源码分析Android中View的绘制流程
Posted 加冰雪碧
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在开发中自定义控件的使用是比较频繁的，而自定义控件的基础之一就是View的测量以及绘制。这篇文章从源码的角度简要分析一下View的测量绘制。
在了解View绘制流程之前，必须先要了解一个类，MeasureSpec，它是View的内部类，专门来进行对测量的数据和类型进行打包和解包，看一下源码就会清楚不少：
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)
return size + mode;
public static int getMode(int measureSpec)
return (measureSpec & MODE_MASK);
public static int getSize(int measureSpec)
return (measureSpec & ~MODE_MASK);
public static String toString(int measureSpec)
int mode = getMode(measureSpec);
int size = getSize(measureSpec);
StringBuilder sb = new StringBuilder("MeasureSpec: ");
if (mode == UNSPECIFIED)
sb.append("UNSPECIFIED ");
else if (mode == EXACTLY)
sb.append("EXACTLY ");
else if (mode == AT_MOST)
sb.append("AT_MOST ");
else
sb.append(mode).append(" ");
sb.append(size);
return sb.toString();
类还是比较简单，使用getMode和getSize方法对一个32位的int值进行解包，解释出前2位和后30位分别作为测量模式和大小的标注。使用makeMeasureSpec来进行打包。这里暂时只需要知道可以用一个int值来存储这两种类型的数据就可以了，后面还会接触到。
ViewGroup中存放了许多的View，那么对View的测量一定会经过ViewGroup，在ViewGroup中的measureChildren方法完成了对子View的测量，来看一下：
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);
在方法中只是遍历了一下然后调用了measureChild方法，进入看一下：
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);
可以看到方法中生成了子View的MeasureSpec，然后简单的调用了子View的measure方法。来看一下getChildMeasureSpec方法：
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);
先解释一下参数，spec是当前ViewGroup的MeasureSpec，padding是当前ViewGroup的padding，childDimension是子View的大小（可以为MATCH_PARENT和WRAP_CONTENT）。
当ViewGroup的MeasureSpec是EXACTLY时：
①子View的大小是大于等于0的，那么给子View设置的大小就是childDimension，MeasureSpec为EXACTLY。
②子View的大小是MATCH_PARENT，给子View设置的大小是当前ViewGroup大小-padding大小，MeasureSpec为EXACTLY。
③子View的大小是WRAP_CONTENT，给子View设置的大小是当前ViewGroup的大小-padding大小，MeasureSpec为AT_MOST。
当ViewGroup的MeasureSpec是AT_MOST时：
①子View的大小是大于等于0的，那么给子View设置的大小就是childDimension，MeasureSpec为EXACTLY。
②子View的大小是MATCH_PARENT，给子View设置的大小是当前ViewGroup大小-padding大小，MeasureSpec为AT_MOST。
③子View的大小是WRAP_CONTENT，给子View设置的大小是当前ViewGroup的大小-padding大小，MeasureSpec为AT_MOST。
剩下的一种情况是ViewGroup的MeasureSpec为UNSPECIFIED，这种情况基本不会出现，在这里就不深入考虑了。
了解了这些后ViewGroup对子View的测量就完成了，可以看到，子View宽高的测量值是由父ViewGroup和其自身的LayoutParams共同决定的。看完了ViewGroup对子View的分发测量再来看一下View的measure方法：
public final void measure(int widthMeasureSpec, int heightMeasureSpec)
if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec)
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET)
throw new IllegalStateException("onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
在这里参数就是ViewGroup中创建的，因为这个方法是final的，我们并不能修改，所以我们只看一下能修改的onMeasure方法：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
在方法中直接调用getDefaultSize生成了测量结果，然后进行赋值，跟进一下这个方法：
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;
由前面的分析我们知道，基本上不管子View中的LayoutParams设置的大小为什么，它的MeasureSpec都会是AT_MOST和EXACTLY中的一个，那么它最后的测量大小必定等于specSize。并且还由刚才的分析可以知道，当我们把子View指定为WRAP_CONTENT时它的默认大小就是覆盖了整个ViewGroup。大家可以简单的做个实验，在这里就不贴实验的代码了。
如果想要让WRAP_CONTENT起作用，我们需要在onMeasure中加入逻辑的判断以确定其大小。
在onMeasure方法结束后，调用了setMeasuredDimension方法为mMeasuredWidth和mMeasuredHeight赋值：
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight)
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
我们可以调用getMeasuredWidth对这个测量结果进行获取（但是必须在onMeasure调用结束后），getMeasuredHeight同理。
既然已经测量过了，现在就来看一下layout方法：
ViewGroup的layout方法也是简单的调用了父类View的layout方法，所以这里只看一下View的方法。
public void layout(int l, int t, int r, int b)
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
boolean changed = 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;
在layout中首先判断了一下View的大小位置是否发生了改变，并且在setFram方法时将传入的参数赋给本地变量进行保存，而后调用onLayout方法去处理子View，因为onLayout方法和具体布局相关，所以在View和ViewGroup中都没有对其给出实现。
layout基本看完了，下面就是和绘画相关的draw。
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)
final Drawable background = mBackground;
if (background != null)
final int scrollX = mScrollX;
final int scrollY = mScrollY;
if (mBackgroundSizeChanged)
background.setBounds(0, 0, mRight - mLeft, mBottom - mTop);
mBackgroundSizeChanged = false;
if ((scrollX | scrollY) == 0)
background.draw(canvas);
else
canvas.translate(scrollX, scrollY);
background.draw(canvas);
canvas.translate(-scrollX, -scrollY);
// 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);
// we're done...
return;
注释写的也比较清楚，绘制背景->绘制自己->绘制子View->绘制ScrollBar，值得一提的是在View中dispatchDraw方法是空的，在ViewGroup中给出了实现。
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