Jetpack -- ViewModel狀態保存

簡介

• Activity對應的ViewModel，當Activity進入後臺，屏幕翻轉，語言切換等Activity會重建但是ViewModel還存活着，所以數據並不會丟失，當進程被系統殺死，Activity被回收，那麼ViewModel中的數據就會丟失，這時候就需要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處理

class MainActivity : AppCompatActivity() {
    private lateinit var mViewModel: MyViewModel
    private lateinit var activityMainBinding: ActivityMainBinding
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        // 將視圖綁定
        activityMainBinding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        // vm和控制器綁定
        mViewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)
        if (savedInstanceState != null) {
            // 之前的寫法
            mViewModel.setLikedNumber(savedInstanceState.getInt(KEY))
            // 現在的寫法
            mViewModel.setLikedNumber(mViewModel.getLikedNumber().value!!)
        }
        // 取回數據, this LifeCycleOwner
        activityMainBinding.setWjx(mViewModel)
        // 進行數據觀察
        activityMainBinding.setLifecycleOwner(this)
    }

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

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