Android平台上为已经开发者提供了 AsyncTask,Handler 等用来做异步操作的类库，那我们为什么还要选择RxJava 呢？答案是简洁！ RxJava 可以用非常简洁的代码逻辑来解决复杂问题；而且即使业务逻辑的越来越复杂，它依然能够保持简洁！再配合上Lambda 用简单的几行代码分分钟就解决你负责的业务问题。

3.什么是观察者模式？

举个栗子，Android 中 View 的点击监听器的实现， View 是被观察者， OnClickListener 对象是观察者， Activity要如何知道 View 被点击了？那就是派一个 OnClickListener 对象，入驻 View ，与 View 达成一个订阅关系，一旦View 被点击了，就通过 OnClickListener 对象的 OnClick 方法传达给 Activity 。采用观察者模式可以避免去轮询检查，节约有限的cpu 资源。

4.RxJava原理理解

被观察者通过订阅将事件按顺序依次传递给观察者

5.实例

1. 建立被观察者（ Observable ） & 生产事件

2. 建立观察者（ Observer ）并定义响应事件的行为

3. 经过订阅（ Subscribe ）链接观察者和被观察者


public class MainActivity extends AppCompatActivity {
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //建立观察者 向被观察者发送消息
        Observable observable=Observable.create(new ObservableOnSubscribe<Object>() {
            @Override
            public void subscribe(ObservableEmitter<Object> emitter) throws Exception {
                emitter.onNext(1);
                emitter.onNext(2);
                emitter.onNext(3);
                emitter.onComplete();
            }
        });
        //建立被观察者 接收观察者的消息进行处理
        Observer observer=new Observer() {
            @Override
            public void onSubscribe(Disposable d) {
                Log.i("onSubscribe","开始连接");
            }
 
            @Override
            public void onNext(Object o) {
                Log.i("onNext","接收到的值是"+o);
            }
 
            @Override
            public void onError(Throwable e) {
                Log.i("onError","失败");
            }
 
            @Override
            public void onComplete() {
                Log.i("onComplete","完成");
            }
        };
            //被观察者向观察者进行订阅
            observable.subscribe(observer);
    }
}

2.Retrofit+OkHTTP+RxJava+RecycleView的实现步骤

第一步：创建项目

第二步：导入依赖


//for rxjava
implementation 'io.reactivex.rxjava2:rxjava:2.2.6'
//for rxandroid
implementation 'io.reactivex.rxjava2:rxandroid:2.1.1'
//衔接 Retrofit & RxJava，此处一定要注意使用RxJava2的版本
implementation 'com.squareup.retrofit2:adapter-rxjava2:2.5.0'
//添加Retrofit依赖
implementation 'com.squareup.retrofit2:retrofit:2.5.0'
//添加Gson解析
implementation 'com.squareup.retrofit2:converter-gson:2.5.0'
//添加图片加载库依赖
implementation 'com.github.bumptech.glide:glide:4.12.0'

第三步：添加网络权限

<uses-permission android:name="android.permission.INTERNET" />

第四步：在activity_main.xml添加Recycle，并给其设置id


<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity2"
    android:padding="15dp"
    android:orientation="vertical">
 
    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/rv_news"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

第五步：创建子布局


<?xml version="1.0" encoding="utf-8"?>
<androidx.cardview.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="150dp"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    app:cardElevation="5dp"
    app:cardCornerRadius="5dp"
    android:layout_marginTop="10dp">
 
 
    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="140dp"
        android:layout_margin="5dp">
 
 
        <ImageView
            android:id="@+id/news_pic"
            android:scaleType="fitXY"
            android:layout_width="140dp"
            android:layout_height="140dp"
            app:srcCompat="@drawable/ic_launcher_background" />
 
        <TextView
            android:id="@+id/news_title"
            android:layout_toRightOf="@+id/news_pic"
            android:layout_marginStart="20dp"
            android:textSize="16sp"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="TextView" />
 
        <TextView
            android:id="@+id/news_desc"
            android:layout_marginTop="20dp"
            android:layout_alignLeft="@+id/news_title"
            android:layout_below="@+id/news_title"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="TextView" />
    </RelativeLayout>
 
