android界面的measure详解
【摘要】
想必大家都遇到过在onCreate中调用getMeasuredHeight和getMeasuredHeight这两个函数得到的返回值是0,0的情况吧，这是为什么呢？当然是android界面初始化的相关机制导致的这个“问题”啦，我们现在就来看一下android的view和viewgroup两个类在初始化中measure相关的机制吧。
我...
想必大家都遇到过在onCreate中调用getMeasuredHeight和getMeasuredHeight这两个函数得到的返回值是0,0的情况吧，这是为什么呢？当然是android界面初始化的相关机制导致的这个“问题”啦，我们现在就来看一下android的view和viewgroup两个类在初始化中measure相关的机制吧。
我们都知道视图的绘制过程要经历三个过程，分别是onMeasure(确定大小),onLayout(确定位置)和onDraw三个方法，我们先从View类中的ViewRootImpl类中开始的，ViewRootImpl对象是view的静态内部类AttachInfo的一个成员变量，上述的绘制过程的三个方法都会在它的performTraversals()中被调用。
那我们就先分步看一下这个函数，由于这个函数比较长，这里就只给出部分代码啦。
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int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
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int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
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//TODO:measureSpec!!!!第一步啊，下一步是OnLayout
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performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
先看一下getRootMeasureSpec方法，那么我们就要先介绍一下View类中的另一个静态内部类MeasureSpec啦，MeasureSpec类封装啦从父节点传递到子节点的布局的需求，每个MeasureSpec既代表了width的需求也代表了height的需求，每个measureSpec有size和mode组成，一共有三种mode：
- UNSPECIFIED，代表父节点不对子节点做任何限制，子节点想多大就多大
- EXACTLY，代表父节点已经决定了准确的大小，子节点必须遵守
- AT_MOST,代表子节点的大小要小于特定的大小，但是具体大小自己决定。
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private static int getRootMeasureSpec(int windowSize, int rootDimension) {
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int measureSpec;
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switch (rootDimension) {
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//TODO：Match_Parent,Wrap_content和exactly,at_most的对应关系
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case ViewGroup.LayoutParams.MATCH_PARENT:
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// Window can't resize. Force root view to be windowSize.
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measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
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break;
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case ViewGroup.LayoutParams.WRAP_CONTENT:
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// Window can resize. Set max size for root view.
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measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
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break;
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default:
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// Window wants to be an exact size. Force root view to be that size.
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measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
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break;
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}
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return measureSpec;
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}
在performMeasure中其实并没有神马复杂的逻辑，只是调用了对象的一个成员变量mView的measure方法，将childWidthMeasureSpec,childHeightMeasureSpec传递过去，也就是MeasureSpec类中所说的一样，将父节点对子节点的布局要求传递过去。
然后我们就来到了VIew.measure(int,int)这个方法啦，我们可以注意到这是个final方法，也就是说我们不能重载这个方法，在文档中也说明啦，这个方法是为了得出这个view需要多大，而且父节点给出了宽和高的限制信息，但是实际的measure工作是onMeasure方法完成的，子类可以重载这个方法。这就是说明，view的设计师不希望用户重载measure方法，而是重载onMeasure方法。
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protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
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setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
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getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
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}
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protected int getSuggestedMinimumWidth() {
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return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
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}
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public static int getDefaultSize(int size, int measureSpec) {
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//TODO:onMeasure真正做事情的地方
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int result = size;
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int specMode = MeasureSpec.getMode(measureSpec);
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int specSize = MeasureSpec.getSize(measureSpec);
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switch (specMode) {
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case MeasureSpec.UNSPECIFIED:
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result = size; //这是原size
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break;
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case MeasureSpec.AT_MOST:
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case MeasureSpec.EXACTLY:
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result = specSize; //
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break;
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}
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return result;
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}
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protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
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//TODO: getMeasuredWith==0的原因啊，在这个方法调用之前都是为0的。
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mMeasuredWidth = measuredWidth;
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mMeasuredHeight = measuredHeight;
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mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
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}
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protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
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//这是测量子view的大小的一个方法
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final int size = mChildrenCount; //用于for循环啊
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final View[] children = mChildren;
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for (int i = 0; i < size; ++i) {
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final View child = children[i];
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if ((child.mViewFlags & VISIBILITY_MASK) != GONE) { //针对VISIBILITY的view
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measureChild(child, widthMeasureSpec, heightMeasureSpec); //所有的spec都是一样的
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}
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}
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}
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protected void measureChild(View child, int parentWidthMeasureSpec,
