scala - 如何理解这个语法？ case streamingRelation@StreamingRelation(dataSourceV1, sourceName, 输出)

Question

在阅读structured Streaming的源代码时，我对这个语法感到困惑。

在 microBatchExecution.scala 中

val _logicalPlan = analyzedPlan.transform {
     case streamingRelation@StreamingRelation(dataSourceV1, sourceName, 
output) =>
    toExecutionRelationMap.getOrElseUpdate(streamingRelation, {
      // Materialize source to avoid creating it in every batch
      val metadataPath = s"$resolvedCheckpointRoot/sources/$nextSourceId"
      val source = dataSourceV1.createSource(metadataPath)
      nextSourceId += 1
      logInfo(s"Using Source [$source] from DataSourceV1 named 
'$sourceName' [$dataSourceV1]")
      StreamingExecutionRelation(source, output)(sparkSession)
    })
……
 }

My Question:

1. how to understand case streamingRelation@StreamingRelation(dataSourceV1, sourceName, output)?

2. what's the function of "@" here?

Answer 1

有许多不同的方法可以进行模式匹配:

您可以通过类型捕获整个对象到变量:

 case streamingRelation: StreamingRelation => 
     //do something with object of type StreamingRelation bound to variable streamingRelation

或者你可以解构它:

case StreamingRelation(dataSourceV1, sourceName, output) => 
    //do something with members of an object like dataSourceV1, sourceName etc.

@ 的语法结合了两者:

case streamingRelation@StreamingRelation(dataSourceV1, sourceName, output) =>
  //both whole object is available as streamingRelation and all matched members like dataSourceV1, sourceName