Android的RxJava和Retrofit
在这个教程中，我们将在Android应用中使用RxJava来实现Retrofit调用。我们将创建一个使用Retrofit和RxJava来填充RecyclerView的应用程序。我们将使用一个加密货币API。
你将学到什么？
- Making a Retrofit call using RxJava.
- Doing Multiple Retrofit Calls Using RxJava
- Transforming the Retrofit POJO response using RxJava
我们的安卓应用将使用Java 8来引入lambda表达式。
总览
Retrofit是一个REST客户端，它使用OkHttp作为HttpClient和JSON解析器来解析响应。我们在这里将使用gson作为JSON解析器。以下是创建Retrofit实例的方法：
HttpLoggingInterceptor interceptor = new HttpLoggingInterceptor();
interceptor.setLevel(HttpLoggingInterceptor.Level.BODY);
OkHttpClient client = new OkHttpClient.Builder().addInterceptor(interceptor).build();
Gson gson = new GsonBuilder()
.setLenient()
.create();
Retrofit retrofit = new Retrofit.Builder()
.baseUrl(BASE_URL)
.client(client)
.addConverterFactory(GsonConverterFactory.create(gson))
.build();
使用HttpLoggingInterceptor来记录网络调用过程中的数据。RxJava是一个用于异步和响应式编程的库，以流的形式展现。在RxJava中，我们使用不同的线程。网络调用使用后台线程，更新UI使用主线程。在RxJava中，Schedulers负责使用不同的线程执行操作。RxAndroid是RxJava的扩展，包含了用于在Android环境中使用的Android线程。在Retrofit环境中使用RxJava只需要进行两个主要的更改。
- Add the RxJava in Retrofit Builder.
- Use Observable type in the interface instead of Call
使用RxJava操作符可以进行多个调用或者对响应进行转换。让我们来看看通过下面的示例应用程序是如何完成的。
项目结构
在我们的build.gradle文件中添加以下依赖项。
implementation 'com.android.support:cardview-v7:27.1.0'
implementation 'com.android.support:design:27.1.0'
implementation('com.squareup.retrofit2:retrofit:2.3.0')
{
exclude module: 'okhttp'
}
implementation 'com.squareup.retrofit2:converter-gson:2.3.0'
implementation 'io.reactivex.rxjava2:rxjava:2.1.9'
implementation 'com.squareup.retrofit2:adapter-rxjava2:2.3.0'
implementation 'io.reactivex.rxjava2:rxandroid:2.0.1'
implementation 'com.squareup.okhttp3:logging-interceptor:3.9.1'
代码
以下是activity_main.xml布局的代码。
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="https://schemas.android.com/apk/res/android"
xmlns:tools="https://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<android.support.v7.widget.RecyclerView
android:id="@+id/recyclerView"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</android.support.constraint.ConstraintLayout>
以下是CryptocurrencyService.java类的代码。
package com.Olivia.rxjavaretrofit;
import com.Olivia.rxjavaretrofit.pojo.Crypto;
import io.reactivex.Observable;
import retrofit2.http.GET;
import retrofit2.http.Path;
public interface CryptocurrencyService {
String BASE_URL = "https://api.cryptonator.com/api/full/";
@GET("{coin}-usd")
Observable<Crypto> getCoinData(@Path("coin") String coin);
}
@Path传递我们指定的路径进入花括号。注意：@Path参数的名称必须与@GET中的一致。下面是POJO类Crypto.java。
package com.Olivia.rxjavaretrofit.pojo;
import com.google.gson.annotations.SerializedName;
import java.util.List;
public class Crypto {
@SerializedName("ticker")
public Ticker ticker;
@SerializedName("timestamp")
public Integer timestamp;
@SerializedName("success")
public Boolean success;
@SerializedName("error")
public String error;
public class Market {
@SerializedName("market")
public String market;
@SerializedName("price")
public String price;
@SerializedName("volume")
public Float volume;
public String coinName;
}
public class Ticker {
@SerializedName("base")
public String base;
@SerializedName("target")
public String target;
@SerializedName("price")
public String price;
@SerializedName("volume")
public String volume;
@SerializedName("change")
public String change;
@SerializedName("markets")
public List<Market> markets = null;
}
}
coinName是我们设置的一个字段。借助于RxJava的魔力，我们将在这个字段上设置一个值来转换响应。
使用RxJava创建一个单一的调用
CryptocurrencyService cryptocurrencyService = retrofit.create(CryptocurrencyService.class);
Observable<Crypto> cryptoObservable = cryptocurrencyService.getCoinData("btc");
cryptoObservable.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.map(result -> result.ticker)
.subscribe(this::handleResults, this::handleError);
subscribeOn() 创建一个调度线程，用于进行网络请求。我们可以在其中传递以下任何调度器。
- trampoline(): This runs the tasks on the current thread. So it’ll run your code after the current task on the thread is complete. Useful for queueing operations.
