Android开发中偶尔会用到自定义View，一般情况下，自定义View都需要继承View类的onMeasure方法，那么，为什么要继承 onMeasure()函数呢？什么情况下要继承onMeasure()？系统默认的onMeasure()函数行为是怎样的 ？本文就探究探究这些问题。

 

首先，我们写一个自定义View，直接调用系统默认的onMeasure函数，看看会是怎样的现象：

 

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          `package` `com.titcktick.customview;`
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          `import` `android.content.Context;`
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          `import` `android.util.AttributeSet;`
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          `import` `android.view.View;`
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          `public` `class` `CustomView ``extends` `View {`
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          `    `
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          `    ``public` `CustomView(Context context) {`
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          `        ``super``(context); `
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          `    ``}`
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          `    ``public` `CustomView(Context context, AttributeSet attrs) {`
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          `        ``super``(context, attrs);      `
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          `    ``}`
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          `    `
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          `    ``@Override`
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          `    ``protected` `void` `onMeasure(``int` `widthMeasureSpec, ``int` `heightMeasureSpec) {        `
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          `        ``super``.onMeasure(widthMeasureSpec, heightMeasureSpec);`
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          `    ``}`
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          `}`
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1. 父控件使用match_parent，CustomView使用match_parent

 

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          `&lt;LinearLayout xmlns:android=``"http://schemas.android.com/apk/res/android"`
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          `    ``xmlns:tools=``"http://schemas.android.com/tools"`
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          `    ``android:layout_width=``"match_parent"`
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          `    ``android:layout_height=``"match_parent"`
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          `    ``android:orientation=``"vertical"``&gt;`
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          `    ``&lt;com.titcktick.customview.CustomView`
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          `        ``android:layout_width=``"match_parent"`
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          `        ``android:layout_height=``"match_parent"`
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          `        ``android:layout_margin=``"10dp"`
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          `        ``android:background=``"@android:color/black"``/&gt;`
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          `&lt;/LinearLayout&gt;`
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这里加了10dp的margin并且把View的背景设置为了黑色，是为了方便辨别我们的CustomView，效果如下：

 

wKioL1Pso77ROkIvAAAxTKcLymg465.jpg

 

我们可以看到，默认情况下，如果父控件和CustomView都使用match_parent，则CustomView会充满父控件。

 

2.  父控件使用match_parent，CustomView使用wrap_content

 

把layout文件中，CustomView的layout_width/layout_height替换为wrap_content，你会发现，结果依然是充满父控件。

 

3.  父控件使用match_parent，CustomView使用固定的值

 

把layout文件中，CustomView的layout_width/layout_height替换为50dp，你会发现，CustomView的显示结果为50dpx50dp，如图所示：

 

wKiom1PsoqaShr74AAA8MgI5FPY164.jpg

 

4.  父控件使用固定的值，CustomView使用match_parent或者wrap_content

 

那么，如果把父控件的layout_width/layout_height替换为50dp，CustomView设置为match_parent或者wrap_content，你会发现，CustomView的显示结果也是为50dpx50 dp。

 

5  结论

 

如果自定义的CustomView采用默认的onMeasure函数，行为如下：

 

（1） CustomView设置为 match_parent 或者 wrap_content 没有任何区别，其显示大小由父控件决定，它会填充满整个父控件的空间。

 

（2） CustomView设置为固定的值，则其显示大小为该设定的值。

 

如果你的自定义控件的大小计算就是跟系统默认的行为一致的话，那么你就不需要重写onMeasure函数了。

 

6. 怎样编写onMeasure函数

 

系统默认的onMeasure函数的行为就讨论到这，下面也说说怎样重写onMeasure函数，以及onMeasure函数的基本原理，关键部分在代码中以注释的形式给出了，仅供参考：

 

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          `package` `com.titcktick.customview;`
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          `import` `android.content.Context;`
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          `import` `android.util.AttributeSet;`
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          `import` `android.view.View;`
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          `public` `class` `CustomView ``extends` `View {`
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          `    `
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          `    ``private` `static` `final` `int` `DEFAULT_VIEW_WIDTH = ``100``;`
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          `    ``private` `static` `final` `int` `DEFAULT_VIEW_HEIGHT = ``100``;`
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          `    `
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          `    ``public` `CustomView(Context context) {`
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          `        ``super``(context); `
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          `    ``}`
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          `    ``public` `CustomView(Context context, AttributeSet attrs) {`
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          `        ``super``(context, attrs);      `
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          `    ``}`
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          `    `
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          `    ``@Override`
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          `    ``protected` `void` `onMeasure(``int` `widthMeasureSpec, ``int` `heightMeasureSpec) {`
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          `        `
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          `        ``int` `width  = measureDimension(DEFAULT_VIEW_WIDTH, widthMeasureSpec);`
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          `        ``int` `height = measureDimension(DEFAULT_VIEW_HEIGHT, heightMeasureSpec);`
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          `        `
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          `        ``setMeasuredDimension(width, height);                `
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          `    ``}`
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          `    `
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          `    ``protected` `int` `measureDimension( ``int` `defaultSize, ``int` `measureSpec ) {`
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          `        `
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          `        ``int` `result = defaultSize;`
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          `        `
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          `        ``int` `specMode = MeasureSpec.getMode(measureSpec);`
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          `        ``int` `specSize = MeasureSpec.getSize(measureSpec);`
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          `                `
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          `        ``//1. layout给出了确定的值，比如：100dp`
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          `        ``//2. layout使用的是match_parent，但父控件的size已经可以确定了，比如设置的是具体的值或者match_parent`
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          `        ``if` `(specMode == MeasureSpec.EXACTLY) {      `
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          `            ``result = specSize; ``//建议：result直接使用确定值`
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          `        ``} `
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          `        ``//1. layout使用的是wrap_content`
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          `        ``//2. layout使用的是match_parent,但父控件使用的是确定的值或者wrap_content`
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          `        ``else` `if` `(specMode == MeasureSpec.AT_MOST) {             `
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          `            ``result = Math.min(defaultSize, specSize); ``//建议：result不能大于specSize`
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          `        ``} `
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          `        ``//UNSPECIFIED,没有任何限制，所以可以设置任何大小`
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          `        ``//多半出现在自定义的父控件的情况下，期望由自控件自行决定大小`
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          `        ``else` `{      `
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          `            ``result = defaultSize; `
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          `        ``}`
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          `        `
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          `        ``return` `result;`
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          `    ``}`
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          `}`
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这样重载了onMeasure函数之后，你会发现，当CustomView使用match_parent的时候，它会占满整个父控件，而当CustomView使用wrap_content的时候，它的大小则是代码中定义的默认大小100×100像素。当然，你也可以根据自己的需求改写measureDimension()的实现。

 

转自：http://ticktick.blog.51cto.com/823160/1540134

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