如何正确的获得一个view的宽和高

http://www.jcodecraeer.com/a/anzhuokaifa/androidkaifa/2014/0802/1641.html

们都知道在onCreate()里面获取控件的高度是0,这是为什么呢?我们来看一下示例:

首先我们自己写一个控件,这个控件非常简单:

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public class MyImageView extends ImageView {     public MyImageView(Context context, AttributeSet attrs) {         super(context, attrs);     }     public MyImageView(Context context) {         super(context);     }                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           @Override     protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {         super.onMeasure(widthMeasureSpec, heightMeasureSpec);         System.out.println("onMeasure 我被调用了"+System.currentTimeMillis());     }                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           @Override     protected void onDraw(Canvas canvas) {         super.onDraw(canvas);         System.out.println("onDraw 我被调用了"+System.currentTimeMillis());     } }

布局文件:

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<com.test.MyImageView     android:id="@+id/imageview"     android:layout_width="wrap_content"     android:layout_height="wrap_content"     android:src="@drawable/test" />

测试的Activity的onCreate():

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@Override public void onCreate(Bundle savedInstanceState) {     super.onCreate(savedInstanceState);     setContentView(R.layout.main);           System.out.println("执行完毕.."+System.currentTimeMillis()); }

现在我们现在来看一下结果:

说明等onCreate方法执行完了,我们定义的控件才会被度量(measure),所以我们在onCreate方法里面通过view.getHeight()获取控件的高度或者宽度肯定是0,因为它自己还没有被度量,也就是说他自己都不知道自己有多高,而你这时候去获取它的尺寸,肯定是不行的.

有如下两种方法可以解决这个问题:

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方法一：使用view的measure方法。

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优点：可以立即获得宽和高

缺点：人为的多了一次测量过程

这种方法适用于需要在onCreate完成之前就获得一个view的宽和高的情况。

比如获得一个LinearLayout宽和高

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//宽 public int getViewWidth(LinearLayout view){     view.measure(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);     return view.getMeasuredWidth(); } //高 public int getViewHeight(LinearLayout view){     view.measure(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);     return view.getMeasuredHeight(); }

这种方法的原理是直接调用一个view或者viewgroup的measure方法去测量，测量之后该view的getMeasuredHeight()就会返回刚才测量所得的高，getMeasuredWidth返回测量所得宽。本来在布局加载的过程中，view的measure方法一定会被系统调用，但这发生在我们所不知道的某个时间点，为了在这之前提前得到测量结果，我们主动调用measure方法，但是这样做的好处是可以立即获得宽和高，坏处是多了一次测量过程。

至于为什么参数是LayoutParams.WRAP_CONTENT，那是因为我假设这个view的layout_width和layout_height为wrap_content,因为如果为一个确切的值，还有必要测量吗？

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方法二：布局监听类ViewTreeObserver的OnGlobalLayoutListener

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当一个view的布局加载完成或者布局发生改变时OnGlobalLayoutListener可以监听到，利用这点我们可以在布局加载完成的瞬间获得一个view的宽高。

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int mHeaderViewHeight; mHeaderView.getViewTreeObserver().addOnGlobalLayoutListener(         new OnGlobalLayoutListener() {             @Override             public void onGlobalLayout() {                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         mHeaderViewHeight = mHeaderView.getHeight();                 getViewTreeObserver()                         .removeGlobalOnLayoutListener(this);             } });

这种方法无法像第一种方法那样通过一个函数返回值，因为他是基于listener的，OnGlobalLayoutListener的onGlobalLayout被回调之前是没有值的。由于布局状态可能会发生多次改变，因此OnGlobalLayoutListener的onGlobalLayout可能被回调多次，所以我们在 第一次获得值之后就将listener注销掉。

优点：不需要额外的测量过程

缺点：只有在布局加载完成后，才能得到宽和高

其实在activity的onResume中可以直接调用view.getWidth获得宽，那是不是第二种方法就失去意义了呢？

当然不是，如果我们自定义一个view，需要在view的内部获得某个子view的宽和高，而view本身又没有onResume这样的生命周期方法，这时OnGlobalLayoutListener的onGlobalLayout就起作用了， 可以认为onGlobalLayout就是相当于一个view的生命周期。