View的工作原理（一）——从ViewRoot和DecorView说起
前言
本文参考《Android开发艺术与探索》第四章内容及网上几篇博客，里面融入笔者的个人理解。希望能对大家理解View有所帮助。
基本概念介绍
介绍View的工作原理之前我们首先要理解DecorView和ViewRoot两个概念：
1、DecorView
DecorView是Windows中的View的最顶层View。我们可以根据下面一副图来认识它：
由这幅图我们可以看到 ，其实DecorView是一个FrameLayout，里面是一个垂直的线性布局，在线性布局中分上下两部分FrameLayout,上面一部分是TitleBar，下面是android.R.id.content,我们平常的setContentView就是将布局加载在android.R.id.content中。
2、ViewRoot
ViewRoot是连接WindowsManager和DecorView的桥梁对应于ViewRootImpl。
View的绘制流程就是从ViewRootImpl的performTraversala（）方法开始的，包含三大流程：
1、Measure（）：［ 测量流程］
2、Layout（）：［布局流程］
3、Draw（）：［绘制流程］
这三大流程也就是View绘制的三大流程。我们可以通过下面两幅图来理解performTrarsala（）方法；
什么是MeasureSpec
MeasureSpec 从名字上来看看起来是“测量规格“或是”测量说明书“。大致意思就是决定View的Measure过程。我们可以这样来理解：”MeasureSpec在很大程度上决定了一个View的尺寸规格，之所以这样说是很大程度上是因为这个过程还受父容器的影响，因为父容器影响View的MeasureSpec的创建过程。“
1、MeasureSpec
MeasureSpec代表一个32位int值，高2位代表SPecMode（测量模式），低30位代表SpecSize（在某种测量模式下的规格大小）。
- UNSPECIFIED（未指定模式）
- EXACTLY（确定模式）
- AT_MOST（最多模式）
2、MeasureSPec 和LayoutParams的对应关系
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
host.measure(childWidthMeasureSpec, childHeightMeasureSpec);
我们来再看看getRootMeasureSpec方法：
private 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.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}在android.view.ViewRootImpl 中可以看到其对应关系LayoutParams 中这三个值在内部有个对应关系，那就是
LayoutParams.MATCH_PARENT 对应 MeasureSpec.EXACTLY
.LayoutParams.WRAP_CONTENT对应 MeasureSpec.AT_MOST
默认值（也就是具体值） 对应 MeasureSpec.EXACTLY
也就是内部只有两种模式 EXACTLY 精确模式 和 AT_MOST 最大模式！
对于普通View它的measure是由ViewGroup传递而来，我们看一下ViewGroup的measureChildWidthMargins方法:
/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding
* and margins. The child must have MarginLayoutParams The heavy lifting is
* done in getChildMeasureSpec.
*
* @param child The child to measure
* @param parentWidthMeasureSpec The width requirements for this view
* @param widthUsed Extra space that has been used up by the parent
* horizontally (possibly by other children of the parent)
* @param parentHeightMeasureSpec The height requirements for this view
* @param heightUsed Extra space that has been used up by the parent
* vertically (possibly by other children of the parent)
*/
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 // 考虑上parent的padding和child的margin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);
// 这里如果child是个ViewGroup类型，则实际会递归下去。。。
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}我们可以清楚的看到子View的MeasureSpec创建与父容器的MeasureSpec和子元素本身的LayoutParams有关，此外还和VIew的margin 和 padding 有关，我们来看下ViewGroup的getChildMeasureSpec:
/**
* Does the hard part of measureChildren: figuring out the MeasureSpec to
* pass to a particular child. This method figures out the right MeasureSpec
* for one dimension (height or width) of one child view.
*
* The goal is to combine information from our MeasureSpec with the
* LayoutParams of the child to get the best possible results. For example,
* if the this view knows its size (because its MeasureSpec has a mode of
* EXACTLY), and the child has indicated in its LayoutParams that it wants
* to be the same size as the parent, the parent should ask the child to
* layout given an exact size.
*
* @param spec The requirements for this view
* @param padding The padding of this view for the current dimension and
* margins, if applicable
* @param childDimension How big the child wants to be in the current
* dimension
* @return a MeasureSpec integer for the child
*/
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
// 这个方法是协商型的，最终结果既可能直接由spec（parent提供的），也可能由childDimension决定
// 所以我们知道了，一个View的大小不是简单的单方面决定的，而是通过一系列条件协商的结果，
// 有时会尊重parent的spec，有时会坚持自己的dimension要求
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding); // 可用的大小
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY: // parent说child你应该是个确定的大小
if (childDimension >= 0) { // child正好设置了确定的大小
resultSize = childDimension; // 让child是那个确定的大小
resultMode = MeasureSpec.EXACTLY; // 设置mode为EXACTLY
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size; // 其他情况下都是parent spec中的大小，只是mode不同
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; // 不能超过size
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST: // parent说child你应该最大是某个值。。。
if (childDimension >= 0) { // child指定确定值了，则听child的
// 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: // parent没对child的大小有啥要求
if (childDimension >= 0) { // child指定了确定的值，听child的
// 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);
}| parentSpecMode childLayoutParams |
EXACTLY |
AT_MOST |
UNSPECIFIED |
| dp/px |
EXACTLY chiladSize |
EXACTLY chiladSize |
EXACTLY childSize |
|
match_parent |
EXACTLY parentSize |
AT_MOST parentSize |
UNSPECIFIED 0 |
| wrap_content |
EXACTLY parentSize |
AT_MOST parentSize |
UNSPECIFIED 0 |