01retrofit---使用retrofit完成一次网络请求方法调用链

public class Biz {
    public static Single<String> method() {
        RespTransformer<String> transformer = RespTransform.newInstance();
        return ApiGenerator.createApi(Api.class)
                           .method()
                           .compose(transformer)
                           .subscribeOn(Schedulers.io())
                           .observeOn(AndroidSchedulers.mainThread())
    }
}

public class presenter {
    public void method() {
        Biz.method()
           .subscribe(new Consumer<List<Resp>>() {
               @Override
               public void accept(List<Resp> resps) {
                   ...
               }
           }, new BaseRespThrowableObserver() {
               @Override
               public void onV2Error(String code, String errorMsg)  {
                   ...
               } 
           }));
    }
}
public class ApiGenerator {
    public static <S> createApi(Class<S> apiClass) {
        // serviceCreator()返回Retrofit实例;
        return NetManager.getInstance().serviceCreator().create(apiClass);
    }
}
复制代码

上面就是Retrofit的调用流程, 下面根据这个调用流程来进行分析, Retrofit是如何完成一次网络请求的,

2.1 Retrofit.create()

@SuppressWarnings("unchecked") // Single-interface proxy creation guarded by parameter safety.
public <T> T create(final Class<T> service) {
    validateServiceInterface(service);
    return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
        new InvocationHandler() {
            private final Platform platform = Platform.get();
            private final Object[] emptyArgs = new Object[0];
            @Override 
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                // If the method is a method from Object then defer to normal invocation.
                if (method.getDeclaringClass() == Object.class) {
                    return method.invoke(this, args);
                }
                if (platform.isDefaultMethod(method)) {
                    return platform.invokeDefaultMethod(method, service, proxy, args);
                }
                return loadServiceMethod(method).invoke(args != null ? args : emptyArgs);
            }
        });
}
复制代码

retrofit使用到 动态代理 的技术, 使得不同的接口调用, 最终都走到这里, 进行网络请求.

2.2 Retrofit.loadServiceMethod

Retrofit.loadServiceMethod
--->ServiceMethod.parseAnnotations
    --->HttpServiceMethod.parseAnnotations
复制代码

static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
      Retrofit retrofit, Method method, RequestFactory requestFactory) {
    boolean isKotlinSuspendFunction = requestFactory.isKotlinSuspendFunction;
    boolean continuationWantsResponse = false;
    boolean continuationBodyNullable = false;
    Annotation[] annotations = method.getAnnotations();
    Type adapterType;
    adapterType = method.getGenericReturnType();
    // 1.获取adapter;
    CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit, method, adapterType, annotations);
    Type responseType = callAdapter.responseType();
    // 2.获取convert;
    Converter<ResponseBody, ResponseT> responseConverter = createResponseConverter(retrofit, method, responseType);
    okhttp3.Call.Factory callFactory = retrofit.callFactory;
    // 3.构建CallAdapter;
    return new CallAdapted<>(requestFactory, callFactory, responseConverter, callAdapter);
}
复制代码

retrofit的好处, 支持自定义convert与adapter, 默认okhttp只是单纯的网络请求, 与rxjava并没有关系, 但是通过Retrofit可以实现okhttp与rxjava的结合. 贴出来一段代码retrofit如何将okhttp与rxjava结合

2.2.1 自定义Adapter与GsonConvert

public void initRetrofit() {
    Retrofit retrofit = new Retrofit.Builder()
                .addConverterFactory(GsonConverterFactory.create(/*自定义Gson*/))
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .build();
}
public final class RxJava2CallAdapterFactory extends CallAdapter.Factory {
    public static RxJava2CallAdapterFactory create() {
        return new RxJava2CallAdapterFactory(null, false);
    }
}
复制代码

• 初始化Retrofit时, 通过Retrofit对外暴露的接口, 传入自定义的Gson和Adapter, 平时在开发时, 应当注意这个思想, 通过建造者的模式, 将能力以接口的形式对外暴露, 支持用户自定义, 提高了扩展性.
• 同时这里也用到了适配器模式, 之前每次提到适配器模式, 可能都会说ListView与Adapter, 但是ListView代码量比较多, 可能理解起来没有那么容易, 这里就很好理解了, okhttp与rxjava本来是不相干的两个东西, 这里使用适配器模式, 将二者结合起来, 接下来看适配器模式如何将okhttp与rxjava结合.

2.2.2 构建RxJava2CallAdapter

class HttpServiceMethod::createCallAdapter
private static <ResponseT, ReturnT> CallAdapter<ResponseT, ReturnT> createCallAdapter(
      Retrofit retrofit, Method method, Type returnType, Annotation[] annotations) {
    return (CallAdapter<ResponseT, ReturnT>) retrofit.callAdapter(returnType, annotations);
}

class Retrofit::callAdapter
public CallAdapter<?, ?> nextCallAdapter(CallAdapter.Factory skipPast, Type returnType, Annotation[] annotations) {
    Objects.requireNonNull(returnType, "returnType == null");
    Objects.requireNonNull(annotations, "annotations == null");
    int start = callAdapterFactories.indexOf(skipPast) + 1;
    for (int i = start, count = callAdapterFactories.size(); i < count; i++) {
        CallAdapter<?, ?> adapter = callAdapterFactories.get(i).get(returnType, annotations, this);
        if (adapter != null) {
            return adapter;
        }
    }
}

