一、measure过程
1、View的measure过程
View的measure方法是一个final方法，不可重写，在measure中会去调用自身的onMeasure方法。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
/**
* Utility to return a default size. Uses the supplied size if the
* MeasureSpec imposed no constraints. Will get larger if allowed
* by the MeasureSpec.
*
* @param size Default size for this view
* @param measureSpec Constraints imposed by the parent
* @return The size this view should be.
*/
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;
}
setMeasuredDimension方法会设置View宽高的测量值。
在getDefaultSize方法中，在AT_MOST与EXACTLY情况下，都返回measureSpec的尺寸，即测量后的尺寸。
UNSPECIFIED一般用于系统内部的测量过程，返回getSuggestedMinimumWidth作为尺寸
#View.java
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
#Drawable.java
public int getMinimumWidth() {
final int intrinsicWidth = getIntrinsicWidth();
return intrinsicWidth > 0 ? intrinsicWidth : 0;
}
从源码看出，在getSuggestedMinimumWidth方法中，如果View没有设置背景，则返回mMinWidth，否则返回minWidth与mBackground的宽度的最大值。minWidth默认为0。
getSuggestedMinimumWidth总结：
（1）如果View没有设置背景，则返回minWidth，这个可以为0
（2）如果View设置了背景，则返回背景宽度与minWidth的最大值。
注意
直接继承View的自定义控件需要重写onMeasure方法并设置wrap_content时的自身大小，否则在布局中使用wrap_content就相当于使用match_parent。
原因：当View使用wrap_content时，它的specMode是AT_MOST模式，specSize为parentSize，即父布局当前剩余的空间大小，同时在AT_MOST模式下，onMeasure会直接返回specSize值。
2、ViewGroup的measure过程
对于ViewGroup来说，除了完成自己的measure过程外，还会遍历调用所有子元素的measure方法，各个子元素再递归执行这个过程。ViewGroup并没有重写onMeasure方法，但它提供了一个measureChildren的方法。之所以没有重写onMeasure方法，我个人的想法是因为它的子类都把onMeasure方法都重写了，所以它写没啥意义。
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);
}
}
}
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);
}
measureChild的思想他就是取出子元素的LayoutParams，然后再通过getChildMeasureSpec来创建子元素的MeasureSpec，接着将MeasureSpc直接传递给View的measure方法进行测量。
不同的ViewGroup子类有不同的measure实现方式，以下针对LinearLayout分析。
3、LinearLayout measure过程
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
//TODO 省略部分代码
// See how tall everyone is. Also remember max width.
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
if (child.getVisibility() == View.GONE) {
i += getChildrenSkipCount(child, i);
continue;
}
...
if (heightMode == MeasureSpec.EXACTLY && useExcessSpace) {
...
} else {
...
// Determine how big this child would like to be. If this or
// previous children have given a weight, then we allow it to
// use all available space (and we will shrink things later
// if needed).
//TODO 步骤1
final int usedHeight = totalWeight == 0 ? mTotalLength : 0;
measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
heightMeasureSpec, usedHeight);
final int childHeight = child.getMeasuredHeight();
if (useExcessSpace) {
// Restore the original height and record how much space
// we've allocated to excess-only children so that we can
// match the behavior of EXACTLY measurement.
lp.height = 0;
consumedExcessSpace += childHeight;
}
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child));
if (useLargestChild) {
largestChildHeight = Math.max(childHeight, largestChildHeight);
}
}
...
//TODO 步骤2
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
int heightSize = mTotalLength;
// Check against our minimum height
heightSize = Math.max(heightSize, getSuggestedMinimumHeight());
// Reconcile our calculated size with the heightMeasureSpec
int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
...
