JetPack -- ViewModel状态保存

简介

• 一个Activity对应一个ViewModel，当Activity进入后台，屏幕翻转，语言切换等Activity会重建但是ViewModel还存活着，所以数据并不会丢失，当进程被系统杀死，Activity被回收，那么ViewModel中的数据就会丢失，这时候就需要ViewModelStore

• 

使用

class MyViewModel(handle: SavedStateHandle) : ViewModel() {
    private var handle: SavedStateHandle

    init {
        this.handle = handle
    }

    fun getLikedNumber(): MutableLiveData<Int> {
        if (!handle.contains(MainActivity.KEY)) {
            handle.set(MainActivity.KEY, 0)
        }
        return handle.getLiveData(MainActivity.KEY)
    }

    fun setLikedNumber(value: Int) {
        handle.set(MainActivity.KEY, getLikedNumber().value!! + 1)
    }
}

• Activity处理

class MainActivity : AppCompatActivity() {
    private lateinit var mViewModel: MyViewModel
    private lateinit var activityMainBinding: ActivityMainBinding
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        // 将视图绑定
        activityMainBinding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        // vm和控制器绑定
        mViewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)
        if (savedInstanceState != null) {
            // 之前的写法
            mViewModel.setLikedNumber(savedInstanceState.getInt(KEY))
            // 现在的写法
            mViewModel.setLikedNumber(mViewModel.getLikedNumber().value!!)
        }
        // 取回数据, this LifeCycleOwner
        activityMainBinding.setWjx(mViewModel)
        // 进行数据观察
        activityMainBinding.setLifecycleOwner(this)
    }

    /**
     * 之前的写法
     */
    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        outState.putInt(KEY, mViewModel.getLikedNumber().value!!)
    }

    companion object {
        const val KEY : String = "KEY";
    }
}