自定义View之onMeasure()

可以说重载onMeasure()，onLayout()，onDraw()三个函数构建了自定义View的外观形象。再加上onTouchEvent()等重载视图的行为，可以构建任何我们需要的可感知到的自定义View。

本节我们探索自定义View中onMeasure()起到了什么样的作用，题外要插的一句是，Activity框架，View框架中大量的on函数基本上都应用到了Template模式，掌握这一模式对于理解这些框架大有裨益。

我们知道，不管是自定义View还是系统提供的TextView这些，它们都必须放置在LinearLayout等一些ViewGroup中，因此理论上我们可以很好的理解onMeasure()，onLayout()，onDraw()这三个函数：1.View本身大小多少，这由onMeasure()决定；2.View在ViewGroup中的位置如何，这由onLayout()决定；3.绘制View，onDraw()定义了如何绘制这个View。

首先我们看看TextView.java中的onMeasure()源码：

    @Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        int widthMode = MeasureSpec.getMode(widthMeasureSpec);        int heightMode = MeasureSpec.getMode(heightMeasureSpec);        int widthSize = MeasureSpec.getSize(widthMeasureSpec);        int heightSize = MeasureSpec.getSize(heightMeasureSpec);        int width;        int height;        ...         if (widthMode == MeasureSpec.EXACTLY) {            // Parent has told us how big to be. So be it.            width = widthSize;        } else {            if (mLayout != null && mEllipsize == null) {                des = desired(mLayout);            }        ...        setMeasuredDimension(width, height); 

了解了这两个参数的来源，还要知道这两个值的作用。我们只取heightMeasureSpec作说明。这个值由高32位和低16位组成，高32位保存的值叫specMode，可以通过如代码中所示的MeasureSpec.getMode()获取；低16位为specSize，同样可以由MeasureSpec.getSize()获取。那么specMode和specSize的作用有是什么呢？要想知道这一点，我们需要知道代码中的最后一行，所有的View的onMeasure()的最后一行都会调用setMeasureDimension()函数的作用——这个函数调用中传进去的值是View最终的视图大小。也就是说onMeasure()中之前所作的所有工作都是为了最后这一句话服务的。

我们知道在ViewGroup中，给View分配的空间大小并不是确定的，有可能随着具体的变化而变化，而这个变化的条件就是传到specMode中决定的，specMode一共有三种可能：

MeasureSpec.EXACTLY：父视图希望子视图的大小应该是specSize中指定的。

MeasureSpec.AT_MOST：子视图的大小最多是specSize中指定的值，也就是说不建议子视图的大小超过specSize中给定的值。

MeasureSpec.UNSPECIFIED：我们可以随意指定视图的大小。

由TextView中源码也可以知道这个值的设计意义是为了根据ViewGroup中具体能够提供的空间大小来指定子View的视图大小。

通过以上这些分析，可以知道视图最终的大小由父视图，子视图以及程序员根据需要决定，良好的设计一般会根据子视图的measureSpec设置合适的布局大小。

讲到上述这些内容，可能已经了解了如何去使用onMeasure来设置我们的视图的大小，但还有一个疑惑的地方，EXACTLY，AT_MOST，UNSPECIFIED和layout_是如何对应的呢？什么情况下对应什么值呢？

我们通过如下例子，稍作了解：

package com.sean.myview;import android.content.Context;import android.util.AttributeSet;import android.util.Log;import android.view.View;public class MyView extends View {	public MyView(Context context, AttributeSet attrs) {		super(context, attrs);		// TODO Auto-generated constructor stub	}	@Override	protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {		// TODO Auto-generated method stub		Log.d("MyView","------", new Throwable());		int speSize = MeasureSpec.getSize(heightMeasureSpec);		int speMode = MeasureSpec.getMode(heightMeasureSpec);		Log.d("MyView", "---speSize = " + speSize + "");		Log.d("MyView", "---speMode = " + speMode + "");		if(speMode == MeasureSpec.AT_MOST){			Log.d("MyView", "---AT_MOST---");		}		if(speMode == MeasureSpec.EXACTLY){			Log.d("MyView", "---EXACTLY---");		}		if(speMode == MeasureSpec.UNSPECIFIED){			Log.d("MyView", "---UNSPECIFIED---");		}		setMeasuredDimension(MeasureSpec.getSize(widthMeasureSpec), speSize);	}}

