Android观察者模式的理解

观察者模式的定义

观察者定义了一个一对多的依赖关系，让一个或者多个观察者监听一个主题（被观察者）的变化。因此，当主题状态发生变化时，主题会通知观察者，使观察者能够自动更新。

被观察者的特点

• 持有监听的观察者的引用
• 支持增加和删除观察者
• 主题状态改变，通知观察者

观察者的特点

接收到被观察者状态发生变化时做出变化。

如何实现

• 观察者实现Observer接口即可
• 被观察者继承Observable即可

示例代码

    package com.example.mazhihao.observerdemo;
    import android.os.Bundle;
    import android.support.v7.app.AppCompatActivity;
    import android.util.Log;
    import android.view.View;



    public class MainActivity extends AppCompatActivity {

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            findViewById(R.id.btn).setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    Organization organization = new Organization();
                    organization.setMsg("Hello Observer...");
                    Member member = new Member(MainActivity.this);
                    organization.addObserver(member);//添加一个观察者
                    organization.display();
                    Log.i("mzh", "observer count is " + organization.countObservers());
                    organization.deleteObserver(member);
                    Log.i("mzh", "observer count is " + organization.countObservers());
                }
            });

        }
    }

观察者

    package com.example.mazhihao.observerdemo;

    import android.content.Context;
    import android.widget.Toast;

    import java.util.Observable;
    import java.util.Observer;

    /**
     * Created by mazhihao on 2016/7/13.
     */
    public class Member implements Observer {
        private Context context;

        public Member(Context context) {
            this.context = context;
        }

        @Override
        public void update(Observable observable, Object data) {
            String msg = ((Organization) observable).getMsg();
            Toast.makeText(context, msg, Toast.LENGTH_SHORT).show();
        }
    }

被观察者

    package com.example.mazhihao.observerdemo;

    import java.util.Observable;

    /**
     * Created by mazhihao on 2016/7/13.
     */
    public class Organization extends Observable {
        private String msg;

        public String getMsg() {
            return msg;
        }

        public void setMsg(String msg) {
            this.msg = msg;
        }

        public void display() {
            this.setChanged();//改变状态
            this.notifyObservers();
        }
    }

关注要点

主题发生改变时，需要调用setChange()方法，表示状态放生改变。下面看看这个方法的具体实现

    /**
     * Sets the changed flag for this {@code Observable}. After calling
     * {@code setChanged()}, {@code hasChanged()} will return {@code true}.
     */
    protected void setChanged() {
        changed = true;
    }

再来看看notifyObservers()这个方法的具体实现：

    /**
     * If {@code hasChanged()} returns {@code true}, calls the {@code update()}
     * method for every observer in the list of observers using null as the
     * argument. Afterwards, calls {@code clearChanged()}.
     * 

     * Equivalent to calling {@code notifyObservers(null)}.
     */
    public void notifyObservers() {
        notifyObservers(null);
    }

    /**
     * If {@code hasChanged()} returns {@code true}, calls the {@code update()}
     * method for every Observer in the list of observers using the specified
     * argument. Afterwards calls {@code clearChanged()}.
     *
     * @param data
     *            the argument passed to {@code update()}.
     */
    @SuppressWarnings("unchecked")
    public void notifyObservers(Object data) {
        int size = 0;
        Observer[] arrays = null;
        synchronized (this) {
            if (hasChanged()) {
                clearChanged();
                size = observers.size();
                arrays = new Observer[size];
                observers.toArray(arrays);
            }
        }
        if (arrays != null) {
            for (Observer observer : arrays) {
                observer.update(this, data);
            }
        }
    }

总结

当主题状态发生变化时，必须要调用setChange()方法改变标记，然后调用notifyObservers()方法通知所有的观察者才会生效。