拆Retrofit2.0 的create()方法流程

yangxi_001發表於2017-06-22

滾滾長江東逝水，浪花也要乘風迎浪。一直在心裡感激在網上寫部落格的各路大神們，在開發過程中幫助很大，自己也有心向先輩們學習，一來對自己也是一種督促，寫部落格也是鞏固知識的過程，二來也希望能將自己開發中的所學記錄下來，說不定能幫助到某些程式猿呢？這也是我的第一篇部落格，內容如果有誤還請指出來。好了，廢話少說，進入正文。

Retrofit不用多介紹了，誰用誰知道。我在使用過程中會產生疑問：

1.宣告的介面NetService怎麼經過create就能呼叫發起網路請求？

2.介面中的方法可以返回Call，也可以返回其他型別，比如Observable，這之間有什麼關係，或者經過怎樣的轉化過程？

帶著這兩個疑問去探索下create()方法的執行流程。下面是該方法的原始碼，比較短我們直接貼上過來：

public  T create(final Class service) {
        Utils.validateServiceInterface(service);
        if (validateEagerly) {
            eagerlyValidateMethods(service);
        }
        return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class[]{service},
                new InvocationHandler() {
                    private final Platform platform = Platform.get();
                    @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);
                        }
                        ServiceMethod serviceMethod = loadServiceMethod(method);
                        OkHttpCall okHttpCall = new OkHttpCall(serviceMethod, args);
                        return serviceMethod.callAdapter.adapt(okHttpCall);
                    }
                });
    }

我們的第一個疑問有解了，在create中會通過Java動態代理的方式來處理對方法的呼叫問題。這樣說可能比較抽象，簡單舉個栗子，通過動態代理就會返回Call物件。

public interface MyService{
  @Get("info")
  Call getUser();
}

重點是最後的三行程式碼。

1. ServiceMethod serviceMethod =(ServiceMethod)loadServiceMethod(method);

ServiceMethod官方解釋：

Adapts an invocation of an interface method into an HTTP call

有四個重要的成員：

//封裝call，預設是okHttpCall
this.callFactory = builder.retrofit.callFactory();
//最著名的RxJava，預設是不支援，通過新增.addCallAdapterFactory(RxJavaCallAdapterFactory.create())增加對Obsevable的支援
this.callAdapter = builder.callAdapter;
//addConverterFactory(GsonConverterFactory.create())將伺服器返回的response，提取出ResponseBody(json)轉化為User
this.responseConverter = builder.responseConverter;
// 負責解析API定義時每個方法的引數
this.parameterHandlers = builder.parameterHandlers;

2. okHttpCall = new OkHttpCall(serviceMethod, args);

OkHttpCall實現Call介面，主要會用到的方法execute和enqueue,一個同步一個非同步。

@Override
public Response execute() throws IOException {
    okhttp3.Call call;
    synchronized (this) {
     if (executed) throw new IllegalStateException("Already executed.");
        executed = true;
        if (creationFailure != null) {
            if (creationFailure instanceof IOException) {
                throw (IOException) creationFailure;
            } else {
                throw (RuntimeException) creationFailure;
            }
        }
        call = rawCall;
        if (call == null) {
            try {
                call = rawCall = createRawCall();
            } catch (IOException | RuntimeException e) {
                creationFailure = e;
                throw e;
            }
        }
    }
        if (canceled) {
            call.cancel();
        }
        return parseResponse(call.execute());
    }

可以看出，會通過createRawCall建立okhttp,建立過程先通過serviceMethod建立request，request傳入callFactory建立okhttp3.Call。最後通過okhttpCall呼叫execute,enqueue的呼叫過程類似，最後也是通過okhttp呼叫enqueue。

private okhttp3.Call createRawCall() throws IOException {
        Request request = serviceMethod.toRequest(args);
        okhttp3.Call call = serviceMethod.callFactory.newCall(request);
        if (call == null) {
            throw new NullPointerException("Call.Factory returned null.");
        }
        return call;
}

3. serviceMethod.callAdapter.adapt(okHttpCall)

有幾個概念需要注意下：

responseType:這個返回的是關注的型別，比如Call ,返回的是User的型別

returnType:這個是介面中方法的返回型別，比如Call那麼返回的就是Call型別

public interface CallAdapter {
        Type responseType();
         T adapt(Call call);
        public abstract CallAdapter get(Type returnType, Annotation[] annotations, Retrofit retrofit);
protected static Type getParameterUpperBound(int index, ParameterizedType type) {
            return Utils.getParameterUpperBound(index, type);
        }
        protected static Class getRawType(Type type) {
            return Utils.getRawType(type);
        }
    }

callAdapter是介面public interface CallAdapter的實現類

serviceMethod中的callAdapter通過callAdapter = createCallAdapter()建立

private CallAdapter createCallAdapter() {
        Type returnType = method.getGenericReturnType();
        if (Utils.hasUnresolvableType(returnType)) {
            throw methodError("Method return type must not include a type       variable or wildcard: %s", returnType);
        }
        if (returnType == void.class) {
            throw methodError("Service methods cannot return void.");
        }
        Annotation[] annotations = method.getAnnotations();
        try {
            return retrofit.callAdapter(returnType, annotations);
        } catch (RuntimeException e) {
          // Wide exception range because   factories are user code.
        throw methodError(e, "Unable to create call adapter for %s", returnType);
        }
    }

