总结对比不同方法获取控件的宽和高

我们都知道在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:

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());
    }

}

测试的Activity的onCreate():

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.

现在碰到这个问题我们不能不解决,在网上找到了如下办法:

//------------------------------------------------方法一
int w = View.MeasureSpec.makeMeasureSpec(0,View.MeasureSpec.UNSPECIFIED);
int h = View.MeasureSpec.makeMeasureSpec(0,View.MeasureSpec.UNSPECIFIED);
imageView.measure(w, h);
int height =imageView.getMeasuredHeight();
int width =imageView.getMeasuredWidth();
textView.append("\n"+height+","+width);




//-----------------------------------------------方法二
ViewTreeObserver vto = imageView.getViewTreeObserver();
vto.addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
    public boolean onPreDraw() {
        int height = imageView.getMeasuredHeight();
        int width = imageView.getMeasuredWidth();
        textView.append("\n"+height+","+width);
        return true;
    }
});
//-----------------------------------------------方法三   
ViewTreeObserver vto2 = imageView.getViewTreeObserver();  
vto2.addOnGlobalLayoutListener(new OnGlobalLayoutListener() {
    @Override  
    public void onGlobalLayout() {
        imageView.getViewTreeObserver().removeGlobalOnLayoutListener(this);  
        textView.append("\n\n"+imageView.getHeight()+","+imageView.getWidth());
    }  
});

现在要讨论的是当我们需要时候使用哪个方法呢?

现在把测试的Activity改成如下:

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    final ImageView imageView = (ImageView) findViewById(R.id.imageview);      
    
    //------------------------------------------------方法一
    int w = View.MeasureSpec.makeMeasureSpec(0,View.MeasureSpec.UNSPECIFIED);
    int h = View.MeasureSpec.makeMeasureSpec(0,View.MeasureSpec.UNSPECIFIED);
    imageView.measure(w, h);
    int height =imageView.getMeasuredHeight();
    int width =imageView.getMeasuredWidth();
    textView.append("\n"+height+","+width);
    
    System.out.println("执行完毕.."+System.currentTimeMillis());
}

接着来看下面几种方式输出结果:

把测试Activity改成如下:

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    final ImageView imageView = (ImageView) findViewById(R.id.imageview);
    //-----------------------------------------------方法二
    ViewTreeObserver vto = imageView.getViewTreeObserver();
    vto.addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
        public boolean onPreDraw() {
            int height = imageView.getMeasuredHeight();
            int width = imageView.getMeasuredWidth();
            textView.append("\n"+height+","+width);
            return true;
        }
    });
}

结果如下:

　　

方法三：

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    final ImageView imageView = (ImageView) findViewById(R.id.imageview);
    //-----------------------------------------------方法三   
    ViewTreeObserver vto2 = imageView.getViewTreeObserver();  
    vto2.addOnGlobalLayoutListener(new OnGlobalLayoutListener() {
        @Override  
        public void onGlobalLayout() {
            imageView.getViewTreeObserver().removeGlobalOnLayoutListener(this);  
            textView.append("\n\n"+imageView.getHeight()+","+imageView.getWidth());
        }  
    });  
}

方法一: 比其他的两个方法多了一次计算,也就是多调用了一次onMeasure()方法,该方法虽然看上去简单,但是如果要目标控件计算耗时比较大的话(如listView等),不建议使用.
方法二,它的回调方法会调用很多次,并且滑动TextView的时候任然会调用,所以不建议使用.
方法三,比较合适.
当然,实际应用的时候需要根据实际情况而定.