public class UserActivity extends AppCompatActivity {
    // 数据提供
    private UserViewModel mViewModel;
    // 管理subscriptions订阅的释放
    private final CompositeDisposable mDisposable = new CompositeDisposable();

    @Override
    protected void onStart() {
        super.onStart();
        // 发起网络请求
        mDisposable.add(mViewModel.getUserName()
                .subscribeOn(Schedulers.io()) // 请求在IO线程运行
                .observeOn(AndroidSchedulers.mainThread()) // 回调到Android主线程
                .subscribe(userName -> mUserName.setText(userName), // 回调监听
                        throwable -> Log.e(TAG, "Unable to update username", throwable)));
    }

    @Override
    protected void onStop() {
        super.onStop();
        // 释放请求
        mDisposable.clear();
    }
}