Android

Question

我正在尝试了解 Flow，但我不太清楚。我有一个简单的界面 :

interface Operation<T> {
    fun performAsync(callback: (T? , Throwable?) -> Unit)
    fun cancel()
}

然后我有一个经理class，其功能为：

fun<T : Any> Operation<T>.perform(): Flow<T> =
        callbackFlow {
            performAsync {
                value , exception ->
                when {
                    exception !=null -> close(exception)   //operation had failed
                    value == null -> close() //operation had succeeded
                    else -> offer(value as T)
                }
            }
            awaitClose { cancel() }
        }

假设我有一个非常简单的操作 - 尝试使用 gson 将对象序列化为 JSON :

fun convert () {
try {
  val carJSON = gson.toJson(carObj)
  //send Car value
} catch (e : Exception) {
  //here I want to send exception and receive it in callback in activity/fragment.
}
}

你能给我解释一下，如何在 Activity/Fragment 中观察异常（或 T 值）和 send/receive 吗？

Answer 1

你用密封的怎么样class。

sealed class Operation<out V, out E> {

data class Result<out V> (val data: V) : Operation<V, Nothing>()

data class Error<out E> (val error: E) : Operation<Nothing, E>()

companion object {
    inline fun <V> build(operation : () -> V) : Response<V, Exception> {
        return try {
            Value(operation.invoke())
        } catch (e: Exception) {
            Error(e)
        }
    }
}

然后当你需要一些回调函数作为流程的响应时，执行以下操作。

fun someFunction() : Operation<Flow<String>, Exception> {

val flow = callbackFlow<String> { 

val callback = object : someCallback {

       onResult(result: String) {
          offer(result)
       }
       onError(e: Exception) {
          throw e 
       }
}

awaitClose { callback.remove() } //Just an example of a callback

}

return Operation.build { flow }

}

现在 Activity，

简单地在一些方法中得到结果，比如，

when(someFunction) {

is Operation.Result -> collect(someFunction().data)
is Operation.Error  -> handleErrorAsHoweverYouWant(someFunction().error)

}

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