源码阅读--Retrofit

1.使用方法

1、构造retrofit实例（建造者模式，类似于Dialog）

        Retrofit retrofit = new Retrofit.Builder()                .baseUrl("https://api.github.com/")                .build();        //https://api.github.com/repos/square/retrofit/contributors        GitHubApi repo = retrofit.create(GitHubApi.class);

2、创建同步或异步HTTP请求到远程网络服务器

Call<ResponseBody> call = repo.contributorsBySimpleGetCall("square", "retrofit");

3、网络请求

call.enqueue(new Callback<ResponseBody>() {            @Override            public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {                try {                    Gson gson = new Gson();                    ArrayList<Contributor> contributorsList = gson.fromJson(response.body().string(), new                            TypeToken<List<Contributor>>() {                            }.getType());                    for (Contributor contributor : contributorsList) {                        Log.d("login", contributor.getLogin());                        Log.d("contributions", contributor.getContributions() + "");                    }                } catch (IOException e) {                    e.printStackTrace();                }            }            @Override            public void onFailure(Call<ResponseBody> call, Throwable t) {            }        });

2.源码解析

看看retrofit.create干了些啥

public <T> T create(final Class<T> service) {    Utils.validateServiceInterface(service);    if (validateEagerly) {      eagerlyValidateMethods(service);    }    //--------------------------------------newProxyInstance跟踪进去就是通过native函数实现的一个代理。不管怎样，成功后会调用invoke函数    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);            }            //--------------------------------------------重要的是下面三句            ServiceMethod<Object, Object> serviceMethod =                (ServiceMethod<Object, Object>) loadServiceMethod(method);            OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);            return serviceMethod.callAdapter.adapt(okHttpCall);          }    });}

先看一下Platform.get

  private static final Platform PLATFORM = findPlatform();  static Platform get() {    return PLATFORM;  }  private static Platform findPlatform() {    try {      Class.forName("android.os.Build");      if (Build.VERSION.SDK_INT != 0) {        return new Android();//--------------------------看一下Android的实现      }    } catch (ClassNotFoundException ignored) {    }    try {      Class.forName("java.util.Optional");      return new Java8();    } catch (ClassNotFoundException ignored) {    }    return new Platform();  }

Android平台的实现

  static class Android extends Platform {    @Override public Executor defaultCallbackExecutor() {      return new MainThreadExecutor();    }    @Override CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {      return new ExecutorCallAdapterFactory(callbackExecutor);    }    //-------------------------------------------handler用来切换线程    static class MainThreadExecutor implements Executor {      private final Handler handler = new Handler(Looper.getMainLooper());      @Override public void execute(Runnable r) {        handler.post(r);      }    }  }

接下来看最重要的三句话：

ServiceMethod<Object, Object> serviceMethod =(ServiceMethod<Object, Object>) loadServiceMethod(method);OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);//----------------------------生成OkHttpCall接口return serviceMethod.callAdapter.adapt(okHttpCall);//--------------------------这段代码很简单，只要知道serviceMethodCache就是ConcurrentHashMapServiceMethod<?, ?> loadServiceMethod(Method method) {    ServiceMethod<?, ?> result = serviceMethodCache.get(method);    if (result != null) return result;    synchronized (serviceMethodCache) {      result = serviceMethodCache.get(method);      if (result == null) {        result = new ServiceMethod.Builder<>(this, method).build();        serviceMethodCache.put(method, result);      }    }    return result;}

最后serviceMethod.callAdapter.adapt(okHttpCall);在Android中callAdapter默认的是ExecutorCallAdapterFactory。adapt函数返回ExecutorCallbackCall实体类最后看一下这段代码：    @Override public void enqueue(final Callback<T> callback) {      checkNotNull(callback, "callback == null");      delegate.enqueue(new Callback<T>() {        @Override public void onResponse(Call<T> call, final Response<T> response) {          callbackExecutor.execute(new Runnable() {            @Override public void run() {              if (delegate.isCanceled()) {                // Emulate OkHttp's behavior of throwing/delivering an IOException on cancellation.                callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));              } else {                //-------------------------------------回调onResponse                callback.onResponse(ExecutorCallbackCall.this, response);              }            }          });        }