1,什么是viewmodel

1),在日常开发中,一般比较简单的页面都会全部写在activity或fragment中,稍微复杂点的都会用mvp来写.viewmodel从字面上看的工作就是维护view和model之间的数据交换,我愚昧的觉得就是类似mvp的p层.google那边给出的介绍说viewmodel就是view和model之间的桥梁.

2,viewmodel两大特性

1,数据操作不受activity或fragment生命周期影响,并activity销毁时系统回调viewmodel的onCleared()方法
2,可以容易在activity和fragment之间通信
1),接入viewmodel

   implementation 'androidx.lifecycle:lifecycle-viewmodel:2.2.0'

2),相关代码
a,创建继承viewmodel的子类CounterViewModel

public class CounterViewModel extends ViewModel {
    /***
     * 对应的activity销毁时系统会调用此方法
     */
    @Override
    protected void onCleared() {
        super.onCleared();
    }
}

继续补全代码:

public class CounterViewModel extends ViewModel {
    private Timer timer;
    private int count=60;
    private onCountChangeListenner mOnCountChangeListenner;

    public void start(){
        Log.i("viewmodel_tag","开始计时");
        timer=new Timer();
        timer.schedule(new TimerTask() {
            @Override
            public void run() {
                count--;
                if (mOnCountChangeListenner!=null){
                    mOnCountChangeListenner.countChange(count);
                }
                if (count<=0){
                    timer.cancel();
                }
            }
        },1000,1000);
    }
    public interface onCountChangeListenner{
        void countChange(int count);
    }

    public void setOnCountChangeListenner(onCountChangeListenner mOnCountChangeListenner) {
        this.mOnCountChangeListenner = mOnCountChangeListenner;
    }

    /***
     * 对应的activity销毁时系统会调用此方法
     */
    @Override
    protected void onCleared() {
        super.onCleared();
        Log.i("viewmodel_tag","onCleared");
        timer.cancel();
        mOnCountChangeListenner=null;
    }
}

CounterViewModel 中有个start方法启动计时器开始计时,并通过onCountChangeListenner将计时回调,在onCleared时取消timer
b,在activity中调用

public class ViewModelActivity extends AppCompatActivity {
 private Button btn_start;
 private TextView tv_show;
 private CounterViewModel counterViewModel;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_view_model);
        btn_start=(Button)findViewById(R.id.btn_start);
        tv_show=(TextView) findViewById(R.id.tv_show);
        initViewModel();
    }

    private void initViewModel() {
        counterViewModel=new ViewModelProvider(this).get(CounterViewModel.class);
        counterViewModel.setOnCountChangeListenner(new CounterViewModel.onCountChangeListenner() {
            @Override
            public void countChange(int count) {
                Log.i("viewmodel_tag","countChange :"+count);
                tv_show.setText("剩余"+count+"秒");
            }
        });
        btn_start.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                counterViewModel.start();
            }
        });
    }

    @Override
    public void onConfigurationChanged(@NonNull Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        Log.i("viewmodel_tag","onConfigurationChanged");
    }

    @Override
    public void onBackPressed() {
        super.onBackPressed();
        Log.i("viewmodel_tag","放回键退出应用");
    }
}

通过一句代码,便可创建实例,很方便

counterViewModel=new ViewModelProvider(this).get(CounterViewModel.class);

设置监听后并点击btn_start按钮开始计时,期间会旋转屏幕,观察日志打印

image.png

很明显,屏幕旋转并不影响计时操作,生命周期重走没有让counterViewModel重新实例化,重新start计时.最后双击返回键退出应用也走了onCleared()函数