关于Observable的调用流程探究，自解惑.

    # gradle引用
    implementation 'io.reactivex.rxjava2:rxandroid:2.1.1'
    implementation 'io.reactivex.rxjava2:rxjava:2.2.7'
    implementation 'com.squareup.retrofit2:retrofit:2.5.0'
    implementation 'com.squareup.okhttp3:okhttp:3.14.0'
    implementation 'com.squareup.okio:okio:2.2.2'
    implementation 'com.squareup.retrofit2:converter-gson:2.5.0'
    implementation 'com.squareup.okhttp3:logging-interceptor:3.14.0'
    implementation 'com.squareup.retrofit2:adapter-rxjava2:2.5.0'

# 一般流程如下
mOkHttpClient = new OkHttpClient.Builder()
        .addNetworkInterceptor(
                new Interceptor() {
                    @Override
                    public Response intercept(Chain chain) throws IOException {
                        Request request = chain.request();

                        //在这里获取到request后就可以做任何事情了
                        Response response = chain.proceed(request);
                        Log.d(TAG, "intercept: url "+request.url().toString());
                        return response;
                    }
                }
        ).build();

mRetrofit = new Retrofit.Builder()
        .client(mOkHttpClient)
        .addConverterFactory(GsonConverterFactory.create())
        .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
        .baseUrl(Constant.API_BASE_URL)
        .build();

Single<> xx = mRetrofit.create(targetClass.class)
xx.scribeOn(io())
. ...
.subscribe();

# Json解析代码器 GsonConverterFactory.create()
# 转换为Observable及真正执行的地方 GsonConverterFactory.create()

Retrofit.create(targetClass.class)的时候会生成代理，其内部逻辑如下:

# 重点部分是serviceMethod的生成及adapt()的最终调用
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);
            }
            ServiceMethod<Object, Object> serviceMethod =
                (ServiceMethod<Object, Object>) loadServiceMethod(method);
            OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);
            return serviceMethod.callAdapter.adapt(okHttpCall);
          }
        })

# serviceMethod生成的地方.
callAdapter生成位置是ServcieMethod.createCallAdapter(), 里面调用Retrofit.nextCallAdapter(),
最终根据注解及方法名获取对应的CallAdapter, adapterFactories.get(i).get(xxx)：其实现为RxJava2CallAdapterFactory.java,函数返回值为new RxJava2CallAdapter(...);
其中的adapt(Call)返回的是目标Observable. （此处是:CallExecuteObservable）
Observable又在subscribe的时候调用subscribeActual, 最终调用observer.onNext/Complete/Error()