getLocationOffset(child) ,
childWidth , childHeight);
childTop += childHeight + lp. bottomMargin + getNextLocationOffset(child);
~~~
top会逐渐增大，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 + width , top + height);
}
~~~
通过子元素的layout来确定自身。
## draw过程
它有以下几步：
* 绘制背景，(canvas)
* 绘制自己。(onDraw)
* 绘制children(dispatchDraw)
* 绘制装饰（onDrawScrollBars）
看下view的draw源码：
~~~
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK ) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo .mIgnoreDirtyState );
mPrivateFlags = (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' layers to prepare for fading
* 3\. Draw view'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 needed
int saveCount;
if (!dirtyOpaque) {
drawBackground(canvas);
}
// skip step 2 & 5 if possible (common case)
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL ) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL ) != 0;
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas) ;
// Step 4, draw the children
dispatchDraw(canvas) ;
// Step 6, draw decorations (scrollbars)
onDrawScrollBars(canvas) ;
if ( mOverlay != null && !mOverlay.isEmpty()) {
mOverlay .getOverlayView().dispatchDraw(canvas) ;
}
// we'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，因为activity的生命周期和view不同步，在oncreate中无法获取到view的宽高，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，宽高己经准备好。当Activity得到焦点和失去焦点均会被调用，所以它会调用多次。
* 通过view.post，将一个runnable投弟到消息队列尾部，等待looper调用时，view己经初始化好。
~~~
protected void onStart(){
super.onStart();
view.post(new Runnable(){
public void run(){
int width = view.getMeasuredWidth();
int height = new .getMeasuredHeight();
}
})
}
~~~
* ViewTreeObserver:
使用ViewTreeObserver众多回调接口来完成，如OnGlobalLayoutListener，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 = view.getViewObserver ();
obserber.addOnGlobalLayoutListener(new OnGlobalLayoutListener(){
public void onGlobalLayout(){
obserber.removeOnGlobalLayoutListener(this);
int width = view.getMeasuredWidth();
int height = new .getMeasuredHeight();
}
})
~~~
* 通过view进行measure来得到view的宽高。
~~~
int width = MeasureSpec.makeMeasureSpec(100,Measure.EXACTLY);//确定值
int height= MeasureSpec.makeMeasureSpec(100,Measure.EXACTLY);//确定值
view.measure(width,height);
对于wrap_content:
int width = MeasureSpec.makeMeasureSpec((1<<30)-1,Measure.AT_MOST);//wrap_content
int height= 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：
* 如果是继承view或者viewGroup，让view支持wrap_content。
* 如果有必要，让view支持padding。
* view中如果有动画或者线程，要在onDetachedFromWindow中及时停止。当view的Activity退出或者当前view被remove时，调用它。