- newThread(): Creates and returns a Scheduler that creates a new Thread for each unit of work. This is costly since it creates a separate thread everytime.
- computation(): Creates and returns a Scheduler intended for computational work. This should be used for parallel work since the thread pool is bound. I/O operations shouldn’t be done here.
- io(): Creates and returns a Scheduler intended for IO-bound work. Again it’s bounded like computation. Typically this is used for network calls.
“subscribeOn()和observeOn()的区别”
- subscribeOn works downstream and upstream. All the tasks above and below it would use the same thread.
- observeOn works downstream only.
- consecutive subscribeOn methods won’t change the thread. Only the first subscribeOn thread would be used.
- consecutive observeOn methods will change the thread.
- After an observeOn(), putting a subscribeOn() won’t change the thread. Hence observeOn should generally come after a subscribeOn.
AndroidSchedulers.mainThread()是RxAndroid的一部分，用于仅在主线程上观察数据。subscribe方法是触发retrofit调用并在handleResults方法中获取数据的方式，我们很快就会看到。
多个电话
我们使用RxJava操作符merge来依次执行两个retrofit调用。
Observable<List<Crypto.Market>> btcObservable = cryptocurrencyService.getCoinData("btc");
Observable<List<Crypto.Market>> ethObservable = cryptocurrencyService.getCoinData("eth");
Observable.merge(btcObservable, ethObservable)
.subscribeOn(Schedulers.computation())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(this::handleResults, this::handleError);
改变回应。
要转换POJO响应，我们可以执行以下操作：
Observable<List<Crypto.Market>> btcObservable = cryptocurrencyService.getCoinData("btc")
.map(result -> Observable.fromIterable(result.ticker.markets))
.flatMap(x -> x).filter(y -> {
y.coinName = "btc";
return true;
}).toList().toObservable();
Observable<List<Crypto.Market>> ethObservable = cryptocurrencyService.getCoinData("eth")
.map(result -> Observable.fromIterable(result.ticker.markets))
.flatMap(x -> x).filter(y -> {
y.coinName = "eth";
return true;
}).toList().toObservable();
Observable.merge(btcObservable, ethObservable)
.subscribeOn(Schedulers.computation())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(this::handleResults, this::handleError);
我们使用Observable.fromIterable将map结果转换为Observable流。flatMap逐个操作元素。因此将ArrayList转换为单个元素。在filter方法中，我们改变了响应。toList()用于将flatMap的结果转换回List。toObservable()将它们包装成Observable流。
主活动.java
以下给出了MainActivity.java类的代码：
package com.Olivia.rxjavaretrofit;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.widget.Toast;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.Olivia.rxjavaretrofit.pojo.Crypto;
import java.util.List;
import io.reactivex.Observable;
import io.reactivex.android.schedulers.AndroidSchedulers;
import io.reactivex.schedulers.Schedulers;
import okhttp3.OkHttpClient;
import okhttp3.logging.HttpLoggingInterceptor;
import retrofit2.Retrofit;
import retrofit2.adapter.rxjava2.RxJava2CallAdapterFactory;
import retrofit2.converter.gson.GsonConverterFactory;
import static com.Olivia.rxjavaretrofit.CryptocurrencyService.BASE_URL;
public class MainActivity extends AppCompatActivity {
RecyclerView recyclerView;
Retrofit retrofit;
RecyclerViewAdapter recyclerViewAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
recyclerView = findViewById(R.id.recyclerView);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
recyclerViewAdapter = new RecyclerViewAdapter();
recyclerView.setAdapter(recyclerViewAdapter);
HttpLoggingInterceptor interceptor = new HttpLoggingInterceptor();
interceptor.setLevel(HttpLoggingInterceptor.Level.BODY);
OkHttpClient client = new OkHttpClient.Builder().addInterceptor(interceptor).build();
Gson gson = new GsonBuilder()
.setLenient()
.create();
retrofit = new Retrofit.Builder()
.baseUrl(BASE_URL)
.client(client)
.addCallAdapterFactory(RxJava2CallAdapterFactory.create())
.addConverterFactory(GsonConverterFactory.create(gson))
.build();
callEndpoints();
}
private void callEndpoints() {
CryptocurrencyService cryptocurrencyService = retrofit.create(CryptocurrencyService.class);
//Single call
/*Observable<Crypto> cryptoObservable = cryptocurrencyService.getCoinData("btc");
cryptoObservable.subscribeOn(Schedulers.newThread()).observeOn(AndroidSchedulers.mainThread()).map(result -> result.ticker).subscribe(this::handleResults, this::handleError);*/
Observable<List<Crypto.Market>> btcObservable = cryptocurrencyService.getCoinData("btc")
.map(result -> Observable.fromIterable(result.ticker.markets))
