我正在使用两个 LiveDatas 从我的服务器获取数据，并且想在两个 LiveData 完成后得到结果？LiveData live1 = ...;LiveData live2 = ...;MutableLiveData live3 = ...;live1.observe(this, value -> { live3.postValue(value);});live2.observe(this, value -> { live3.postValue(value);});live3.observe(this, value -> {// TODO: Get both values from live1, live2}我期望 live1 和 live2 的两个值
5 回答
呼唤远方
TA贡献1856条经验 获得超11个赞
你需要的是所谓的zip功能：
fun <A, B> zip(first: LiveData<A>, second: LiveData<B>): LiveData<Pair<A, B>> {
val mediatorLiveData = MediatorLiveData<Pair<A, B>>()
var isFirstEmitted = false
var isSecondEmitted = false
var firstValue: A? = null
var secondValue: B? = null
mediatorLiveData.addSource(first) {
isFirstEmitted = true
firstValue = it
if (isSecondEmitted) {
mediatorLiveData.value = Pair(firstValue!!, secondValue!!)
isFirstEmitted = false
isSecondEmitted = false
}
}
mediatorLiveData.addSource(second) {
isSecondEmitted = true
secondValue = it
if (isFirstEmitted) {
mediatorLiveData.value = Pair(firstValue!!, secondValue!!)
isFirstEmitted = false
isSecondEmitted = false
}
}
return mediatorLiveData
}
现在，您可以调用zip(firstLiveData,secondLiveData)并观察它。
守着星空守着你
TA贡献1799条经验 获得超8个赞
这是一个更通用的版本，它允许您观察多个LiveData.
fun zipLiveData(vararg liveItems: LiveData<*>): LiveData<ArrayList<Any>> {
//MediatorLiveData used to merge multiple LiveDatas
return MediatorLiveData<ArrayList<Any>>().apply {
val zippedObjects = ArrayList<Any>()
liveItems.forEach {
//Add each LiveData as a source for the MediatorLiveData
addSource(it) { item ->
//Add value to list
item?.let { it1 -> zippedObjects.add(it1) }
if (zippedObjects.size == liveItems.size) {
//If all the LiveData items has returned a value, save that value in MediatorLiveData.
value = zippedObjects
//Clear the list for next time
zippedObjects.clear()
}
}
}
}
}
上面的函数不会将null值添加到列表中，假设您null也想添加值，您需要按照以下几行做一些事情，
fun zipLiveData(vararg liveItems: LiveData<*>): LiveData<ArrayList<Any?>> {
return MediatorLiveData<ArrayList<Any?>>().apply {
val zippedObjects = ArrayList<Any?>()
liveItems.forEach {
addSource(it) { item ->
zippedObjects.add(item)
if (zippedObjects.size == liveItems.size) {
value = zippedObjects
zippedObjects.clear()
}
}
}
}
}
更新- 我刚刚意识到上述方法不保留项目的“顺序” LiveData（例如，如果第二个LiveData在第一个之前发布了一个值，您将获得值[secondLiveDataValue, firstLiveDataValue]而不是预期值[firstLiveDataValue, secondLiveDataValue]）。如果您希望保留项目值的“顺序”，则可以改用以下两个函数之一LiveData。
//If you know the LiveDatas won't get null values
fun zipLiveData(vararg liveItems: LiveData<*>): LiveData<ArrayList<Any?>> {
return MediatorLiveData<ArrayList<Any?>>().apply {
var zippedObjects = arrayOfNulls<Any>(liveItems.size)
liveItems.forEachIndexed { index, liveData ->
addSource(liveData) { item ->
zippedObjects[index] = item
if (!zippedObjects.contains(null)) {
value = zippedObjects.toCollection(ArrayList())
zippedObjects = arrayOfNulls(liveItems.size)
}
}
}
}
}
//Incase your LiveDatas might have null values
fun zipLiveDataWithNull(vararg liveItems: LiveData<*>): LiveData<ArrayList<Any?>> {
return MediatorLiveData<ArrayList<Any?>>().apply {
val zippedObjects = arrayOfNulls<Any>(liveItems.size)
val zippedObjectsFlag = BooleanArray(liveItems.size)
liveItems.forEachIndexed { index, liveData ->
addSource(liveData) { item ->
zippedObjects[index] = item
zippedObjectsFlag[index] = true
if (!zippedObjectsFlag.contains(false)) {
value = zippedObjects.toCollection(ArrayList())
for(i in 0 until liveItems.size) {
zippedObjectsFlag[i] = false
}
}
}
}
}
}
牧羊人nacy
TA贡献1862条经验 获得超7个赞
这个扩展功能对我有用
fun <A, B> LiveData<A>.zipWith(stream: LiveData<B>): LiveData<Pair<A, B>> {
val result = MediatorLiveData<Pair<A, B>>()
result.addSource(this) { a ->
if (a != null && stream.value != null) {
result.value = Pair(a, stream.value!!)
}
}
result.addSource(stream) { b ->
if (b != null && this.value != null) {
result.value = Pair(this.value!!, b)
}
}
return result
}
蛊毒传说
TA贡献1895条经验 获得超3个赞
我的解决方案有点受到 rxjava zip 运算符的启发，
inline fun <reified I1, I2, O> biZip(inputLiveData1: LiveData<I1>, inputLiveData2: LiveData<I2>, crossinline tranform: (data1: I1, data2: I2) -> O): LiveData<O> {
var input1: I1? = null
var input2: I2? = null
val mediatorLiveData = MediatorLiveData<O>()
mediatorLiveData.addSource(inputLiveData1) {
input1 = it
if (input1 != null && input2 != null) {
mediatorLiveData.value = tranform.invoke(input1!!, input2!!)
}
}
mediatorLiveData.addSource(inputLiveData2) {
input2 = it
if (input1 != null && input2 != null) {
mediatorLiveData.value = tranform.invoke(input1!!, input2!!)
}
}
return mediatorLiveData
}
现在你可以像这样使用它
val liveDataString = MutableLiveData<String>()
val liveDataInt = MutableLiveData<Int>()
val liveDataofTest = biZip<String, Int, Test>(liveDataString, liveDataInt) { data1:String, data2:Int ->
return@biZip Test(data1, data2)
}
测试 pojo 就像
data class Test(val name:String,value:Int)
沧海一幻觉
TA贡献1824条经验 获得超5个赞
您可以像扩展功能一样直接使用它
fun <A, B, C> LiveData<A>.zip(stream: LiveData<B>, func: (source1: A?, source2: B?) -> C): LiveData<C> {
val result = MediatorLiveData<C>()
result.addSource(this) { a ->
result.setValue(func.invoke(a,stream.value))
}
result.addSource(stream) { b ->
result.setValue(func.invoke(this.value, b))
}
return result
}
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