View的基本概念与MeasureSpec

1.基本概念

View的绘制是由measuer、layout、draw三个过程才能完整的绘制一个View，其中measure是测量View的宽、高，layout是为了确认View在父容器所在的位置，draw是负责在屏幕上将View绘制出来。View的绘制流程是从ViewRoot的performTraversals开始的，ViewRoot对应ViewRootImpl类，是连接WindowManager和DecorView的纽带，View的三大流程都是通过ViewRoot来完成的。

从图中可以得知，performTraversals会依次调用performMeasure，performLayout，performDraw这三个方法，这三个方法分别完成顶级View的measure，layout，draw这三大流程，来完成整个View的绘制，以performMeasure为例，performMeasure会调用measure，measure又会调用onMeasure，在onMeasure方法中则会对所有子View进行measure过程，这时measure的流程就从父容器传递给子元素了，这就完成了一次measure过程。子元素重复父容器的measure过程就完成了整个View树的遍历，那么performLayout和performDraw也是差不多的，但是performDraw有一点不同的是performDraw的传递过程是在draw方法中通过dispatchDraw来实现的不过没什么区别。

measure过程决定了View的宽/高，Measure完成以后，可以通过getMeasureWidth和getMeasureHeight获取测量后View的宽，高，并且在大多数情况下都等同于View的最终宽/高。Layout过程完成以后，可以知道View的四个顶点坐标和View的实际宽/高，通过getLeft、getTop、getRight、getBottom即可获得，宽/高可以通过getWidth，getHeight获得，并且是View的最终宽/高。Draw过程决定了View的显示，只有draw方法完成以后才可以将View的内容完全呈现出来。

2.理解MeasureSpec

这一段内容主要是加深对View测量过程的理解。MeasureSpec在决定了View的尺寸规格，但是会受到父容器的影响，因为父容器影响View的MeasureSpec的创建过程。在测量过程中，系统会将View的LayoutParams根据父容器所施加的规则转换成MeasureSpec，然后再根据这个MeasureSpec来测量View的宽/高。这里的宽/高是测量宽/高，不一定等于View的最终宽/高。

(1).MeasureSpec

MeasureSpec代表一个32位的int值，其中高2位代表SpecMode，低30位代表SpecSize，SpecMode代表测量模式，SpecSize代表在某种测量模式下的规格大小。SpecMode有三种模式：

UNSPECIFIED ：父容器不对View有任何限制，有多大给多大，用于系统内部，表示一种测量状态。

AT_MOST ：父容器已经指定了可用大小SpecSize，View的大小不能大于这个值，具体的值要看不同View的具体实现，对应LayoutParams的wrap_content。

EXACTLY：父容器已经检测出View的精确大小，这个时候View的最终值就是SpecSize所指定的值，对应LayoutParams的match_parent和具体的大小。

(2).MeasureSpec和layoutParams的对应关系

在View测量的时候，系统会将LayoutParams在父容器的约束下转换成MeasureSpec，然后再根据这个MeasureSpec来确定View在测量后的宽/高。需要注意的是，LayoutParams并不是唯一决定MeasureSpec的，需要由LayoutParams和父容器一起才能确定View的MeasureSpec，从而进一步确定View的宽/高。另外对于顶级View(DecorView)和普通View来说，MeasureSpec的转换过程略有不同。MeasureSpec一旦确定后，onMeasure就可以确定View的测量宽/高。

DecorView：其MeasureSpec是根据窗口尺寸和自身的LayoutParams来共同确定的；

普通View：其measureSpec是根据父容器的MeasureSpec和自身的LayoutParams来共同确定的。

①.DecorView的MeasureSpec是如何产生的呢？

对于DecorView来说，在ViewRootImpl中的measureHierarchy方法有如下一段代码，

childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);复制代码

再看一下getRootMeasureSpec的代码

/*** Figures out the measure spec for the root view in a window based on it's* layout params.** @param windowSize*            The available width or height of the window** @param rootDimension*            The layout params for one dimension (width or height) of the*            window.** @return The measure spec to use to measure the root view.*/privatestaticintgetRootMeasureSpec(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;
}
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从源码来看，DecorView的MeasureSpec的产生过程就明确了，主要遵守如下规则：

LayoutParams.MATCH_PARENT：精确模式，大小就是窗口大小；

LayoutParams.WRAP_CONTENT：最大模式，尺寸不限但是不能大于当前窗口

固定大小：精确模式，大小就是LayoutParams中所设置的大小。

②.普通View的MeasureSpec是如何产生的？

这里是指布局中的View，View的measure由ViewGroup传递而来，先看一下ViewGroup的measureChildWithMargins

/*** 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)*/protectedvoidmeasureChildWithMargins(View child,int parentWidthMeasureSpec, int widthUsed,int parentHeightMeasureSpec, int heightUsed) {finalMarginLayoutParamslp= (MarginLayoutParams) child.getLayoutParams();finalintchildWidthMeasureSpec= getChildMeasureSpec(parentWidthMeasureSpec,mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin+ widthUsed, lp.width);finalintchildHeightMeasureSpec= getChildMeasureSpec(parentHeightMeasureSpec,mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin+ heightUsed, lp.height);child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
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上述方法会对子元素进行measure，在调用子元素的measure方法之前会先通过getChildMeasureSpec方法得到子元素的MeasureSpec。从代码中可以知道，子元素的MeasureSpec的创建与父元素的MeasureSpec和子元素本身的LayoutParams有关，此外还和View的margin，padding有关，具体可以查看getChildMeasureSpec的源码

/*** 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*/publicstaticintgetChildMeasureSpec(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 uscase MeasureSpec.EXACTLY:if (childDimension >= 0) {resultSize = childDimension;resultMode = MeasureSpec.EXACTLY;} elseif (childDimension == LayoutParams.MATCH_PARENT) {// Child wants to be our size. So be it.resultSize = size;resultMode = MeasureSpec.EXACTLY;} elseif (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 uscase MeasureSpec.AT_MOST:if (childDimension >= 0) {// Child wants a specific size... so be itresultSize = childDimension;resultMode = MeasureSpec.EXACTLY;} elseif (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;} elseif (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 becase MeasureSpec.UNSPECIFIED:if (childDimension >= 0) {// Child wants a specific size... let him have itresultSize = childDimension;resultMode = MeasureSpec.EXACTLY;} elseif (childDimension == LayoutParams.MATCH_PARENT) {// Child wants to be our size... find out how big it should// beresultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;resultMode = MeasureSpec.UNSPECIFIED;} elseif (childDimension == LayoutParams.WRAP_CONTENT) {// Child wants to determine its own size.... find out how// big it should beresultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;resultMode = MeasureSpec.UNSPECIFIED;}break;}//noinspection ResourceTypereturn MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
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上述代码主要作用是根据父容器的MeasureSpec同时结合View本身的LayoutParams来确定子元素的MeasureSpec，参数中的padding表示父容器已经占用的空间，因此子元素可用的空间大小为父容器减去padding。