View绘制流程
一图胜千言
DecorView添加至窗口的流程
这里说明下Acitivity的onResume是在handleResumeActivity之前执行的，所以在onResume中获取View的宽高为0。
实际上通用的绘制流程应该是从WindowManager#addView开始。
View measure()分析
首先View的Measure方法声明为final，子类无法继承，故关于View多态的实现就只能在onMeasure方法中实现
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
//1.判断View本身LayoutMode是否是视觉边界布局，仅用于检测ViewGroup
boolean optical = isLayoutModeOptical(this);
//2.判断父容器是否是视觉布局边界，是则重新调整测量规格（mParent可能是ViewRootImpl）
if (optical != isLayoutModeOptical(mParent)) {
//View的background需要设置.9背景图才会生效，否则insets的left、right全为0
Insets insets = getOpticalInsets();
int oWidth = insets.left + insets.right;
int oHeight = insets.top + insets.bottom;
widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth);
heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
}
// Suppress sign extension for the low bytes,禁止低位进行符号扩展
//3.根据当前测量规格生成一个与之对应的key（相同的测量规格产生的测量值肯定一样的），供后续索引缓存测量值
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
//4.初始化测量缓存稀疏数组，该数组可自行扩容，只是初始化为2
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
//5.如果强制layout（eg.view.forceLayout（））或者本次测量规格与上次测量规格不同，进入该if语句
if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) {
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
//6.如果需要强制layout则进行重新测量，反之则从缓存中查询是否有与目前测量规格对应的key，
//如果有则取用缓存中的测量值，反之则执行onMeasure方法重新测量,未索引到返回一个负数
int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
//7.targetSDK小于Kitkat版本（API20），sIgnoreMeasureCache则为true
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {
long value = mMeasureCache.valueAt(cacheIndex);
// Casting a long to int drops the high 32 bits, no mask needed
setMeasuredDimensionRaw((int) (value >> 32), (int) value);
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
// 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("View with id " + getId() + ": "
+ getClass().getName() + "#onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
//8.标志位赋值，表明需要在layout方法中执行onlayout方法
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
//9.缓存本次测量规格，及测量宽高
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
/**
* Return true if o is a ViewGroup that is laying out using optical bounds.
* @hide
*/
public static boolean isLayoutModeOptical(Object o) {
return o instanceof ViewGroup && ((ViewGroup) o).isLayoutModeOptical();
}
关于LayoutMode请参考Android LayoutMode需要翻墙
关于MeasureSpec请参考Android MeasureSpec
小结:
View的measure方法只是一个测量优化者，主要做了2级测量优化：
1.如果flag不为强制Layout或者与上次测量规格相比未改变，那么将不会进行重新测量（执行onMeasure方法），直接使用上次的测量值；
2.如果满足非强制测量条件，即前后测量规格发生变化，则会先根据目前测量规格生成的key索引缓存数据，索引到就无需进行重新测量;如果targetSDK小于API 20则二级测量优化无效，依旧会重新测量，不会采用缓存测量值。
View onMeasure()分析
View的onMeasure方法比较简单，目的是将测量值赋给mMeasuredWidth和mMeasuredHeight
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
//未指定则使用最小值
case MeasureSpec.UNSPECIFIED:
result = size;
break;
//解析测量规格获得宽、高，这就是为何View无论你填match_parent还是wrap_content，它始终是填满父容器的原因
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
//表明测量尺寸已经设置，与measure方法中的first clears the measured dimension flag相呼应
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
小结:
View的onMeasure方法才是真正的测量者，它根据测量规格以及其他条件来决定自己最终的测量大小。
需要注意，自定义View重写该方法时，务必保证调用setMeasuredDimension()将测量宽、高存起来，measure方法分析中有提到，如果不调用该方法将会抛出非法状态异常。
ViewGroup onMeasure分析
ViewGroup继承至View实现了ViewParent接口，是一个抽象类。前面也提到View的measure方法不能被继承，所以ViewGroup没有measure方法。查看源码发现它并没有重写onMeasure方法，那就去看看其实现类是否有重写，就看最简单的实现类FrameLayout。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
//判断是否要再次测量layout_width/height属性为match_parent的child
//即FrameLayout的layout_width/height属性为wrap_content，则会再次测量属性为match_parent的child
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
//首次遍历测量子控件
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
//GONE类型child不测量，这就是为何GONE不会占用位置，因为没有测量
if (mMeasureAllChildren || child.getVisibility() != GONE) {
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
..................
}
}
..................
//保存自身的测量宽高
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
//再次测量layout_width/height属性为match_parent的child,部分View三次执行onMeasure的原因
count = mMatchParentChildren.size();
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);
..........
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
进入MeasureChildWithMargins函数
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
//根据ViewGroup自身的测量规格生成child的测量规格
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);
//调用child的measure方法，并将child的测量规格传递给child
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
进入getChildMeasureSpec方法，看看到底是如何测量child的规格的
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
//获取父容器的测量模式、测量大小
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
//计算出父容器允许child的最大尺寸
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// 父容器测量模式为精确模式
case MeasureSpec.EXACTLY:
//如果child的layout_width/height为具体的数值eg.20dp，那么child的测量规格就为大小20dp，模式为精确模式
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
}
//如果child的layout_width/height为MATCH_PARENT，那么child的测量规格就为大小父容器允许的最大值，模式为精确模式
else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
}
//如果child的layout_width/height为WRAP_CONTENT，那么child的测量规格就为大小父容器允许的最大值，模式为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;
小结：
ViewGroup的测量主要是根据其自身测量规格，结合child的LayoutParams进行判断分析，生成一个child的测量规格信息，传递给child的measure方法。所谓的测量规格即是一个建议，建议view的宽、高应该为多少，至于采取与否完全取决于view自己（嗯应该是取决于程序员O(∩_∩)O！）。
另ViewGroup提供了三个测量方法供我们使用，在实际运用中可以偷偷懒，不用自己去实现测量逻辑：
- measureChildWithMargins 测量单个child，margin参数有效
- measureChild 测量单个child，margin参数无效
- measureChildren 测量所有child内部调用measureChild
延伸阅读：
【View为什么会至少执行2次onMeasure、onLayout】暂未发布
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