Retrofit2线程切换

一、retrofit使用

interface RequestService {
  @GET
  operator fun get(@Url url: String,@QueryMap params: HashMap<String, @JvmSuppressWildcards Any>): Call<String>
}

val client = Retrofit.Builder().apply {
   baseUrl("xxx")
   client(OkHttpHolder.okHttpClient)
}.build()

val service: RequestService = client.create(RequestService::class.java)
val call: Call<String> = service.get("xxx",HashMap<String, String>())
call.enqueue(object :Callback<String>{
              override fun onResponse(call: Call<String>, response: Response<String>) {
              }
              override fun onFailure(call: Call<String>, t: Throwable) {
              }
          })

二、DefaultCallAdapterFactory.ExecutorCallbackCall

service.get("xxx",HashMap<String, String>())

通过源码可知，在调用get时，通过代理，会得到DefaultCallAdapterFactory.ExecutorCallbackCall，而ExecutorCallbackCall就是接口Call的实现类，那么，call.enqueue()时，将会执行ExecutorCallbackCall的enqueue方法，看下ExecutorCallbackCall的enqueue(final Callback<T> callback)源码：

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

      delegate.enqueue(
          new Callback<T>() {
            @Override
            public void onResponse(Call<T> call, final Response<T> response) {
              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));
            }
          });
    }

显然，线程就是通过callbackExecutor.execute()切换的，看下callbackExecutor怎么来的。
通过代码追踪，发现，callbackExecutor来自Retrofit中的build()，而当我们创建Retrofit的时候，就调用了这个方法。

三、Retrofit.build()

public final class Retrofit {
    public Retrofit build() {
        //这里仅贴出获取callbackExecutor的代码
        ...
        Executor callbackExecutor = this.callbackExecutor;
        if (callbackExecutor == null) {
           callbackExecutor = platform.defaultCallbackExecutor();
        }
        ...
     }

    public static final class Builder {
         public Builder() {
            this(Platform.get());
         }
    }
}
class Platform {
  private static final Platform PLATFORM = findPlatform();

  static Platform get() {
    return PLATFORM;
  }

  private static Platform findPlatform() {
    return "Dalvik".equals(System.getProperty("java.vm.name"))
        ? new Android() //
        : new Platform(true);
  }

   static final class Android extends Platform {
       Android() {
         super(Build.VERSION.SDK_INT >= 24);
       }

      @Override
       public Executor defaultCallbackExecutor() {
          return new MainThreadExecutor();
       }
   }

  static final class MainThreadExecutor implements Executor {
      private final Handler handler = new Handler(Looper.getMainLooper());

      @Override
      public void execute(Runnable r) {
        handler.post(r);
      }
    }
}

platform是通过Platform.get()获取的，System.getProperty("java.vm.name")获取的是java VM的名称，显然，Android系统下将返回Android(),Android是Platform实现类，当调用latform.defaultCallbackExecutor()时，将返回一个MainThreadExecutor对象，而，当执行MainThreadExecutor.execute的时候，通过new Handler(Looper.getMainLooper())讲子线程切到了UI线程。

四、如何让Retrofit请求结束后的回调中还在子线程

   val client = Retrofit.Builder().apply {
                baseUrl("")
                callbackExecutor(ThreadExecutor())
                addConverterFactory(ScalarsConverterFactory.create())
                client(OkHttpHolder.okHttpClient)
            }.build()
    class ThreadExecutor : Executor {
        override fun execute(r: Runnable) {
            r.run()
        }
    }

给Retrofit重新创造一个Executor就行了。