Android中實(shí)現(xiàn)觀察者模式的原理是什么

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.進(jìn)來(lái)的人給了一張座子
  public interface OnAListener{
    //可以看畫家繪制
    void look();
  }

  //2.一個(gè)房間
  private ArrayList<OnAListener> lists = new ArrayList<>();

  //3.進(jìn)入房間的通道
  public void register(OnAListener listener){
    lists.add(listener);
  }

  @Override
  protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    canvas.drawText("畫家畫畫", 50, 50, mPaint);

    //4.只有房間的人能看畫家繪制
    for(int i=0 ;i<lists.size(); i++){
      lists.get(i).look();
    }
  }

  //5.出去房間的通道
  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.創(chuàng)建一個(gè)人
    mListener = new A.OnAListener() {

      //3.當(dāng)進(jìn)入房間之后，人才能看到畫家繪制
      @Override
      public void look() {
        Toast.makeText(B.this, "我注冊(cè)了，就可以監(jiān)聽A中畫家是否在繪制了", Toast.LENGTH_SHORT).show();
      }
    };
    //2.進(jìn)入房間
    mA.register(mListener);

    setContentView(mA);
  }

  @Override
  protected void onDestroy() {
    super.onDestroy();
    //4.出去房間
    if(mListener != null)
      mA.unregister(mListener);
  }
}