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
heightSizeAndState);
if (matchWidth) {
forceUniformWidth(count, heightMeasureSpec);
}
}
（1）LinearLayout会遍历子元素并对每个子元素执行measureChildBeforeLayout方法，这个方法实际上执行的是ViewGroup.measureChildWithMargins方法，且LinearLayout通过mTotalLength来存储LinearLayout在竖直方向的初步高度。增加的部分主要包括子元素的高度以及子元素在竖直方向上的margin等。
（2）当子元素测量完毕后，LinearLayout会根据子元素的情况测量自己的大小，在竖直方向上，如果它的布局中高度采用的是match_parent或者具体数值，那么它的测量过程和View一致，高度为specSize；如果它的布局高度采用wrap_content，那么它的高度是所有子元素所占高度的总和，但仍然不超过它的父容器的剩余空间。
public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
final int specMode = MeasureSpec.getMode(measureSpec);
final int specSize = MeasureSpec.getSize(measureSpec);
final int result;
switch (specMode) {
case MeasureSpec.AT_MOST:
if (specSize < size) {
result = specSize | MEASURED_STATE_TOO_SMALL;
} else {
result = size;
}
break;
case MeasureSpec.EXACTLY:
result = specSize;
break;
case MeasureSpec.UNSPECIFIED:
default:
result = size;
}
return result | (childMeasuredState & MEASURED_STATE_MASK);
}
3、获取View的measuredWidth/measuredHeight时机
（1）Activity/View#onWindowFocusChanged
这个方法的含义是：View已经初始化完毕了，宽高已经准备好了。当Activity的窗口得到或失去焦点时均会被调用一次。
（2）view.post()
通过post将一个runnable投递到消息队列的尾部，等待Looper调用此runnable。
（3）ViewTreeObserver
ViewTreeObserver里有众多回调可以完成这个功能。
（4）view.measure()
通过手动对View进行measure来得到View的宽高。
二、layout过程
layout的作用是ViewGroup用来确定子元素的位置，当ViewGroup的位置被确定后，它在onLayout中会遍历所有的子元素并调用其layout方法。
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);
if (shouldDrawRoundScrollbar()) {
if(mRoundScrollbarRenderer == null) {
mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
}
} else {
mRoundScrollbarRenderer = null;
}
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList listenersCopy =
(ArrayList)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);
}
}
}
final boolean wasLayoutValid = isLayoutValid();
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
...
}
通过setFrame方法设定View的四个顶点位置，即初始化mLeft，mRight，mTop，mBottom，View的四个顶点一旦确定，那么View在容器中的位置也就确定；接着会调用onLayout方法，用于确定子元素的位置。
不同的View的onLayout规则不一样，以下以LinearLayout举例：
1、LinearLayout#onLayout
@Override
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);
}
}
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();
...
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);
}
}
}
private void setChildFrame(View child, int left, int top, int width, int height) {
child.layout(left, top, left + width, top + height);
}
layoutVertical方法中，会遍历所有子元素并调用setChildFrame来为子元素指定对应的位置，其中childTop会逐渐增大，这就意味着后面的子元素被放置在靠下的位置。
2、getMeasuredWidth与getWidth区别
public final int getWidth() {
return mRight - mLeft;
}
public final int getMeasuredWidth() {
return mMeasuredWidth & MEASURED_SIZE_MASK;
}
在View的默认实现中，View的测量宽高和最终宽高是相等的，只不过测量宽高形成于View的measure过程，最终宽高形成于View的layout过程，赋值时机不同。可以认为View的测量宽高与最终宽高相等。
三、draw过程
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
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;
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
onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
drawAutofilledHighlight(canvas);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
// Step 7, draw the default focus highlight
drawDefaultFocusHighlight(canvas);
if (debugDraw()) {
debugDrawFocus(canvas);
}
// we're done...
return;
}
...
}
View的绘制过程遵循如下几步：
（1）绘制背景background.draw(canvas)
（2）绘制自己（onDraw）
（3）绘制children（dispatchDraw）
（4）绘制装饰（onDrawScrollBars）
View的绘制过程的传递是通过dispatchDraw来实现的，dispatchDraw会遍历调用所有元素的draw方法，如此draw事件就一层层的传递了下去。
View#setWillNotDraw
/**
* If this view doesn't do any drawing on its own, set this flag to
* allow further optimizations. By default, this flag is not set on
* View, but could be set on some View subclasses such as ViewGroup.
*
* Typically, if you override {@link #onDraw(android.graphics.Canvas)}
* you should clear this flag.
*
* @param willNotDraw whether or not this View draw on its own
*/
public void setWillNotDraw(boolean willNotDraw) {
setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);
}
如果一个View不需要绘制任何内容，那么设置这个标记为true以后，系统会进行相应的优化。默认情况下View没有启用这个优化标记位，但是ViewGroup会默认启用这个标记位。
这个标记位对实际开发的意义是：当我们的自定义控件继承于ViewGroup并且本身不具备绘制功能时，就可以开启这个标记位从而便于系统进行后续的优化。
结尾
摘抄自《Andorid开发艺术探索》