implementation 'com.android.support:appcompat-v7:27.1.0'
implementation 'com.android.support.constraint:constraint-layout:1.0.2'

implementation 'io.reactivex.rxjava2:rxandroid:2.0.2'
implementation 'io.reactivex.rxjava2:rxjava:2.1.10'

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  tools:context="com.haocent.android.countdown.MainActivity">

  <TextView
    android:id="@+id/tv_count_down"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textSize="16sp"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    tools:text="Hello World!" />

</android.support.constraint.ConstraintLayout>

public class MainActivity extends AppCompatActivity {

  private static final String TAG = MainActivity.class.getSimpleName();

  private Disposable mDisposable;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main_activity);

    final TextView tvCountDown = findViewById(R.id.tv_count_down);

    // 倒計(jì)時(shí) 10s
    mDisposable = Flowable.intervalRange(0, 11, 0, 1, TimeUnit.SECONDS)
        .observeOn(AndroidSchedulers.mainThread())
        .doOnNext(new Consumer<Long>() {
          @Override
          public void accept(Long aLong) throws Exception {
            Log.d(TAG, "倒計(jì)時(shí)");

            tvCountDown.setText("倒計(jì)時(shí) " + String.valueOf(10 - aLong) + " 秒");
          }
        })
        .doOnComplete(new Action() {
          @Override
          public void run() throws Exception {
            Log.d(TAG, "倒計(jì)時(shí)完畢");

            Toast.makeText(MainActivity.this, "倒計(jì)時(shí)完畢", Toast.LENGTH_SHORT).show();
          }
        })
        .subscribe();
  }

  @Override
  protected void onDestroy() {
    super.onDestroy();

    if (mDisposable != null) {
      mDisposable.dispose();
    }
  }
}