[移动开发]Extension method - 一对多依赖 - MediatorLiveData

一对多依赖 —— MediatorLiveData
MediatorLiveData 允许您将一个或多个数据源添加到单个 LiveData 可观察对象。

示例：观察两个LiveData同时为 true 时, 执行相应的操作：

postLiveDataExample(liveData1, liveData2).observe(this, Observer {
    if (it){
        // TODO
    }
})

fun postLiveDataExample(
    liveData1: LiveData<Boolean>,
    liveData2: LiveData<Boolean>
): LiveData<Boolean> {
    val result = MediatorLiveData<Boolean>()

    result.addSource(liveData1) { value ->
        result.value = combineLatestData(liveData1, liveData2)
    }
    result.addSource(liveData2) { value ->
        result.value = combineLatestData(liveData1, liveData2)
    }
    return result
}

private fun combineLatestData(
    liveData1: LiveData<Boolean>,
    liveData2: LiveData<Boolean>
): Boolean {

    val value1 = liveData1.value
    val value2 = liveData2.value

    // Don't send a success until we have both results that are true
//    if (value1 && value2) {
//        return true
//    }else{
//        return false
//    }
    return value1 && value2
}

上面的 MediatorLiveData 示例显示了一些代码重复，因此我们可以利用 Kotlin 的扩展功能：
扩展写法：

/**
 * 将值设置为一个函数的结果，该函数在两个 LiveData 都有数据或之后接收到更新时调用
 */
fun <T, A, B> LiveData<A>.combineAndCompute(
    other: LiveData<B>,
    onChange: (A?, B?) -> T
): MediatorLiveData<T> {

    var source1emitted = false
    var source2emitted = false

    val result = MediatorLiveData<T>()

    val mergeF = {
        if (source1emitted && source2emitted) {
            result.value = onChange.invoke(this.value, other.value)
        }
    }

    result.addSource(this) { source1emitted = true; mergeF.invoke() }
    result.addSource(other) { source2emitted = true; mergeF.invoke() }
    return result
}

调用示例：

liveData1.combineAndCompute(liveData2){
        value1, value2 -> value1 == true && value2 == true
    }.observe(this){
        if (it){
            // TODO
        }
    }