Android ViewModel与LiveData组件组合使用详解

/**
 * @Author: ly
 * @Date: 2022/9/13
 * @Description: 继承ViewModel类，将视图与数据分离
 */
public class TimerViewModel extends ViewModel {
    private Timer timer;
    private int currentSecond;
    /**
     * 开始计时
     */
    public void startTiming() {
        if (timer == null) {
            currentSecond = 0;
            timer = new Timer();
            TimerTask timerTask = new TimerTask() {
                @Override
                public void run() {
                    currentSecond++;
                    if (onTimeChangeListener != null) {
                        onTimeChangeListener.onTimeChanged(currentSecond);
                    }
                }
            };
            timer.schedule(timerTask, 1000, 1000);
        }
    }
    /**
     * 通过接口的方式，完成对调用者的通知
     * 实际上这种方式不是很好，更好的方式是通过LiveData组件来实现
     */
    public interface OnTimeChangeListener {
        void onTimeChanged(int second);
    }
    private OnTimeChangeListener onTimeChangeListener;
    public void setOnTimeChangeListener(OnTimeChangeListener onTimeChangeListener) {
        this.onTimeChangeListener = onTimeChangeListener;
    }
    @Override
    protected void onCleared() {
        super.onCleared();
        timer.cancel();
    }
}

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        iniComponent();
    }
    private void iniComponent() {
        final TextView textView = findViewById(R.id.textView);
        TimerViewModel timerViewModel = new ViewModelProvider(this).get(TimerViewModel.class);
        timerViewModel.setOnTimeChangeListener(new TimerViewModel.OnTimeChangeListener() {
            @Override
            public void onTimeChanged(int second) {
                //更新UI界面
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        textView.setText(String.valueOf(second));
                    }
                });
            }
        });
        timerViewModel.startTiming();
    }
}

/**
 * @Author: ly
 * @Date: 2022/9/1
 * @Description: LiveData是一个抽象类，不能直接使用，我们使用它的直接子类MutableLiveData
 */
public class TimerWithLiveDataViewModel extends ViewModel {
    //将currentSecond这个字段用MutableLiveData包装起来
    private MutableLiveData<Integer> currentSecond;
    private Timer timer;
    private int second = 0;
    public LiveData<Integer> getCurrentSecond() {
        if (currentSecond == null) {
            currentSecond = new MutableLiveData<>();
        }
        return currentSecond;
    }
    /**
     * 开始计时
     */
    public void startTiming() {
        if (timer == null) {
            timer = new Timer();
            second = 0;
            TimerTask timerTask = new TimerTask() {
                @Override
                public void run() {
                    currentSecond.postValue(second++);
                }
            };
            timer.schedule(timerTask, 1000, 1000);
        }
    }
    @Override
    protected void onCleared() {
        super.onCleared();
        timer.cancel();
    }
}

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        iniComponent();
    }
        final TextView textView = findViewById(R.id.textView);
        final Button btn = findViewById(R.id.btn);
        TimerWithLiveDataViewModel timerWithLiveDataViewModel = new ViewModelProvider(this).get(TimerWithLiveDataViewModel.class);
        //得到ViewModel中的LiveData
        final MutableLiveData<Integer> liveData = (MutableLiveData<Integer>) timerWithLiveDataViewModel.getCurrentSecond();
        //通过LiveData.observe()观察ViewModel中数据的变化
        liveData.observe(this, new Observer<Integer>() {
            @Override
            public void onChanged(Integer integer) {
                //收到回调更新UI界面
                textView.setText("Time:" + integer);
            }
        });
        //重置计时器
        btn.setOnClickListener((v) -> {
            //完成对ViewModel中数据的更新
            //setValue(T) 必须在主线程中调用 ,
            // 而 postValue(T) 既可以在主线程中调用, 也可以在子线程中调用
            liveData.setValue(0);
        });
        //计时开始
        timerWithLiveDataViewModel.startTiming();
    }
}