LiveData : Transformations.map and Transformations.switchMap usage

项目背景 : 某个int值,在APage needs to be obtained it showsint数值,在BPage need toint值转换为String字符串

1. Transformations.map

基于此背景,This article introduceTransformations.map和Transformations.switchMap,数据源A监听数据源B的内容,Changes will beB
The content of the other contents into corresponding and notice toAData source of the observer.

案例

class CarViewModel : ViewModel() {
    
	// 数据源 B
    private var initialData = MutableLiveData<Int>()

    /** * @describe 数据源A */
    private var transformLiveData = Transformations.map(getInitialData()) {
    
        when (it) {
    
            0 -> "on"
            else -> "off"
        }
    }

    fun getInitialData(): MutableLiveData<Int> {
    
        return initialData
    }
}

2.Transformations.switchMap

数据源A监听数据源B的内容的变化, Change fromBContent to obtain the corresponding data sourceC, 添加到A的监听列表
即 A 同时监听B和C ,BThe change of the content will only(switch)更换AListen to the list ofC,CThe change of the content will informA监听器

class CarViewModel : ViewModel() {
    
    
    // 数据源B
    private val userLivedata = MutableLiveData<User>()
    
    // 数据源C1
    private val cFirstLiveData = MutableLiveData<Int>()
	
	// 数据源C2
    private val cSecondLiveData = MutableLiveData<Int>()

    /** * 数据源A */
    val mapLiveData = Transformations.switchMap(userLivedata) {
    
        when (it.name) {
    
            "test" -> cFirstLiveData
            else -> cSecondLiveData
        }
    }

    data class User(val name: String = "test", var age: Int = 18)

}

3.MediatorLiveData

合并两个LiveData

     var count = 0
     
    // A 数据
    private val liveData1 = MutableLiveData<String>()
    // B 数据
    private val liveData2 = MutableLiveData<Int>()
    val liveCombind = MediatorLiveData<String>()
    init {
    
        liveCombind.addSource(liveData1) {
    
            addData()
        }
        liveCombind.addSource(liveData2) {
    
            addData()
        }
    }
    private fun  addData() {
    
        count++
        if (count == 10) {
    
            count = 0
            //TODO
        }
    }