.flatMap(x -> x).filter(y -> {
y.coinName = "btc";
return true;
}).toList().toObservable();
Observable<List<Crypto.Market>> ethObservable = cryptocurrencyService.getCoinData("eth")
.map(result -> Observable.fromIterable(result.ticker.markets))
.flatMap(x -> x).filter(y -> {
y.coinName = "eth";
return true;
}).toList().toObservable();
Observable.merge(btcObservable, ethObservable)
.subscribeOn(Schedulers.computation())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(this::handleResults, this::handleError);
}
private void handleResults(List<Crypto.Market> marketList) {
if (marketList != null && marketList.size() != 0) {
recyclerViewAdapter.setData(marketList);
} else {
Toast.makeText(this, "NO RESULTS FOUND",
Toast.LENGTH_LONG).show();
}
}
private void handleError(Throwable t) {
Toast.makeText(this, "ERROR IN FETCHING API RESPONSE. Try again",
Toast.LENGTH_LONG).show();
}
}
在Java 8中使用::调用handleResults和handleError，handleResults中我们将转换后的响应设置在ReyclerViewAdapter上。如果响应中有错误，则调用handleError()。下面是recyclerview_item_layout布局的代码。
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="https://schemas.android.com/apk/res/android"
xmlns:app="https://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="wrap_content">
<android.support.v7.widget.CardView
android:id="@+id/cardView"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:layout_margin="16dp">
<android.support.constraint.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:padding="8dp">
<TextView
android:id="@+id/txtCoin"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginLeft="8dp"
android:layout_marginRight="8dp"
android:layout_marginTop="8dp"
android:textAllCaps="true"
android:textColor="@android:color/black"
app:layout_constraintHorizontal_bias="0.023"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/txtMarket"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginLeft="8dp"
android:layout_marginRight="8dp"
android:layout_marginTop="8dp"
app:layout_constraintHorizontal_bias="0.025"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toBottomOf="@+id/txtCoin" />
<TextView
android:id="@+id/txtPrice"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginLeft="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
app:layout_constraintHorizontal_bias="0.025"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toBottomOf="@+id/txtMarket" />
</android.support.constraint.ConstraintLayout>
</android.support.v7.widget.CardView>
</LinearLayout>
以下是RecyclerViewAdapter.java类的代码：
package com.Olivia.rxjavaretrofit;
import android.graphics.Color;
import android.support.v4.content.ContextCompat;
import android.support.v7.widget.CardView;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import com.Olivia.rxjavaretrofit.pojo.Crypto;
import java.util.ArrayList;
import java.util.List;
public class RecyclerViewAdapter extends RecyclerView.Adapter<RecyclerViewAdapter.ViewHolder> {
private List<Crypto.Market> marketList;
public RecyclerViewAdapter() {
marketList = new ArrayList<>();
}
@Override
public RecyclerViewAdapter.ViewHolder onCreateViewHolder(ViewGroup parent,
int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.recyclerview_item_layout, parent, false);
RecyclerViewAdapter.ViewHolder viewHolder = new RecyclerViewAdapter.ViewHolder(view);
return viewHolder;
}
@Override
public void onBindViewHolder(RecyclerViewAdapter.ViewHolder holder, int position) {
Crypto.Market market = marketList.get(position);
holder.txtCoin.setText(market.coinName);
holder.txtMarket.setText(market.market);
holder.txtPrice.setText("$" + String.format("%.2f", Double.parseDouble(market.price)));
if (market.coinName.equalsIgnoreCase("eth")) {
holder.cardView.setCardBackgroundColor(Color.GRAY);
} else {
holder.cardView.setCardBackgroundColor(Color.GREEN);
}
}
@Override
public int getItemCount() {
return marketList.size();
}
public void setData(List<Crypto.Market> data) {
this.marketList.addAll(data);
notifyDataSetChanged();
}
public class ViewHolder extends RecyclerView.ViewHolder {
public TextView txtCoin;
public TextView txtMarket;
public TextView txtPrice;
public CardView cardView;
public ViewHolder(View view) {
super(view);
txtCoin = view.findViewById(R.id.txtCoin);
txtMarket = view.findViewById(R.id.txtMarket);
txtPrice = view.findViewById(R.id.txtPrice);
cardView = view.findViewById(R.id.cardView);
}
}
}
上述应用程序的输出如下：以上输出将通过Retrofit合并比特币和以太坊市场价格的结果。本教程到此结束。您可以通过下方链接下载Android RxJavaRetrofit项目。
RxJavaRetrofit 在中国的本地化翻译可能是：响应式Java和网络请求库
