错误及分析

异常如下：

java.lang.IllegalArgumentException: Unable to create call adapter for io.reactivex.Observable<me.fallblank.weiworld.bean.ContentResponse>
    for method IWeiboContent.listLastWeibo
    at retrofit2.ServiceMethod$Builder.methodError(ServiceMethod.java:751)
    at retrofit2.ServiceMethod$Builder.createCallAdapter(ServiceMethod.java:236)
    at retrofit2.ServiceMethod$Builder.build(ServiceMethod.java:161)
    at retrofit2.Retrofit.loadServiceMethod(Retrofit.java:169)
    at retrofit2.Retrofit$1.invoke(Retrofit.java:146)
    at com.sun.proxy.$Proxy5.listLastWeibo(Unknown Source)
    ...

问题原因：<br />
retrofit没有默认提供给RxJava 2.x.y的适配器，无法把call转换为相应的Observable，但这在RxJava1中却是可以的。出现这个错误是升级了Rxjava却没有配置retrofit的adapter导致。<br />

修正

引入相应依赖：

compile 'com.squareup.retrofit2:adapter-rxjava2:2.2.0'
//也可以用Jackwharton写的adapter

添加adapter到retrofit：

Retrofit mRetrofit = new Retrofit.Builder()
            ...
            .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
            ...
            .build();

至此，问题修复！