既然来了简书，总要留下点什么.......

TIM图片20200402165033.jpg

定义

当一个对象的状态发生改变，与它相关联的部分对象的状态同时也会发生改变。

举例子：

我MY订阅了Android进阶之旅的公众号，每更新一篇文章都会及时的通知我。

角色划分

被观察者：公众号
具体的被观察者：Android进阶之旅微信公众号
观察者：用户
具体的观察者：订阅Android进阶之旅微信公众号的用户MY

伪代码1

//观察者
public interface IWXUser {
    void push(String article);
}

public class WXRealUser implements IWXUser{
    private String name;

    public WXRealUser(String name) {
        this.name = name;
    }

    @Override
    public void push(String article) {
        System.out.println(name+"收到文章"+article+"的推送");
    }
}

//被观察者
public class WXPublicObservable {

    public List<IWXUser> mList;

    public WXPublicObservable() {
        this.mList = new ArrayList<>();
    }

    //订阅
    public void register(IWXUser user){
        mList.add(user);
    }

    //取消订阅
    public void unregister(IWXUser user){
        mList.remove(user);
    }

    public void update(String article){
        for (IWXUser user : mList) {
            user.push(article);
        }
    }
}

public class WXRealObservable extends WXPublicObservable {
    private String article;

    public String getArticle(){
        return article;
    }

    public void setArticle(String article){
        this.article = article;
        //推送
        update(article);
    }
}

//调用
public class Client {
    public static void main(String[] args) {
        WXRealObservable wxRealObservable = new WXRealObservable();
        WXRealUser wxRealUser = new WXRealUser("fxy");
        wxRealObservable.register(wxRealUser);
        wxRealObservable.setArticle("志气");
    }
}

推拉模式

订阅号主动推送一篇文章给我们(推模式)(伪代码见上方代码)；
用户主动拉取一篇历史文章查看(拉模式)；
推拉模式的结合。

伪代码2

public interface PullIWXUser {
    void pull(PullWXPublicObservable observable);
}

public class PullWXRealUser implements PullIWXUser {
    private String name;

    public PullWXRealUser(String name) {
        this.name = name;
    }

    @Override
    public void pull(PullWXPublicObservable observable) {
        PullWXRealObservable realObservable = (PullWXRealObservable) observable;
        System.out.println(name+"拉取了文章"+realObservable.getArticle());
    }
}

public class PullWXPublicObservable {

    public List<PullIWXUser> mList;

    public PullWXPublicObservable() {
        this.mList = new ArrayList<>();
    }

    //订阅
    public void register(PullIWXUser user){
        mList.add(user);
    }

    //取消订阅
    public void unregister(PullIWXUser user){
        mList.remove(user);
    }

    public void update(String article){
        for (PullIWXUser user : mList) {
            user.pull(this);
        }
    }
}
其余的代码不变

区别：
拉模式：主动；
推模式：被动。

Java中自带的观察者(推拉)模式

java.util.Observable

伪代码3：

//被观察者
public class WXRealObservable extends Observable {
    private String article;

    public String getArticle(){
        return article;
    }

    public void setArticle(String article){
        this.article = article;
        setChanged();
        notifyObservers(article);
    }
}
//观察者
public class WXRealUser implements Observer {
    private String name;

    public WXRealUser(String name) {
        this.name = name;
    }


    @Override
    public void update(Observable o, Object arg) {

        System.out.println(name+"收到文章"+arg+"的推送");

        WXRealObservable wxRealObservable = (WXRealObservable) o;

        System.out.println(name+"主动拉取了一篇文章"+wxRealObservable.getArticle());

    }
}
//调用
public class Client {

    public static void main(String[] args) {
        WXRealObservable wxRealObservable = new WXRealObservable();
        WXRealUser wxRealUser = new WXRealUser("fxy");
        WXRealUser wx2RealUser = new WXRealUser("my");
        wxRealObservable.addObserver(wxRealUser);
        wxRealObservable.addObserver(wx2RealUser);
        wxRealObservable.setArticle("争气");
        wxRealObservable.deleteObserver(wx2RealUser);
        wxRealObservable.setArticle("不放弃");
    }
}