Retrofit 在kotlin中使用及解析

build.gradle

dependencies {    
    //retrofit + gson + rxjava
    implementation 'com.squareup.retrofit2:retrofit:2.9.0'
    implementation 'com.squareup.retrofit2:converter-gson:2.5.0'
    implementation 'com.squareup.retrofit2:adapter-rxjava2:2.9.0'
}

interface ApiService {
 
    @GET("getUserData")
    fun getUserData1(): Call
}
 
fun main1() {
    val retrofit = Retrofit.Builder()
   .baseUrl("http://mockapi.eolinker.com/9IiwI82f58c23411240ed608ceca204b2f185014507cbe3/")
        .build()
    val service = retrofit.create(ApiService::class.java)
    val call: Call = service.getUserData1()
    call.enqueue(object : Callback {
        override fun onResponse(call: Call<ResponseBody>, response: Response<ResponseBody>) {
            val userBean = response.body()?.string()
            println("userBean: $userBean")
        }
 
        override fun onFailure(call: Call<ResponseBody>, t: Throwable) {
            println("onFailure: $t")
        }
    })
}

Retrofit 是建立在 OkHttp 之上的一个网络请求封装库，内部依靠 okhttp 来完成实际网络请求。Retrofit 在使用上很简洁， API 通过 interface 来声明。我只需要通过 interface 来声明 API路径、请求方式、请求参数、返回值类型等各个配置项。

可以看到，getUserData() 方法的请求结果是一个 json 格式的字符串，其返回值类型被定义为 Call ， 此处的 ResponseBody 即 okHttp3.ResponseBody ,是 okhttp 提供的对网络请求结果的包装类，Call 即 retrofit2.Call ，是 Retrofit 对 okhttp3.Call 做的一层包装，OkHttp在实际发起请求的时候使用的回调是 okhttp3.Call ，回调内部会中转调用 retrofit2.Call，以便将请求结果转交给外部。

1、converter-gson

API 返回值 Json 转换 ：

interface ApiService {
 
    @GET("getUserData")
    fun getUserData2(): Call
}
 
data class UserBean(val status: Int, val msg: String, val data: Data)
 
data class Data(val userName: String, val userAge: Long)
 
fun main2() {
    val retrofit = Retrofit.Builder()
        .baseUrl("http://mockapi.eolinker.com/9IiwI82f58c23411240ed608ceca204b2f185014507cbe3/")
        .addConverterFactory(GsonConverterFactory.create())
        .build()
    val service = retrofit.create(ApiService::class.java)
    val call: Call = service.getUserData2()
    call.enqueue(object : Callback {
        override fun onResponse(call: Call<UserBean>, response: Response<UserBean>) {
            val userBean = response.body()
            println("userBean: $userBean")
        }
 
        override fun onFailure(call: Call<UserBean>, t: Throwable) {
            println("onFailure: $t")
        }
    })
}

2、adapter-rxjava2

adapter-rxjava2 转换返回值为被观察者

interface ApiService {
 
    @GET("getUserData")
    fun getUserData3(): Observable
}
 
data class UserBean(val status: Int, val msg: String, val data: Data)
 
data class Data(val userName: String, val userAge: Long)
 
fun main3() {
    val retrofit = Retrofit.Builder()
        .baseUrl("http://mockapi.eolinker.com/9IiwI82f58c23411240ed608ceca204b2f185014507cbe3/")
        .addConverterFactory(GsonConverterFactory.create())
        .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
        .build()
    val service = retrofit.create(ApiService::class.java)
    val call: Observable = service.getUserData3()
    call.subscribe({ user ->
        println("userBean: $user")
    }, { t ->
        println("onFailure: $t")
    })
}

一、Retrofit请求过程 

1、Retrofit.create()

 @SuppressWarnings("unchecked") // Single-interface proxy creation guarded by parameter safety.
  public  T create(final Class 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;
  }

2、ServiceMethod 

abstract class ServiceMethod<T> {
  static  ServiceMethod 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);
}

 3、HttpServiceMethod

abstract class HttpServiceMethod<ResponseT, ReturnT> extends ServiceMethod<ReturnT> {
 
 
static  HttpServiceMethod parseAnnotations(
      Retrofit retrofit, Method method, RequestFactory requestFactory) {
    boolean isKotlinSuspendFunction = requestFactory.isKotlinSuspendFunction;
    boolean continuationWantsResponse = false;
    boolean continuationBodyNullable = false;
 
    Annotation[] annotations = method.getAnnotations();
    Type adapterType;
    if (isKotlinSuspendFunction) {
      Type[] parameterTypes = method.getGenericParameterTypes();
      Type responseType =
          Utils.getParameterLowerBound(
              0, (ParameterizedType) parameterTypes[parameterTypes.length - 1]);
      if (getRawType(responseType) == Response.class && responseType instanceof ParameterizedType) {
        // Unwrap the actual body type from Response.
        responseType = Utils.getParameterUpperBound(0, (ParameterizedType) responseType);
        continuationWantsResponse = true;
      } else {
        // TODO figure out if type is nullable or not
        // Metadata metadata = method.getDeclaringClass().getAnnotation(Metadata.class)
        // Find the entry for method
        // Determine if return type is nullable or not
      }
 
      adapterType = new Utils.ParameterizedTypeImpl(null, Call.class, responseType);
      annotations = SkipCallbackExecutorImpl.ensurePresent(annotations);
    } else {
      adapterType = method.getGenericReturnType();
    }
 
