Retrofit 创建网络请求接口实例过程

implementation 'com.squareup.retrofit2:retrofit:2.9.0'
implementation 'com.squareup.retrofit2:converter-gson:2.9.0'

interface NetApi {
    @GET("/hotkey/json")
    fun getHotKey(): Call<Response>
    companion object {
        private const val BASE_URL = "https://www.wanandroid.com/"
        fun createApi(): NetApi =
            Retrofit.Builder().baseUrl(BASE_URL).addConverterFactory(GsonConverterFactory.create())
                .build().create(NetApi::class.java)
    }
}
data class HotWords(
    val id: String,
    val name: String,
)
data class Response(
    val errorCode: Int,
    val errorMsg: String,
    val data: List<HotWords>
)

NetApi.createApi().getHotKey().enqueue(object : Callback<Response> {
    override fun onResponse(call: Call<Response>, response: retrofit2.Response<Response>) {
        Log.i(tag, "onResponse: ${response.body()?.data}")
    }
    override fun onFailure(call: Call<Response>, t: Throwable) {
        Log.i(tag, "onFailure: ${t.message}")
    }
})

public static final class Builder {
    private final Platform platform;
    // 网络请求工厂，工厂方法模式
    private @Nullable okhttp3.Call.Factory callFactory;
    // 网络请求地址
    private @Nullable HttpUrl baseUrl;
    // 数据转换器工厂的集合
    private final List<Converter.Factory> converterFactories = new ArrayList<>();
    // 网络请求适配器工厂的集合，默认是 ExecutorCallAdapterFactory
    private final List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>();
    // 回调方法执行器，用于切换线程
    private @Nullable Executor callbackExecutor;
    // 一个开关，为 true 则会缓存创建的 ServiceMethod
    private boolean validateEagerly;
    ...
    public Builder baseUrl(String baseUrl) {
      Objects.requireNonNull(baseUrl, "baseUrl == null");
      return baseUrl(HttpUrl.get(baseUrl));
    }
    public Builder baseUrl(HttpUrl baseUrl) {
      Objects.requireNonNull(baseUrl, "baseUrl == null");
      List<String> pathSegments = baseUrl.pathSegments();
      if (!"".equals(pathSegments.get(pathSegments.size() - 1))) {
        throw new IllegalArgumentException("baseUrl must end in /: " + baseUrl);
      }
      this.baseUrl = baseUrl;
      return this;
    }
    // 将一个含有 Gson 对象实例的 GsonConverterFactory 放入数据转换器工厂
    public Builder addConverterFactory(Converter.Factory factory) {
      converterFactories.add(Objects.requireNonNull(factory, "factory == null"));
      return this;
    }
    ...
 }

    public Retrofit build() {
      if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
      }
      okhttp3.Call.Factory callFactory = this.callFactory;
      if (callFactory == null) {
        callFactory = new OkHttpClient();
      }
      Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {
        callbackExecutor = platform.defaultCallbackExecutor();
      }
      // Make a defensive copy of the adapters and add the default Call adapter.
      List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
      callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));
      // Make a defensive copy of the converters.
      List<Converter.Factory> converterFactories =
          new ArrayList<>(
              1 + this.converterFactories.size() + platform.defaultConverterFactoriesSize());
      // Add the built-in converter factory first. This prevents overriding its behavior but also
      // ensures correct behavior when using converters that consume all types.
      converterFactories.add(new BuiltInConverters());
      converterFactories.addAll(this.converterFactories);
      converterFactories.addAll(platform.defaultConverterFactories());
      return new Retrofit(
          callFactory,
          baseUrl,
          unmodifiableList(converterFactories),
          unmodifiableList(callAdapterFactories),
          callbackExecutor,
          validateEagerly);
    }

  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 @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;
                return platform.isDefaultMethod(method)
                    ? platform.invokeDefaultMethod(method, service, proxy, args)
                    : loadServiceMethod(method).invoke(args);
              }
            });
  }

  ServiceMethod<?> loadServiceMethod(Method method) {
    ServiceMethod<?> result = serviceMethodCache.get(method);
    if (result != null) return result;
    synchronized (serviceMethodCache) {
      result = serviceMethodCache.get(method);
      if (result == null) {
        result = ServiceMethod.parseAnnotations(this, method);
        serviceMethodCache.put(method, result);
      }
    }
    return result;
  }

abstract class ServiceMethod<T> {
  static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
    RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);
    Type returnType = method.getGenericReturnType();
    if (Utils.hasUnresolvableType(returnType)) {
      throw methodError(
          method,
          "Method return type must not include a type variable or wildcard: %s",
          returnType);
    }
    if (returnType == void.class) {
      throw methodError(method, "Service methods cannot return void.");
    }
    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
  }
  abstract @Nullable T invoke(Object[] args);
}

