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Retrofit源码阅读

2021-04-16  本文已影响0人  崔鹏宇

前言

Retrofit 基本使用

问题分析

Create方法源码

public <T> T create(final Class<T> service) {
  validateServiceInterface(service);
  return (T)
      Proxy.newProxyInstance(
          service.getClassLoader(),
          new Class<?>[] {service},
          new InvocationHandler() {
            //注释①
            private final Platform platform = Platform.get();
            private final Object[] emptyArgs = new Object[0];

            @Override
            public @Nullable Object invoke(Object proxy, Method method, @Nullable 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);
              }
              args = args != null ? args : emptyArgs;
              //注释②
              return platform.isDefaultMethod(method)
                  ? platform.invokeDefaultMethod(method, service, proxy, args)
                  : loadServiceMethod(method).invoke(args);
            }
          });
}

List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
//这添加一个平台默认的CallAdapter并且传一个线程池callbackExecutor
callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));
//可以看到defaultCallbackExecutor也是由平台默认的额
 Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {
        callbackExecutor = platform.defaultCallbackExecutor();
      }

我们看下平台platform这俩个默认方法。

List<? extends CallAdapter.Factory> defaultCallAdapterFactories(
    @Nullable Executor callbackExecutor) {
  DefaultCallAdapterFactory executorFactory = new DefaultCallAdapterFactory(callbackExecutor);
  return hasJava8Types
      ? asList(CompletableFutureCallAdapterFactory.INSTANCE, executorFactory)
      : singletonList(executorFactory);
}

继续看DefaultCallAdapterFactory

final class DefaultCallAdapterFactory extends CallAdapter.Factory {
  private final @Nullable Executor callbackExecutor;

  DefaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
    this.callbackExecutor = callbackExecutor;
  }

  @Override
  public @Nullable CallAdapter<?, ?> get(
      Type returnType, Annotation[] annotations, Retrofit retrofit) {
//省略空检查
    final Type responseType = Utils.getParameterUpperBound(0, (ParameterizedType) returnType);

    final Executor executor =
        Utils.isAnnotationPresent(annotations, SkipCallbackExecutor.class)
            ? null
            : callbackExecutor;

    return new CallAdapter<Object, Call<?>>() {
      @Override
      public Type responseType() {
        return responseType;
      }

      @Override
      public Call<Object> adapt(Call<Object> call) {
        //当时kotlin协程请求时返回okhttpcall 否则返回ExecutorCallbackCall
        return executor == null ? call : new ExecutorCallbackCall<>(executor, call);
      }
    };
  }

  static final class ExecutorCallbackCall<T> implements Call<T> {
    final Executor callbackExecutor;
    final Call<T> delegate;

    ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
      this.callbackExecutor = callbackExecutor;
      this.delegate = delegate;
    }

    @Override
    public void enqueue(final Callback<T> callback) {
      Objects.requireNonNull(callback, "callback == null");

      //（OkHttp封装类）OkHttpCall的enqueue方法进行请求
      delegate.enqueue(
          new Callback<T>() {
            @Override
            public void onResponse(Call<T> call, final Response<T> response) {
              //线程切换
              callbackExecutor.execute(
                  () -> {
                    if (delegate.isCanceled()) {
                      // Emulate OkHttp's behavior of throwing/delivering an IOException on
                      // cancellation.
                      callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
                    } else {
                      callback.onResponse(ExecutorCallbackCall.this, response);
                    }
                  });
            }

            @Override
            public void onFailure(Call<T> call, final Throwable t) {
              callbackExecutor.execute(() -> callback.onFailure(ExecutorCallbackCall.this, t));
            }
          });
    }

    @Override
    public boolean isExecuted() {
      return delegate.isExecuted();
    }

    @Override
    public Response<T> execute() throws IOException {
      return delegate.execute();
    }

    @Override
    public void cancel() {
      delegate.cancel();
    }

    @Override
    public boolean isCanceled() {
      return delegate.isCanceled();
    }

    @SuppressWarnings("CloneDoesntCallSuperClone") // Performing deep clone.
    @Override
    public Call<T> clone() {
      return new ExecutorCallbackCall<>(callbackExecutor, delegate.clone());
    }

    @Override
    public Request request() {
      return delegate.request();
    }

    @Override
    public Timeout timeout() {
      return delegate.timeout();
    }
  }
}

也就是说我们示例中的mApiService?.listRepos("brycecui")?.enqueue（）最终到了这里。delegate他是 Call<T>接口。它的实现类是OkHttpCall。怎么知道不是别的实现类。上面我们通过代码追踪知道了loadserviceMethod.invoke.实际是调用了 抽象类ServiceMethod的实现类HttpServiceMethod的invoke方法。也就是下面这个方法。下面调用了adapt方法并把OkHttpCall传了过去。最终也就是通过DefaultCallAdapterFactory的get方法的adapt创建一个ExecutorCallbackCall传递的。

@Override
final @Nullable ReturnT invoke(Object[] args) {
  Call<ResponseT> call = new OkHttpCall<>(requestFactory, args, callFactory, responseConverter);
  return adapt(call, args);
}

看下 callbackExecutor.execute。线程切换关键所在。

static final class MainThreadExecutor implements Executor {
  private final Handler handler = new Handler(Looper.getMainLooper());
  @Override
  public void execute(Runnable r) {
    handler.post(r);
  }
}

也就是使用handler.post(r);切换到主线程。

总结

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