Android Jetpack 之使用 ViewModel

ViewModel 是 Jetpack MVVM 架構中的 VM 部分。ViewModel 中的資料不會因為 Activity 重建而被銷毀。

以下的 ViewModel 儲存了開始時間戳。

public class ChronometerViewModel extends ViewModel {

    @Nullable
    private Long mStartTime;

    @Nullable
    public Long getStartTime() {
        return mStartTime;
    }

    public void setStartTime(final long startTime) {
        this.mStartTime = startTime;
    }
}
           

在 ChronoActivity2 使用 chronometer 計時器顯示倒計時。當螢幕旋轉時，Activity 重走 onCreate。但是 chronometerViewModel 的 startTime 依然存在，旋轉後會繼續之前的倒計時。

public class ChronoActivity2 extends AppCompatActivity {

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

        // The ViewModelStore provides a new ViewModel or one previously created.
        ChronometerViewModel chronometerViewModel
                = new ViewModelProvider(this).get(ChronometerViewModel.class);

        // Get the chronometer reference
        Chronometer chronometer = findViewById(R.id.chronometer);

        if (chronometerViewModel.getStartTime() == null) {
            // If the start date is not defined, it's a new ViewModel so set it.
            long startTime = SystemClock.elapsedRealtime();
            chronometerViewModel.setStartTime(startTime);
            chronometer.setBase(startTime);
        } else {
            // Otherwise the ViewModel has been retained, set the chronometer's base to the original
            // starting time.
            chronometer.setBase(chronometerViewModel.getStartTime());
        }

        chronometer.start();
    }
}