Android开发中用Kotlin编写LiveData组件教程

class MainViewModel(countReserved:Int) : ViewModel() {
/*当外部调用counter变量时，实际上获得的就是_counter的实例，但是无法给counter设置数据，从而保证了ViewModel的数据的封装性。*/
    val counter:LiveData<Int>
        get()=_counter
    private val _counter = MutableLiveData<Int>()
    init{
        _counter.value=countReserved
    }
    fun plusOne() {
        val count =  _counter.value ?: 0
         _counter.value = count + 1
    }
    fun clear() {
         _counter.value = 0
    }
}

class MainActivity : AppCompatActivity() {
    …
    override fun onCreate(savedInstanceState: Bundle?) {
        …
        plusOneBtn.setOnClickListener {
            viewModel.plusOne()
        }
        clearBtn.setOnClickListener {
            viewModel.clear()
        }
        viewModel.counter.observe(this, Observer { count ->
            infoText.text = count.toString() // 将最新数据更新到界面上
        })
    }
}

data class User(var firstName:String,var lastName:String,var age:Int)

class MainViewModel(countReserved:Int) : ViewModel() {
   private val userLiveData = MutableLiveData<User>()
   val userName:LiveData<String>=Transformations.map(userLiveData){user->
   "${user.firstName} ${user.lastName}"
   }
}

object Repository{
   fun getUser(userId:String):LiveData<User>{
      val liveData=MutableLiveData<User>()
      liveData.value=User(userId,userId,0)
      return liveData
   }
}

class MainViewModel(countReserved:Int) : ViewModel() {
   private val userLiveData = MutableLiveData<User>()
   val user:LiveData<User>=Transformations.SwitchMap(userIdLiveData){userId->
    Repository.getUser(userId)
   }
   fun getUser(userId:String){
     userIdLiveData.value=userId
   }
}