觀察者模式的定義
觀察者定義了一個一對多的依賴關(guān)系，讓一個或者多個觀察者監(jiān)聽一個主題（被觀察者）的變化陨闹。因此薄坏，當(dāng)主題狀態(tài)發(fā)生變化時寨闹，主題會通知觀察者，使觀察者能夠自動更新沈善。
被觀察者的特點
- 持有監(jiān)聽的觀察者的引用
- 支持增加和刪除觀察者
- 主題狀態(tài)改變，通知觀察者
觀察者的特點
接收到被觀察者狀態(tài)發(fā)生變化時做出變化闻牡。
如何實現(xiàn)
- 觀察者實現(xiàn)Observer接口即可
- 被觀察者繼承Observable即可
示例代碼
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);//添加一個觀察者
organization.display();
Log.i("mzh", "observer count is " + organization.countObservers());
organization.deleteObserver(member);
Log.i("mzh", "observer count is " + organization.countObservers());
}
});
}
}
觀察者
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();
}
}
被觀察者
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();//改變狀態(tài)
this.notifyObservers();
}
}
關(guān)注要點
主題發(fā)生改變時玖翅，需要調(diào)用setChange()方法金度，表示狀態(tài)放生改變严沥。下面看看這個方法的具體實現(xiàn)
/**
* Sets the changed flag for this {@code Observable}. After calling
* {@code setChanged()}, {@code hasChanged()} will return {@code true}.
*/
protected void setChanged() {
changed = true;
}
再來看看notifyObservers()這個方法的具體實現(xiàn)：
/**
* 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()}.
* <p>
* 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);
}
}
}
總結(jié)
當(dāng)主題狀態(tài)發(fā)生變化時消玄，必須要調(diào)用setChange()方法改變標(biāo)記，然后調(diào)用notifyObservers()方法通知所有的觀察者才會生效翩瓜。
