[移动开发]Android中观察者模式（Observable）的理解，android应用程序开发第三版答案

定义：“定义对象间的一种一对多的依赖关系，当一个对象的状态发生改变的时候，所有依赖于它的对象都将得到通知，并自动更新”，这就是所谓的观察者模式，照意思理解那么就一定会有观察者和被观察者了，在Java1.2之后，提供了两个类，即Observable被观察者，Observer观察者；

在Android中也提供了一个类Observable用于观察者模式，下面看看Observable 的源码便可以大意理解了。

/*

• Copyright ? 2007 The Android Open Source Project

• Licensed under the Apache License, Version 2.0 (the “License”);

• you may not use this file except in compliance with the License.

• You may obtain a copy of the License at

•  http://www.apache.org/licenses/LICENSE-2.0
  

• Unless required by applicable law or agreed to in writing, software

• distributed under the License is distributed on an “AS IS” BASIS,

• WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

• See the License for the specific language governing permissions and

• limitations under the License.

*/

package android.database;

import java.util.ArrayList;

/**

• Provides methods for (un)registering arbitrary observers in an ArrayList.

*/

public abstract class Observable {

/**

• The list of observers. An observer can be in the list at most

• once and will never be null.

*/

protected final ArrayList mObservers = new ArrayList();

/**

• Adds an observer to the list. The observer cannot be null and it must not already

• be registered.

• @param observer the observer to register

• @throws IllegalArgumentException the observer is null

• @throws IllegalStateException the observer is already registered

*/

public void registerObserver(T observer) {

if (observer == null) {

throw new IllegalArgumentException(“The observer is null.”);

}

synchronized(mObservers) {

if (mObservers.contains(observer)) {

throw new IllegalStateException(“Observer " + observer + " is already registered.”);

}

mObservers.add(observer);

}

}

/**

• Removes a previously registered observer. The observer must not be null and it

• must already have been registered.

• @param observer the observer to unregister

• @throws IllegalArgumentException the observer is null

• @throws IllegalStateException the observer is not yet registered

*/

public void unregisterObserver(T observer) {

if (observer == null) {

throw new IllegalArgumentException(“The observer is null.”);

}

synchronized(mObservers) {

int index = mObservers.indexOf(observer);

if (index == -1) {

throw new IllegalStateException(“Observer " + observer + " was not registered.”);

}

mObservers.remove(index);

}

}

/**

• Remove all registered observer

*/

public void unregisterAll() {

synchronized(mObservers) {

mObservers.clear();

}

}

}

当然这个类只是抽象类，如果我们需要用的话，还需要继承 这个类，这有一个方法，通知被观察者进行更

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新。

观察者模式是软体设计模式的一种。在此种模式中，一个目标物件管理所有相依于它的观察者物件，并且在它本身的状态改变时主动发出通知。