我正在尝试使用GraphStage构造创建Redis Akka流源。这个想法是，只要我从subscription方法获得更新，就将其推送到下一个组件。同样，如果没有拉动信号，则组件应背压。这是代码：
class SubscriberSourceShape(channel: String, subscriber: Subscriber)
extends GraphStage[SourceShape[String]] {
private val outlet: Outlet[String] = Outlet("SubscriberSource.Out")
override def shape: SourceShape[String] = SourceShape(outlet)
override def createLogic(inheritedAttributes: Attributes): GraphStageLogic = {
new GraphStageLogic(shape) {
val callback = getAsyncCallback((msg: String) => push(outlet, msg)
val handler = (msg: String) => callback.invoke(msg)
override def preStart(): Unit = subscriber.subscribe(channel)(handler)
}
}
}
但是，当我用一个简单的接收器运行它时，出现此错误：
Error in stage [akka.http.impl.engine.server.HttpServerBluePrint$ProtocolSwitchStage@58344854]: No handler defined in stage [...SubscriberSourceShape@6a9193dd] for out port [RedisSubscriberSource.Out(1414886352). All inlets and outlets must be assigned a handler with setHandler in the constructor of your graph stage logic.
怎么了？
由于未为Source设置任何输出处理程序，因此出现此错误，因此，当下游组件（Flow或Sink）向该Source发送拉动信号时，将没有处理程序处理该拉动信号。
您可以添加OutputHandler来消除该错误。将onPull方法留空，因为您是在asyncCallback上生成元素的。只需将其添加到GraphStageLogic主体中：
setHandler(outlet, new OutHandler {
override def onPull(): Unit = {}
})
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