两个CloudKit可以互相调用死锁吗？ 怎么修？

Question

在performQuery内部，正在调用另一个performQuery 。 两者的recordType参数相同，但predicate不同。

第二个performQuery永不返回，该应用程序仅运行并等待CloudKit响应。

伪代码是这样的：

publicDatabase.performQuery(CKQuery(recordType: recordTypeSrc, predicate: predicate), inZoneWithID: nil, completionHandler: {records, error in

    if error == nil {

        //.. problem is not name collision or reusing the same parameter, coming codepart is called though other methods

        publicDatabase.performQuery(CKQuery(recordType: recordTypeSrc, predicate: predicate2), inZoneWithID: nil, completionHandler: {records, error in

            //THIS LINE WILL NEVER GET REACHED

            if error == nil {

            } else {
                println(error.localizedDescription)
            }
            dispatch_semaphore_signal(sema2)
        })
        dispatch_semaphore_wait(sema2, DISPATCH_TIME_FOREVER)

        //..

    } else {
        println(error.localizedDescription)
    }
    dispatch_semaphore_signal(sema)
})
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)

Answer 1

看起来是信号量线程问题。 您正在重用sama参数。 最后的dispatch_semaphore_signal必须移到else内部的上方。 然后，可以完全删除第一个dispatch_semaphore_wait。 因此它将是这样的：

publicDatabase.performQuery(CKQuery(recordType: recordTypeSrc, predicate: predicate), inZoneWithID: nil, completionHandler: {records, error in

    if error == nil {

        //.. problem is not name collision, it is nested though other blocks

        publicDatabase.performQuery(CKQuery(recordType: recordTypeSrc, predicate: predicate2), inZoneWithID: nil, completionHandler: {records, error in

            //THIS LINE WILL NEVER GET REACHED

            if error == nil {

            } else {
                println(error.localizedDescription)
            }
            dispatch_semaphore_signal(sema)
        })

        //..

    } else {
        println(error.localizedDescription)
        dispatch_semaphore_signal(sema)
    }
})
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)

Answer 2

死锁是这样修复的：内部查询在外部查询之后移动。 两种信号量均保留。 外部完成处理程序将在内部完成处理程序启动之前完成。 从CloudKit返回的记录存储在局部变量中。

var records: [AnyObject]! = []
publicDatabase.performQuery(CKQuery(recordType: recordTypeSrc, predicate: predicate), inZoneWithID: nil, completionHandler: {records2, error in

    if error == nil {
        records = records2
    } else {
        println(error.localizedDescription)
    }
    dispatch_semaphore_signal(sema)
})
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)

//HERE I can use records

var records3: [AnyObject]! = []
publicDatabase.performQuery(CKQuery(recordType: recordTypeSrc, predicate: predicate2), inZoneWithID: nil, completionHandler: {records4, error in

    if error == nil {
        records3 = records4
    } else {
        println(error.localizedDescription)
    }
    dispatch_semaphore_signal(sema2)
})
dispatch_semaphore_wait(sema2, DISPATCH_TIME_FOREVER)

//HERE I can use records3 too