转自:http://my.oschina.net/xiahuawuyu/blog/167949
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:
首先我们自己写一个控件,这个控件非常简单:
01 | public class MyImageView extends ImageView { |
03 | public MyImageView(Context context, AttributeSet attrs) { |
04 | super (context, attrs); |
06 | public MyImageView(Context context) { |
11 | protected void onMeasure( int widthMeasureSpec, int heightMeasureSpec) { |
12 | super .onMeasure(widthMeasureSpec, heightMeasureSpec); |
13 | System.out.println( "onMeasure 我被调用了" +System.currentTimeMillis()); |
17 | protected void onDraw(Canvas canvas) { |
19 | System.out.println( "onDraw 我被调用了" +System.currentTimeMillis()); |
布局
2 | android:id = "@+id/imageview" |
3 | android:layout_width = "wrap_content" |
4 | android:layout_height = "wrap_content" |
5 | android:src = "@drawable/test" /> |
oncreate:
2 | public void onCreate(Bundle savedInstanceState) { |
3 | super .onCreate(savedInstanceState); |
4 | setContentView(R.layout.main); |
5 | System.out.println( "执行完毕.." +System.currentTimeMillis()); |
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:
02 | int w = View.MeasureSpec.makeMeasureSpec( 0 ,View.MeasureSpec.UNSPECIFIED); |
03 | int h = View.MeasureSpec.makeMeasureSpec( 0 ,View.MeasureSpec.UNSPECIFIED); |
04 | imageView.measure(w, h); |
05 | int height =imageView.getMeasuredHeight(); |
06 | int width =imageView.getMeasuredWidth(); |
07 | textView.append( "\n" +height+ "," +width); |
13 | ViewTreeObserver vto = imageView.getViewTreeObserver(); |
14 | vto.addOnPreDrawListener( new ViewTreeObserver.OnPreDrawListener() { |
15 | public boolean onPreDraw() { |
16 | int height = imageView.getMeasuredHeight(); |
17 | int width = imageView.getMeasuredWidth(); |
18 | textView.append( "\n" +height+ "," +width); |
23 | ViewTreeObserver vto2 = imageView.getViewTreeObserver(); |
24 | vto2.addOnGlobalLayoutListener( new OnGlobalLayoutListener() { |
26 | public void onGlobalLayout() { |
27 | imageView.getViewTreeObserver().removeGlobalOnLayoutListener( this ); |
28 | 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:
01 | public void onCreate(Bundle savedInstanceState) { |
02 | super .onCreate(savedInstanceState); |
03 | setContentView(R.layout.main); |
04 | final ImageView imageView = (ImageView) findViewById(R.id.imageview); |
07 | int w = View.MeasureSpec.makeMeasureSpec( 0 ,View.MeasureSpec.UNSPECIFIED); |
08 | int h = View.MeasureSpec.makeMeasureSpec( 0 ,View.MeasureSpec.UNSPECIFIED); |
09 | imageView.measure(w, h); |
10 | int height =imageView.getMeasuredHeight(); |
11 | int width =imageView.getMeasuredWidth(); |
12 | textView.append( "\n" +height+ "," +width); |
14 | 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:
01 | public void onCreate(Bundle savedInstanceState) { |
02 | super .onCreate(savedInstanceState); |
03 | setContentView(R.layout.main); |
04 | final ImageView imageView = (ImageView) findViewById(R.id.imageview); |
05 | -----------------------------------------------方法二 |
06 | ViewTreeObserver vto = imageView.getViewTreeObserver(); |
07 | vto.addOnPreDrawListener( new ViewTreeObserver.OnPreDrawListener() { |
08 | public boolean onPreDraw() { |
09 | int height = imageView.getMeasuredHeight(); |
10 | int width = imageView.getMeasuredWidth(); |
11 | textView.append( "\n" +height+ "," +width); |
结果如下:
方法三就不再测试了同方法二!!!
那么方法而和方法三在执行上有什么区别呢?
我们在布局文件中加入一个TextView来记录这个控件的宽高.
2 | android:layout_width = "wrap_content" |
3 | android:layout_height = "wrap_content" > |
7 | android:layout_width = "wrap_content" |
8 | android:layout_height = "wrap_content" /> |
先来测试方法二:
01 | public void onCreate(Bundle savedInstanceState) { |
02 | super .onCreate(savedInstanceState); |
03 | setContentView(R.layout.main); |
04 | final ImageView imageView = (ImageView) findViewById(R.id.imageview); |
05 | -----------------------------------------------方法二 |
06 | ViewTreeObserver vto = imageView.getViewTreeObserver(); |
07 | vto.addOnPreDrawListener( new ViewTreeObserver.OnPreDrawListener() { |
08 | public boolean onPreDraw() { |
09 | int height = imageView.getMeasuredHeight(); |
10 | int width = imageView.getMeasuredWidth(); |
11 | textView.append( "\n" +height+ "," +width); |
再来测试方法三
01 | public void onCreate(Bundle savedInstanceState) { |
02 | super .onCreate(savedInstanceState); |
03 | setContentView(R.layout.main); |
04 | final ImageView imageView = (ImageView) findViewById(R.id.imageview); |
06 | ViewTreeObserver vto2 = imageView.getViewTreeObserver(); |
07 | vto2.addOnGlobalLayoutListener( new OnGlobalLayoutListener() { |
09 | public void onGlobalLayout() { |
10 | imageView.getViewTreeObserver().removeGlobalOnLayoutListener( this ); |
11 | textView.append( "\n\n" +imageView.getHeight()+ "," +imageView.getWidth()); |
总结:那么需要获取控件的宽高该用那个方法呢?
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,不见时使用,如listView等.
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.
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