    CallAdapter callAdapter =
        createCallAdapter(retrofit, method, adapterType, annotations);
    Type responseType = callAdapter.responseType();
    if (responseType == okhttp3.Response.class) {
      throw methodError(
          method,
          "'"
              + getRawType(responseType).getName()
              + "' is not a valid response body type. Did you mean ResponseBody?");
    }
    if (responseType == Response.class) {
      throw methodError(method, "Response must include generic type (e.g., Response)");
    }
    // TODO support Unit for Kotlin?
    if (requestFactory.httpMethod.equals("HEAD") && !Void.class.equals(responseType)) {
      throw methodError(method, "HEAD method must use Void as response type.");
    }
 
    Converter responseConverter =
        createResponseConverter(retrofit, method, responseType);
 
    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)
          new SuspendForResponse<>(
              requestFactory,
              callFactory,
              responseConverter,
              (CallAdapter>) callAdapter);
    } else {
      //noinspection unchecked Kotlin compiler guarantees ReturnT to be Object.
      return (HttpServiceMethod)
          new SuspendForBody<>(
              requestFactory,
              callFactory,
              responseConverter,
              (CallAdapter>) callAdapter,
              continuationBodyNullable);
    }
  }
 
 
  @Override
  final @Nullable ReturnT invoke(Object[] args) {
    Call call = new OkHttpCall<>(requestFactory, args, callFactory, responseConverter);
    return adapt(call, args);
  }
}

 4、OkHttpCall

OkHttpCall 是实际发起 okHttp 请求的地方。当我们调用 fun getUserData(): Call 方法时，返回的 Call 对象实际上是 OkHttp 类型。

final class OkHttpCall<T> implements Call<T> {
  private final RequestFactory requestFactory;
  private final Object[] args;
  private final okhttp3.Call.Factory callFactory;
  private final Converter responseConverter;
 
  private volatile boolean canceled;
 
  @GuardedBy("this")
  private @Nullable okhttp3.Call rawCall;
 
  @GuardedBy("this") // Either a RuntimeException, non-fatal Error, or IOException.
  private @Nullable Throwable creationFailure;
 
  @GuardedBy("this")
  private boolean executed;
 
  OkHttpCall(
      RequestFactory requestFactory,
      Object[] args,
      okhttp3.Call.Factory callFactory,
      Converter responseConverter) {
    this.requestFactory = requestFactory;
    this.args = args;
    this.callFactory = callFactory;
    this.responseConverter = responseConverter;
  }
 
  @SuppressWarnings("CloneDoesntCallSuperClone") // We are a final type & this saves clearing state.
  @Override
  public OkHttpCall clone() {
    return new OkHttpCall<>(requestFactory, args, callFactory, responseConverter);
  }
 
  @Override
  public synchronized Request request() {
    try {
      return getRawCall().request();
    } catch (IOException e) {
      throw new RuntimeException("Unable to create request.", e);
    }
  }
 
  @Override
  public synchronized Timeout timeout() {
    try {
      return getRawCall().timeout();
    } catch (IOException e) {
      throw new RuntimeException("Unable to create call.", e);
    }
  }
 
  /**
   * Returns the raw call, initializing it if necessary. Throws if initializing the raw call throws,
   * or has thrown in previous attempts to create it.
   */
  @GuardedBy("this")
  private okhttp3.Call getRawCall() throws IOException {
    okhttp3.Call call = rawCall;
    if (call != null) return call;
 
    // Re-throw previous failures if this isn't the first attempt.
    if (creationFailure != null) {
      if (creationFailure instanceof IOException) {
        throw (IOException) creationFailure;
      } else if (creationFailure instanceof RuntimeException) {
        throw (RuntimeException) creationFailure;
      } else {
        throw (Error) creationFailure;
      }
    }
 
    // Create and remember either the success or the failure.
    try {
      return rawCall = createRawCall();
    } catch (RuntimeException | Error | IOException e) {
      throwIfFatal(e); // Do not assign a fatal error to creationFailure.
      creationFailure = e;
      throw e;
    }
  }
 
  @Override
  public void enqueue(final Callback 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 {
          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 response;
            try {
              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
            }
          }
        });
  }
 
  @Override
  public synchronized boolean isExecuted() {
    return executed;
  }
 
  @Override
  public Response execute() throws IOException {
    okhttp3.Call call;
 
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already executed.");
      executed = true;
 
      call = getRawCall();
    }
 
    if (canceled) {
      call.cancel();
    }
 
    return parseResponse(call.execute());
  }
}

 5、RequestFactory

final class RequestFactory {
  static RequestFactory parseAnnotations(Retrofit retrofit, Method method) {
    return new Builder(retrofit, method).build();
  }
 
  private final Method method;
  private final HttpUrl baseUrl;
  final String httpMethod;
  private final @Nullable String relativeUrl;
  private final @Nullable Headers headers;
  private final @Nullable MediaType contentType;
  private final boolean hasBody;
  private final boolean isFormEncoded;
  private final boolean isMultipart;
  private final ParameterHandler[] parameterHandlers;
  final boolean isKotlinSuspendFunction;
 
  RequestFactory(Builder builder) {
    method = builder.method;
    baseUrl = builder.retrofit.baseUrl;
    httpMethod = builder.httpMethod;
    relativeUrl = builder.relativeUrl;
    headers = builder.headers;
    contentType = builder.contentType;
    hasBody = builder.hasBody;
    isFormEncoded = builder.isFormEncoded;
    isMultipart = builder.isMultipart;
    parameterHandlers = builder.parameterHandlers;
    isKotlinSuspendFunction = builder.isKotlinSuspendFunction;
  }
 
  okhttp3.Request create(Object[] args) throws IOException {
    @SuppressWarnings("unchecked") // It is an error to invoke a method with the wrong arg types.
    ParameterHandler