viewModels

ViewModel

class MyViewModel : ViewModel() {
    var number = 0
}

MainActivity

class MainActivity : AppCompatActivity() {

    private val myViewModel by viewModels<MyViewModel>()

    var mBinding: ActivityMainBinding? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        mBinding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        mBinding!!.tv.text = myViewModel.number.toString()
        mBinding!!.btn.setOnClickListener {
            myViewModel.number++
            mBinding!!.tv.text = myViewModel.number.toString()
        }
    }
}

image.png image.png

页面重建时数据不会丢失

ViewModelSavedState

const val NUMBER_KEY = "number_key"

class MyViewModel(private val state:SavedStateHandle) : ViewModel() {
    var number:Int
        get() {
            return state.get<Int>(NUMBER_KEY)?:0
        }
        set(value) {
            state[NUMBER_KEY] = value
        }
}

MainActivity

class MainActivity : AppCompatActivity() {

    private val myViewModel by lazy {
        ViewModelProvider(this, SavedStateViewModelFactory(application, this)).get(MyViewModel::class.java)
    }

    var mBinding: ActivityMainBinding? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        mBinding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        mBinding!!.tv.text = myViewModel.number.toString()
        mBinding!!.btn.setOnClickListener {
            myViewModel.number++
            mBinding!!.tv.text = myViewModel.number.toString()
        }
    }
}

再次运行程序，数据就不会丢失了。

需要注意的是，这里的数据也不是永久保存的，当手机重启或者用户手动杀掉进程后，数据仍然会丢失。