android View绘制源码分析

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 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_contentint height= MeasureSpec.makeMeasureSpec((1<<30)-1,Measure.AT_MOST);//wrap_contentview.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。