</androidx.cardview.widget.CardView>

第六步：创建实体类


package com.hopu.day23_rxjava.model;
 
import java.util.List;
 
public class News {
    private boolean success;
    private String title;
    private String subtitle;
    private String update_time;
    private List<Data> data;
    public void setSuccess(boolean success) {
        this.success = success;
    }
    public boolean getSuccess() {
        return success;
    }
 
    public void setTitle(String title) {
        this.title = title;
    }
    public String getTitle() {
        return title;
    }
 
    public void setSubtitle(String subtitle) {
        this.subtitle = subtitle;
    }
    public String getSubtitle() {
        return subtitle;
    }
 
    public String getUpdate_time() {
        return update_time;
    }
 
    public void setUpdate_time(String update_time) {
        this.update_time = update_time;
    }
 
    public void setData(List<Data> data) {
        this.data = data;
    }
    public List<Data> getData() {
        return data;
    }
 
    public class Data {
 
        private int index;
        private String title;
        private String desc;
        private String pic;
        private String hot;
        private String url;
        private String mobilUrl;
        public void setIndex(int index) {
            this.index = index;
        }
        public int getIndex() {
            return index;
        }
 
        public void setTitle(String title) {
            this.title = title;
        }
        public String getTitle() {
            return title;
        }
 
        public void setDesc(String desc) {
            this.desc = desc;
        }
        public String getDesc() {
            return desc;
        }
 
        public void setPic(String pic) {
            this.pic = pic;
        }
        public String getPic() {
            return pic;
        }
 
        public void setHot(String hot) {
            this.hot = hot;
        }
        public String getHot() {
            return hot;
        }
 
        public void setUrl(String url) {
            this.url = url;
        }
        public String getUrl() {
            return url;
        }
 
        public void setMobilUrl(String mobilUrl) {
            this.mobilUrl = mobilUrl;
        }
        public String getMobilUrl() {
            return mobilUrl;
        }
    }
}

第七步：创建适配器


public class NewsAdapter extends RecyclerView.Adapter<NewsAdapter.ViewHolder> {
    List<News.Data> list;
 
    public void setResource(List<News.Data> list){
        this.list=list;
 
        notifyDataSetChanged();
    }
 
    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view=LayoutInflater.from(parent.getContext()).inflate(R.layout.news_item,parent,false);
        ViewHolder viewHolder=new ViewHolder(view);
        return viewHolder;
    }
 
    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
        News.Data data=list.get(position);
        holder.news_title.setText(data.getTitle());
        holder.news_desc.setText(data.getDesc());
        Glide.with(holder.view).load(data.getPic()).into(holder.news_pic);
    }
 
    @Override
    public int getItemCount() {
        if (list==null){
            return 0;
        }else {
            return list.size();
        }
    }
 
    public class ViewHolder extends RecyclerView.ViewHolder {
        TextView news_title,news_desc;
        ImageView news_pic;
        View view;
        public ViewHolder(@NonNull View itemView) {
            super(itemView);
            view=itemView;
            news_desc=itemView.findViewById(R.id.news_desc);
            news_title=itemView.findViewById(R.id.news_title);
            news_pic=itemView.findViewById(R.id.news_pic);
        }
    }
}

第八步：设置适配器


private RecyclerView rv_news;
    NewsAdapter newsAdapter;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
 
        rv_news=findViewById(R.id.rv_news);
        newsAdapter=new NewsAdapter();
        LinearLayoutManager layoutManager=new LinearLayoutManager(MainActivity2.this);
        rv_news.setLayoutManager(layoutManager);
        rv_news.setAdapter(newsAdapter);
 
        retrofitGet();
    }

第九步：进行网络请求

1. 创建请求接口


public interface News_Interface {
 
    @GET("api/hotlist?type=baiduRD")
    Observable<News> getNews();
}

2. 开始网络请求

3. 处理返回结果


 private void retrofitGet() {
        Retrofit retrofit=new Retrofit.Builder()
                .baseUrl("https://api.vvhan.com/")
                .addConverterFactory(GsonConverterFactory.create())
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .build();
        News_Interface newsInterface=retrofit.create(News_Interface.class);
        Observable<News> observable=newsInterface.getNews();
 
        observable.subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Observer<News>() {
                    @Override
                    public void onSubscribe(Disposable d) {
 
                    }
 
                    @Override
                    public void onNext(News news) {
                        newsAdapter.setResource(news.getData());
                    }
 
                    @Override
                    public void onError(Throwable e) {
                        Log.i("onError","连接失败");
                    }
 
                    @Override
                    public void onComplete() {
 
                    }
                });

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