Android开发(Kotlin) ViewModel基本用法示例详解

ViewModel基本用法

创建viewmodel

class MyViewModel: ViewModel() {
}

实现一个计数器

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

在activity使用ViewModel，获取ViewModel的实例需要通过ViewModelProvider。点击按钮一次计数器加1

  override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        enableEdgeToEdge()
        binding = ActivityClickBinding.inflate(layoutInflater)
        setContentView(binding.root)
        viewModel = ViewModelProvider(this).get(MyViewModel::class.java)
        binding.clickButC.setOnClickListener {
            viewModel.count++
            refreshCount()
        }
        refreshCount()
    }
    private fun refreshCount() {
        binding.clickTvText.text = viewModel.count.toString()
    }

旋转屏幕后，计数器的数据仍然存在

向ViewModel传递参数

改进上面的例子，保证退出程序后，重新打开，数据任然存在。
修改viewmodel

class MyViewModel(counter: Int) : ViewModel() {
    var count = counter
}

counter记录之前保存的值
创建ViewModelProvider.Factory ，向MyViewModel的构造函数传递数据。添加构造函数，重写create方法，

class MyViewModelFactory(private val counter: Int) : ViewModelProvider.Factory {
    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        return MyViewModel(counter) as T
    }
}

修改activity

class ClickActivity : ComponentActivity() {
    lateinit var viewModel: MyViewModel
    lateinit var binding:ActivityClickBinding
    lateinit var sp: SharedPreferences
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        enableEdgeToEdge()
        binding = ActivityClickBinding.inflate(layoutInflater)
        setContentView(binding.root)
        sp = getSharedPreferences("data",MODE_PRIVATE)
        val countSp = sp.getInt("count",0)
        viewModel = ViewModelProvider(this, MyViewModelFactory(countSp)).get(MyViewModel::class.java)
        binding.clickButC.setOnClickListener {
            viewModel.count++
            refreshCount()
        }
        refreshCount()
    }
    private fun refreshCount() {
        binding.clickTvText.text = viewModel.count.toString()
    }
    override fun onPause() {
        super.onPause()
        sp.edit{
            putInt("count",viewModel.count)
        }
    }
}