CompletableObserver简单的用法跟踪一个任务的结束，不处理复杂的数据传导，例如：

package zhangphil.app;

import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;

import java.util.concurrent.Callable;

import io.reactivex.Completable;
import io.reactivex.CompletableObserver;
import io.reactivex.android.schedulers.AndroidSchedulers;
import io.reactivex.disposables.Disposable;
import io.reactivex.schedulers.Schedulers;

public class MainActivity extends AppCompatActivity {
    private final String TAG = getClass().getSimpleName();

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        test();
    }

    private void test() {
        Completable.fromCallable(new Callable<Object>() {
            @Override
            public Object call() throws Exception {
                try {
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                Log.d(TAG, "任务结束！");

                return null;
            }
        }).subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(getCompletableObserver());
    }

    private CompletableObserver getCompletableObserver() {
        return new CompletableObserver() {
            @Override
            public void onSubscribe(Disposable d) {
                Log.d(TAG, "onSubscribe " + System.currentTimeMillis());
            }

            @Override
            public void onComplete() {
                Log.d(TAG, "onComplete " + System.currentTimeMillis());
            }

            @Override
            public void onError(Throwable e) {

            }
        };
    }
}

代码运行输出：

05-16 14:01:20.908 12292-12292/zhangphil.app D/MainActivity: onSubscribe 1494914480908
05-16 14:01:20.945 12292-12292/zhangphil.app D/InputTransport: Input channel constructed: fd=73
05-16 14:01:20.945 12292-12292/zhangphil.app D/ViewRootImpl@99677a6[MainActivity]: setView = DecorView@de9b5e7[MainActivity] touchMode=true
05-16 14:01:20.949 12292-12292/zhangphil.app W/Activity: AppLock checkAppLockState locked:false verifying:false pkgName = zhangphil.app isInMultiWindowMode:false
05-16 14:01:20.985 12292-12292/zhangphil.app D/ViewRootImpl@99677a6[MainActivity]: MSG_RESIZED_REPORT: ci=Rect(0, 81 - 0, 0) vi=Rect(0, 81 - 0, 0) or=1
05-16 14:01:20.986 12292-12292/zhangphil.app D/ViewRootImpl@99677a6[MainActivity]: MSG_WINDOW_FOCUS_CHANGED 1
05-16 14:01:20.986 12292-12292/zhangphil.app I/InputMethodManager: [IMM] startInputInner - mService.startInputOrWindowGainedFocus
05-16 14:01:20.988 12292-12292/zhangphil.app D/InputTransport: Input channel constructed: fd=74
05-16 14:01:25.911 12292-23777/zhangphil.app D/MainActivity: 任务结束！
05-16 14:01:25.912 12292-12292/zhangphil.app D/MainActivity: onComplete 1494914485912