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Retrofit不用多介绍了，谁用谁知道。我在使用过程中会产生疑问：
1.声明的接口NetService怎么经过create就能调用发起网络请求？
2.接口中的方法可以返回Call，也可以返回其他类型，比如Observable，这之间有什么关系，或者经过怎样的转化过程？
带着这两个疑问去探索下create()方法的执行流程。下面是该方法的源码，比较短我们直接粘贴过来：
public <T> T create(final Class<T> 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);
}
});
}
我们的第一个疑问有解了，在create中会通过Java动态代理的方式来处理对方法的调用问题。这样说可能比较抽象，简单举个栗子，通过动态代理就会返回Call对象。
public interface MyService{
@Get("info")
Call<User> getUser();
}
重点是最后的三行代码。
1. ServiceMethod serviceMethod =(ServiceMethod)loadServiceMethod(method);
ServiceMethod官方解释：
Adapts an invocation of an interface method into an HTTP call
有四个重要的成员：
//封装call，默认是okHttpCall
this.callFactory = builder.retrofit.callFactory();
//最著名的RxJava，默认是不支持，通过添加.addCallAdapterFactory(RxJavaCallAdapterFactory.create())增加对Obsevable的支持
this.callAdapter = builder.callAdapter;
//addConverterFactory(GsonConverterFactory.create())将服务器返回的response，提取出ResponseBody(json)转化为User
this.responseConverter = builder.responseConverter;
// 负责解析API定义时每个方法的参数
this.parameterHandlers = builder.parameterHandlers;
2. okHttpCall = new OkHttpCall<>(serviceMethod, args);
OkHttpCall实现Call接口，主要会用到的方法execute和enqueue,一个同步一个异步。
@Override
public Response<T> 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，会通过createRawCall创建okhttp,创建过程先通过serviceMethod创建request，request传入callFactory创建okhttp3.Call。最后通过okhttpCall调用execute,enqueue的调用过程类似，最后也是通过okhttp调用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)
有几个概念需要注意下：
responseType:这个返回的是关注的类型，比如Call ,返回的是User的类型
returnType:这个是接口中方法的返回类型，比如Call那么返回的就是Call类型
public interface CallAdapter<T> {
Type responseType();
<R> T adapt(Call<R> 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是接口public interface CallAdapter的实现类
serviceMethod中的callAdapter通过callAdapter = createCallAdapter()创建
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中根据returnType从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的时候添加，我们来看下默认的CallAdapter.Factory, 工厂模式，这个也是retrofit中大量使用的设计模式，在public interface Converter 接口中也会使用。DefaultCallAdapterFactory没有多少代码，重点看下get方法
1.默认的模式只支持Call这种returnType,所以一进来会判断是不是Call.class。
2.记下来会根据renturnType来获得responseType，也就是上面例子中Call中的User类型。
3.最后会new一个CallAdapter，因为默认的是支持Call这种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.我们再简单看看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，方法会将call封装成Observable，最后我们就可以快乐的使用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;
}
}
当然还没完，我们在深入到Observable中一探究竟，我们看下异步CallEnqueueObservable的例子。可以看到实际执行还是通过call.enqueue(callback)执行。
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 {
......
}
}
到这里我们对create()方法的分析过程就结束了，Retrofit是通过Java动态代理实现对接口方法的调用处理，通过解析方法的注解和参数封装request，通过request去构造HttpCall，再根据CallAdapter Factory对Call进行代理，其实内部还是通过OkHttp的Call模式做实际工作。Retrofit中比较多的使用工厂模式，实现很好的解耦，很值得借鉴。这方面的文章可以借鉴http://www.jianshu.com/p/45cb536be2f4。
谢谢！
扩展阅读：
https://blog.piasy.com/2016/06/25/Understand-Retrofit/
http://blog.csdn.net/lmj623565791/article/details/51304204
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