Android回调与观察者模式的实现原理

package com.example.view;import android.content.Context;import android.graphics.Canvas;import android.graphics.Paint;import android.view.View;import java.util.ArrayList;/** * user: uidq0530 ,date: 2017-03-29. * description： * * @author xhunmon */public class A extends View {  private Paint mPaint;  public A(Context context) {    super(context);    mPaint = new Paint();  }  //1.进来的人给了一张座子  public interface OnAListener{    //可以看画家绘制    void look();  }  //2.一个房间  private ArrayList<OnAListener> lists = new ArrayList<>();  //3.进入房间的通道  public void register(OnAListener listener){    lists.add(listener);  }  @Override  protected void onDraw(Canvas canvas) {    super.onDraw(canvas);    canvas.drawText("画家画画", 50, 50, mPaint);    //4.只有房间的人能看画家绘制    for(int i=0 ;i<lists.size(); i++){      lists.get(i).look();    }  }  //5.出去房间的通道  public void unregister(OnAListener listener){    if(lists.contains(listener))      lists.remove(listener);  }}

package com.example.test3;import android.app.Activity;import android.os.Bundle;import android.widget.Toast;import com.example.view.A;/** * user: uidq0530 ,date: 2017-03-29. * description： * * @author xhunmon */public class B extends Activity {  private A.OnAListener mListener;  private A mA;  @Override  public void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    mA = new A(this);    //1.创建一个人    mListener = new A.OnAListener() {      //3.当进入房间之后，人才能看到画家绘制      @Override      public void look() {        Toast.makeText(B.this, "我注册了，就可以监听A中画家是否在绘制了", Toast.LENGTH_SHORT).show();      }    };    //2.进入房间    mA.register(mListener);    setContentView(mA);  }  @Override  protected void onDestroy() {    super.onDestroy();    //4.出去房间    if(mListener != null)      mA.unregister(mListener);  }}