在retrofit中根據returnType從adapterFactories(List adapterFactories = new ArrayList();)中得到callAdapter

    public CallAdapter callAdapter(Type returnType, Annotation[] annotations) {
        return nextCallAdapter(null, returnType, annotations);
    }
    public CallAdapter nextCallAdapter(CallAdapter.Factory skipPast, Type returnType, Annotation[] annotations) {
        checkNotNull(returnType, "returnType == null");
        checkNotNull(annotations, "annotations == null");
        int start = adapterFactories.indexOf(skipPast) + 1;
        for (int i = start, count = adapterFactories.size(); i < count; i++) {
            CallAdapter adapter = adapterFactories.get(i).get(returnType, annotations, this);
            if (adapter != null) {
                return adapter;
            }
        }
        …….
    }

factories可以在初始化retrofit的時候新增，我們來看下預設的CallAdapter.Factory, 工廠模式，這個也是retrofit中大量使用的設計模式，在public interface Converter 介面中也會使用。DefaultCallAdapterFactory沒有多少程式碼，重點看下get方法

1.預設的模式只支援Call這種returnType,所以一進來會判斷是不是Call.class。

2.記下來會根據renturnType來獲得responseType，也就是上面例子中Call中的User型別。

3.最後會new一個CallAdapter，因為預設的是支援Call這種returnType,因此在adapt方法中直接返回call。

    final class DefaultCallAdapterFactory extends CallAdapter.Factory {
        static final CallAdapter.Factory INSTANCE = new DefaultCallAdapterFactory();
        @Override
        public CallAdapter get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
            if (getRawType(returnType) != Call.class) {
                return null;
            }
  final Type responseType =Utils.getCallResponseType(returnType);
            return new CallAdapter > () {
                @Override public Type responseType () {
                    return responseType;
                }
                @Override public Call adapt (Call call){
                    return call;
                }
            } ;
        }
    }

4.我們再簡單看看RxJava2CallAdapterFactory

    public final class RxJava2CallAdapterFactory extends CallAdapter.Factory {
        ......
        @Override
        public CallAdapter get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
            Class rawType = getRawType(returnType);
            ......
            return new RxJava2CallAdapter(responseType, scheduler, isAsync, isResult, isBody, isFlowable,isSingle, isMaybe, false);
        }
    }

在RxJava2CallAdapter中，看到熟悉的adapt方法，方法會將call封裝成Observable，最後我們就可以快樂的使用RxJava

    final class RxJava2CallAdapter implements CallAdapter {
        @Override
        public Type responseType() {
            return responseType;
        }
        @Override
        public Object adapt(Call call) {
            Observable > responseObservable = isAsync
                    ? new CallEnqueueObservable(call)
                    : new CallExecuteObservable(call);
            ……
            return observable;
        }
    }

當然還沒完，我們在深入到Observable中一探究竟，我們看下非同步CallEnqueueObservable的例子。可以看到實際執行還是通過call.enqueue(callback)執行。

    final class CallEnqueueObservable extends Observable {
        private final Call originalCall;
        CallEnqueueObservable(Call originalCall) {
            this.originalCall = originalCall;
        }
        @Override
        protected void subscribeActual(Observer>observer) {
            // Since Call is a one-shot type, clone it for each new observer.
            Call call = originalCall.clone();
            CallCallback callback = new CallCallback(call, observer);
            observer.onSubscribe(callback);
            call.enqueue(callback);
        }
        private static final class CallCallback implements Disposable, Callback {
            ......
        }
    }

到這裡我們對create()方法的分析過程就結束了，Retrofit是通過Java動態代理實現對介面方法的呼叫處理，通過解析方法的註解和引數封裝request，通過request去構造HttpCall，再根據CallAdapter Factory對Call進行代理，其實內部還是通過OkHttp的Call模式做實際工作。Retrofit中比較多的使用工廠模式，實現很好的解耦，很值得借鑑。這方面的文章可以借鑑http://www.jianshu.com/p/45cb536be2f4。
謝謝！

擴充套件閱讀：

https://blog.piasy.com/2016/06/25/Understand-Retrofit/

http://blog.csdn.net/lmj623565791/article/details/51304204

轉自：http://www.jianshu.com/p/4d0eae0bc40c

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