class RxJava2CallAdapterFactory::get
@Override 
public CallAdapter<?, ?> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
    Class<?> rawType = getRawType(returnType);
    if (rawType == Completable.class) {
        // Completable is not parameterized (which is what the rest of this method deals with) so it
        // can only be created with a single configuration.
        return new RxJava2CallAdapter(Void.class, scheduler, isAsync, false, true, false, false,
          false, true);
    }
    boolean isFlowable = rawType == Flowable.class;
    boolean isSingle = rawType == Single.class;
    boolean isMaybe = rawType == Maybe.class;
    // ...
    // 这里为okhttp与rxjava结合做了铺垫
    return new RxJava2CallAdapter(responseType, scheduler, isAsync, isResult, isBody, isFlowable,
        isSingle, isMaybe, false);
}
复制代码

2.2.3 rxjava与okhttp相结合

class CallAdapted::invoke
@Override 
final ReturnT invoke(Object[] args) {
    Call<ResponseT> call = new OkHttpCall<>(requestFactory, args, callFactory, responseConverter);
    return adapt(call, args);
}
  
@Override 
protected ReturnT adapt(Call<ResponseT> call, Object[] args) {
    // callAdapter指向的是初始化Retrofit中传入的RxJava2CallAdapter
    return callAdapter.adapt(call);
}
复制代码

• 2.1模块loadServiceMethod 返回CallAdapted对象, 当有网络请求时, 触发CallAdapted.invoke方法的执行

2.2.4 RxJava2CallAdapter.adapt(适配将二者结合)

@Override 
public Object adapt(Call<R> call) {
    Observable<Response<R>> responseObservable = isAsync
        ? new CallEnqueueObservable<>(call)
        : new CallExecuteObservable<>(call);
    Observable<?> observable;
    if (isResult) {
        observable = new ResultObservable<>(responseObservable);
    } else if (isBody) {
        observable = new BodyObservable<>(responseObservable);
    } else {
        observable = responseObservable;
    }
    if (scheduler != null) {
        observable = observable.subscribeOn(scheduler);
    }
    if (isFlowable) {
        return observable.toFlowable(BackpressureStrategy.LATEST);
    }
    if (isSingle) {
        return observable.singleOrError();
    }
    if (isMaybe) {
        return observable.singleElement();
    }
    if (isCompletable) {
        return observable.ignoreElements();
    }
    return RxJavaPlugins.onAssembly(observable);
}
复制代码

• 暴露出来的是Retrofit的接口进行网络请求, 但是通过传入的RxJava2CallAdapter, 将请求交给了rxjava, 然后由rxjava调用okhttp完成网络请求, 并且对结果进行包装, 以rxjava的链式结构进行返回.

三、okhttp端

retrofit端作为网络请求的入口, 将请求权和结果的处理权都交给了rxjava, 实现了rxjava与okhttp的结合.

结合rxjava的源码可知, subsrcibe()会触发CallExecuteObservable.subscribeActual的执行

3.1 CallExecuteObservable.subscribeActual

@Override 
protected void subscribeActual(Observer<? super Response<T>> observer) {
    // Since Call is a one-shot type, clone it for each new observer.
    Call<T> call = originalCall.clone();
    //...
    Response<T> response = call.execute();
    //...
}
复制代码

3.2 OkHttpCall.execute

@Override 
public Response<T> execute() throws IOException {
    okhttp3.Call call;
    synchronized (this) {
        executed = true;
        // call = RealCall;
        call = getRawCall();
    }
    return parseResponse(call.execute());
}
复制代码

• getRawCall流程省略, getRawCall返回RealCall对象.

3.3 RealCall.execute

override fun execute(): Response {
    timeout.enter()
    callStart()
    // 流程省略, client指向OkHttpClient
    client.dispatcher.executed(this)
    // 网络请求的处理使用了责任链模式
    return getResponseWithInterceptorChain()
}
复制代码

3.4 RealCall.getResponseWithInterceptorChain

@Throws(IOException::class)
internal fun getResponseWithInterceptorChain(): Response {
    // Build a full stack of interceptors.
    val interceptors = mutableListOf<Interceptor>()
    interceptors += client.interceptors
    interceptors += RetryAndFollowUpInterceptor(client)
    interceptors += BridgeInterceptor(client.cookieJar)
    interceptors += CacheInterceptor(client.cache)
    interceptors += ConnectInterceptor
    if (!forWebSocket) {
        interceptors += client.networkInterceptors
    }
    interceptors += CallServerInterceptor(forWebSocket)
    val chain = RealInterceptorChain(
        call = this,
        interceptors = interceptors,
        index = 0,
        exchange = null,
        request = originalRequest,
        connectTimeoutMillis = client.connectTimeoutMillis,
        readTimeoutMillis = client.readTimeoutMillis,
        writeTimeoutMillis = client.writeTimeoutMillis
    )
    var calledNoMoreExchanges = false
    val response = chain.proceed(originalRequest)
    return response
}
复制代码

• 目前来看retrofit的作用相当于是一个配置器, 通过建造者的模式对外提供接口, 使得Gson解析, 结果的处理可以自定义, 将rxjava与okhttp结合起来, 请求最终还是通过OkHttp进行;
• 这里使用责任链模式代替了传统的if...else, 在工作中也有借鉴retrofit这个地方的责任链模式, 感觉效果也确实比较好用.