static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
    Retrofit retrofit, Method method, RequestFactory requestFactory) {
  ...
  okhttp3.Call.Factory callFactory = retrofit.callFactory;
  if (!isKotlinSuspendFunction) {
    return new CallAdapted<>(requestFactory, callFactory, responseConverter, callAdapter);
  } else if (continuationWantsResponse) {
    //noinspection unchecked Kotlin compiler guarantees ReturnT to be Object.
    return (HttpServiceMethod<ResponseT, ReturnT>)
        new SuspendForResponse<>(
            requestFactory,
            callFactory,
            responseConverter,
            (CallAdapter<ResponseT, Call<ResponseT>>) callAdapter);
  } else {
    //noinspection unchecked Kotlin compiler guarantees ReturnT to be Object.
    return (HttpServiceMethod<ResponseT, ReturnT>)
        new SuspendForBody<>(
            requestFactory,
            callFactory,
            responseConverter,
            (CallAdapter<ResponseT, Call<ResponseT>>) callAdapter,
            continuationBodyNullable);
  }
}

static final class SuspendForBody<ResponseT> extends HttpServiceMethod<ResponseT, Object> {
  private final CallAdapter<ResponseT, Call<ResponseT>> callAdapter;
  private final boolean isNullable;
  ...
  @Override
  protected Object adapt(Call<ResponseT> call, Object[] args) {
    call = callAdapter.adapt(call);
    Continuation<ResponseT> continuation = (Continuation<ResponseT>) args[args.length - 1];
    try {
      return isNullable
          ? KotlinExtensions.awaitNullable(call, continuation)
          : KotlinExtensions.await(call, continuation);
    } catch (Exception e) {
      return KotlinExtensions.suspendAndThrow(e, continuation);
    }
  }
}

suspend fun <T : Any> Call<T?>.await(): T? {
  return suspendCancellableCoroutine { continuation ->
    //协程取消是调用 cancel
    continuation.invokeOnCancellation {
      cancel()
    }
    enqueue(object : Callback<T?> {
      override fun onResponse(call: Call<T?>, response: Response<T?>) {
        if (response.isSuccessful) {
          //继续执行相应的协程，将 response.body 作为最后一个挂起点的返回值。
          continuation.resume(response.body())
        } else {
          continuation.resumeWithException(HttpException(response))
        }
      }
      override fun onFailure(call: Call<T?>, t: Throwable) {
        continuation.resumeWithException(t)
      }
    })
  }
}

  public void enqueue(final Callback<T> callback) {
    Objects.requireNonNull(callback, "callback == null");
    okhttp3.Call call;
    Throwable failure;
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already executed.");
      executed = true;
      call = rawCall;
      failure = creationFailure;
      if (call == null && failure == null) {
        try {
          // 创建 OkHttp 的 Call 对象
          call = rawCall = createRawCall();
        } catch (Throwable t) {
          throwIfFatal(t);
          failure = creationFailure = t;
        }
      }
    }
    if (failure != null) {
      callback.onFailure(this, failure);
      return;
    }
    if (canceled) {
      call.cancel();
    }
    call.enqueue(
        new okhttp3.Callback() {
          @Override
          public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse) {
            Response<T> response;
            try {
              // 解析网络请求返回的数据
              response = parseResponse(rawResponse);
            } catch (Throwable e) {
              throwIfFatal(e);
              callFailure(e);
              return;
            }
            try {
              callback.onResponse(OkHttpCall.this, response);
            } catch (Throwable t) {
              throwIfFatal(t);
              t.printStackTrace(); // TODO this is not great
            }
          }
          @Override
          public void onFailure(okhttp3.Call call, IOException e) {
            callFailure(e);
          }
          private void callFailure(Throwable e) {
            try {
              callback.onFailure(OkHttpCall.this, e);
            } catch (Throwable t) {
              throwIfFatal(t);
              t.printStackTrace(); // TODO this is not great
            }
          }
        });
  }

  Response<T> parseResponse(okhttp3.Response rawResponse) throws IOException {
    ResponseBody rawBody = rawResponse.body();
    // Remove the body's source (the only stateful object) so we can pass the response along.
    rawResponse =
        rawResponse
            .newBuilder()
            .body(new NoContentResponseBody(rawBody.contentType(), rawBody.contentLength()))
            .build();
    int code = rawResponse.code();
    // 根据响应返回的状态码进行处理 
    if (code < 200 || code >= 300) {
      try {
        // Buffer the entire body to avoid future I/O.
        ResponseBody bufferedBody = Utils.buffer(rawBody);
        return Response.error(bufferedBody, rawResponse);
      } finally {
        rawBody.close();
      }
    }
    if (code == 204 || code == 205) {
      rawBody.close();
      return Response.success(null, rawResponse);
    }
    //包装 RequestBody
    ExceptionCatchingResponseBody catchingBody = new ExceptionCatchingResponseBody(rawBody);
    try {
      // 将响应体转为 Java 对象
      T body = responseConverter.convert(catchingBody);
      return Response.success(body, rawResponse);
    } catch (RuntimeException e) {
      // If the underlying source threw an exception, propagate that rather than indicating it was
      // a runtime exception.
      catchingBody.throwIfCaught();
      throw e;
    }
  }