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int parentHeightMeasureSpec) {
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final LayoutParams lp = child.getLayoutParams(); //获得view的params
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final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
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mPaddingLeft + mPaddingRight, lp.width);//用Spec和viewGroup的paddingleft,right
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//和子view的width来计算
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final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
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mPaddingTop + mPaddingBottom, lp.height);
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child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
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}
在getChildMeasureSeec中和RootViewImpl中的getRootMeasureSpec方法类似，只不过是要考虑布局的实际大小，我在代码中有注释，大家可以自己研究。
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public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
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int specMode = MeasureSpec.getMode(spec);
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int specSize = MeasureSpec.getSize(spec);
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int size = Math.max(0, specSize - padding);
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int resultSize = 0;
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int resultMode = 0;
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//通过viewGroup的Spec和子view的Spec和设定的w,h来决定子view的size和Spec
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switch (specMode) {
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// Parent has imposed an exact size on us
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case MeasureSpec.EXACTLY: //if is exactly
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if (childDimension >= 0) { //这是自己设定了大小
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//这是结果的
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resultSize = childDimension;
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resultMode = MeasureSpec.EXACTLY;
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} else if (childDimension == LayoutParams.MATCH_PARENT) { //values is -1
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// Child wants to be our size. So be it.
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resultSize = size;
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resultMode = MeasureSpec.EXACTLY;
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} else if (childDimension == LayoutParams.WRAP_CONTENT) {
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// Child wants to determine its own size. It can't be
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// bigger than us.
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resultSize = size;
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resultMode = MeasureSpec.AT_MOST;
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}
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break;
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// Parent has imposed a maximum size on us
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case MeasureSpec.AT_MOST:
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if (childDimension >= 0) { //只要是子view设定了specific的值，那么就一定是EXACTLY
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// Child wants a specific size... so be it
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resultSize = childDimension;
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resultMode = MeasureSpec.EXACTLY;
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} else if (childDimension == LayoutParams.MATCH_PARENT) {
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// Child wants to be our size, but our size is not fixed.
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// Constrain child to not be bigger than us.
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resultSize = size;
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resultMode = MeasureSpec.AT_MOST; //继承viewgroup的
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} else if (childDimension == LayoutParams.WRAP_CONTENT) {
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// Child wants to determine its own size. It can't be
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// bigger than us.
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resultSize = size;
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resultMode = MeasureSpec.AT_MOST;
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}
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break;
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// Parent asked to see how big we want to be
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case MeasureSpec.UNSPECIFIED: //完全让子view决定
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if (childDimension >= 0) {
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// Child wants a specific size... let him have it
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resultSize = childDimension;
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resultMode = MeasureSpec.EXACTLY;
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} else if (childDimension == LayoutParams.MATCH_PARENT) {
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// Child wants to be our size... find out how big it should
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// be
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resultSize = 0;
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resultMode = MeasureSpec.UNSPECIFIED;
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} else if (childDimension == LayoutParams.WRAP_CONTENT) {
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// Child wants to determine its own size.... find out how
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// big it should be
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resultSize = 0;
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resultMode = MeasureSpec.UNSPECIFIED;
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}
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break;
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}
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return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
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}
文章来源: blog.csdn.net，作者：程序员历小冰，版权归原作者所有，如需转载，请联系作者。
原文链接：blog.csdn.net/u012422440/article/details/44706055
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