Android Popupwindow弹出窗口的简单使用方法

import android.content.Context;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup.LayoutParams;
import android.widget.Button;
import android.widget.PopupWindow;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
  private Context mContext = null;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mContext = this;

    Button button = (Button) findViewById(R.id.but);
    button.setOnClickListener(newView.OnClickListener() {

      @Override
      public void onClick(View view) {

        showPopupWindow(view);
      }
    });
  }

  private void showPopupWindow(View view) {

    // 一个自定义的布局，作为显示的内容
    View contentView =LayoutInflater.from(mContext).inflate(
        R.layout.popupwindow, null);
    // 设置按钮的点击事件
    Button button = (Button) contentView.findViewById(R.id.button );
      button.setOnClickListener(new View.OnClickListener() {

          @Override
          public void onClick(View v) {
            Toast.makeText(mContext, "button is pressed",
                Toast.LENGTH_SHORT).show();
          }
    });

    final PopupWindow popupWindow = new PopupWindow(contentView,  LayoutParams.WRAP_CONTENT,LayoutParams.WRAP_CONTENT, true);

    // 如果不设置PopupWindow的背景，无论是点击外部区域还是Back键都无法dismiss弹框
    // 我觉得这里是API的一个bug
    popupWindow.setBackgroundDrawable(getResources().getDrawable(R.mipmap.ic_launcher));

    // 设置好参数之后再show
    popupWindow.showAsDropDown(view);
  }
}

 <Button
    android:id="@+id/but"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Hello World!" / 

<?xml version="1.0" encoding="utf-8"? 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="vertical" android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:background="#ffff00" 
<TextView
  android:id="@+id/ttt"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:textSize="20sp"
  android:text="弹出窗口"/ 
  <Button
    android:id="@+id/button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" / 
</LinearLayout