android监听View加载完成的示例讲解

gv_test.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
   @Override
   public void onGlobalLayout() {
    //给GridView设置Adapter，在adapter的getView中获取GridView的高度，在这个回调之前获取的高度都是0
    //处理完后remove掉，至于为什么，后面有解释
    gv_test.getViewTreeObserver()
    .removeOnGlobalLayoutListener(this);
   }
  });

/**
  * Interface definition for a callback to be invoked when the global layout state
  * or the visibility of views within the view tree changes.
  */
 public interface OnGlobalLayoutListener {
  /**
   * Callback method to be invoked when the global layout state or the visibility of views
   * within the view tree changes
   */
  public void onGlobalLayout();
 }

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:app="http://schemas.android.com/apk/res-auto"
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:orientation="vertical"
 tools:context="com.example.myapplication.MainActivity">

 <Button
  android:onClick="test"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:text="test"/>

 <TextView
  android:id="@+id/tv_test"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:text="测试"/>
</LinearLayout>

public class MainActivity extends AppCompatActivity {

 TextView tv_test;
 private static final String TAG = "MainActivity";
 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);

  setContentView(R.layout.activity_main);

  tv_test = (TextView)findViewById(R.id.tv_test);
  //app切换到后台，再点开会调用一次，屏幕关闭运行程序会调用两次
  tv_test.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
   @Override
   public void onGlobalLayout() {
    Log.e(TAG, "onGlobalLayout: ");
   }
  });
 }



 public void test(View v){
  //改变可见性，调用一次
//  tv_test.setVisibility(View.GONE);
  //改变文字布局，没有效果
//  tv_test.setGravity(Gravity.CENTER);

  //修改控件大小，调用一次
//  LinearLayout.LayoutParams para = (LinearLayout.LayoutParams) tv_test.getLayoutParams();
//  para.height = 200;
//  para.weight = 100;
//  tv_test.setLayoutParams(para);

  //修改layoutgravity，这个是在LayoutParams中，调用一次
  LinearLayout.LayoutParams para = (LinearLayout.LayoutParams) tv_test.getLayoutParams();
  para.gravity = Gravity.CENTER_HORIZONTAL;
  tv_test.setLayoutParams(para);
 }
}