widthMeasureSpecD/MyView ( 3506): java.lang.Throwable
D/MyView ( 3506): at com.sean.myview.MyView.onMeasure(MyView.java:18)
D/MyView ( 3506): at android.view.View.measure(View.java:15775)
D/MyView ( 3506): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4942)
D/MyView ( 3506): at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1411)
D/MyView ( 3506): at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1059)
D/MyView ( 3506): at android.widget.LinearLayout.onMeasure(LinearLayout.java:590)
D/MyView ( 3506): at android.view.View.measure(View.java:15775)
D/MyView ( 3506): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4942)
D/MyView ( 3506): at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1411)
D/MyView ( 3506): at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1059)
D/MyView ( 3506): at android.widget.LinearLayout.onMeasure(LinearLayout.java:590)
D/MyView ( 3506): at android.view.View.measure(View.java:15775)
D/MyView ( 3506): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4942)
D/MyView ( 3506): at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
D/MyView ( 3506): at android.view.View.measure(View.java:15775)
D/MyView ( 3506): at android.widget.LinearLayout.measureVertical(LinearLayout.java:850)
D/MyView ( 3506): at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
D/MyView ( 3506): at android.view.View.measure(View.java:15775)
D/MyView ( 3506): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4942)
D/MyView ( 3506): at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
D/MyView ( 3506): at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2193)
D/MyView ( 3506): at android.view.View.measure(View.java:15775)
D/MyView ( 3506): at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2212)
D/MyView ( 3506): at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1291)
D/MyView ( 3506): at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1486)
D/MyView ( 3506): at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1181)
D/MyView ( 3506): at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4942)
D/MyView ( 3506): at android.view.Choreographer$CallbackRecord.run(Choreographer.java:776)
D/MyView ( 3506): at android.view.Choreographer.doCallbacks(Choreographer.java:579)
D/MyView ( 3506): at android.view.Choreographer.doFrame(Choreographer.java:548)
D/MyView ( 3506): at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:762)
D/MyView ( 3506): at android.os.Handler.handleCallback(Handler.java:800)
D/MyView ( 3506): at android.os.Handler.dispatchMessage(Handler.java:100)
D/MyView ( 3506): at android.os.Looper.loop(Looper.java:194)
D/MyView ( 3506): at android.app.ActivityThread.main(ActivityThread.java:5391)
D/MyView ( 3506): at java.lang.reflect.Method.invokeNative(Native Method)
D/MyView ( 3506): at java.lang.reflect.Method.invoke(Method.java:525)
D/MyView ( 3506): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
D/MyView ( 3506): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
D/MyView ( 3506): at dalvik.system.NativeStart.main(Native Method)
D/MyView ( 3506): ---speSize = 940
D/MyView ( 3506): ---speMode = -2147483648
D/MyView ( 3506): ---AT_MOST---

查看onMeasure()的调用堆栈，然后查看源码，可以知道heightMeasureSpec和widthMeasureSpec的值在ViewRootImpl.java中初始化，而初始化又参考了view和LinearLayout中的属性参数的设置最终得到了heightMeasureSpec和widthMeasureSpec的值。

而specMode怎么对应呢？我先给出设置对应值的地方，以下代码皆出自ViewRootImpl.java

            childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);            childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);//这里是赋值的代码

    private static 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;    }

    private void performTraversals() {        // cache mView since it is used so much below......        WindowManager.LayoutParams lp = mWindowAttributes;

    public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {        synchronized (this) {            if (mView == null) {                mView = view;                mViewLayoutDirectionInitial = mView.getRawLayoutDirection();                mFallbackEventHandler.setView(view);                mWindowAttributes.copyFrom(attrs);

D/MyView ( 4249): ---speSize = 940
D/MyView ( 4249): ---speMode = 1073741824
D/MyView ( 4249): ---EXACTLY---

而更改LinearLayout中的这个值对这里的specMode是没有影响的。

下面我们再来通过实验来验证specSize的值由那些属性决定：

我们可以通过控制变量法，逐一改变代码中的LinearLayout和MyView中的相关属性值，看看是哪些影响了specSize，这里我直接给出答案：

530 ＝ 300 * 2.0 - 20 * 2.0 - 15 * 2.0

影响specSize height的因素为：父视图的layout_height和paddingTop以及自身的layout_marginTop。但是我们不要忘记有weight时的影响。