信号量在执行其后的代码块之前不会等待

Question

我对使用信号量相当陌生，所以我不确定我是否正确使用它。

当我尝试这样做时：

var group = dispatch_group_create();
dispatch_async(dispatch_get_global_queue(Int(QOS_CLASS_USER_INITIATED.value), 0)){

    var myRef = self.userRef.childByAppendingPath(self.userRef.authData.uid)
    myRef.observeSingleEventOfType(.Value, withBlock: { snapshot in
        // do some stuff once
        println("WE ARE IN HERE")   
        firstName = snapshot.value["firstName"] as String!
        lastName = snapshot.value["lastName"] as String!
    })
}
dispatch_semaphore_wait(group, DISPATCH_TIME_FOREVER)
dispatch_semaphore_signal(group)
println("WE MADE IT OUT OF THERE")

输出始终是：

"WE MADE IT OUF OF THERE"
"WE ARE IN HERE"

为什么它不等待我的信号量完成后再执行其余代码？

编辑：

dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), { () -> () in
        // do some task
        var myRef = self.userRef.childByAppendingPath(self.userRef.authData.uid)
        myRef.observeSingleEventOfType(.Value, withBlock: { snapshot in
            // do some stuff once
            println("WE ARE IN HERE")
            firstName = snapshot.value["firstName"] as String!
            lastName = snapshot.value["lastName"] as String!
        })
});

dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), { () -> () in
    // do some task
    var standbyData = [
        "firstName":firstName,
        "lastName":lastName,
        "duration":self.helpDurationLabel.text!,
        "radius":self.helpRadiusLabel.text!,
        "categories": categories,
        "time_stamp":self.date.timeIntervalSince1970
    ]
    println("WE ARE OUT HERE")
    var standbyUserRef = self.standbyRef.childByAppendingPath(self.userRef.authData.uid)
    standbyUserRef.updateChildValues(standbyData)
});

因为我将 2 个同步线程放入同一个队列中，所以这不应该起作用吗？

Answer 1

您混淆了几个概念：调度组和信号量用于不同的目的。您需要 (1) 创建一个信号量，(2) 在块中发出信号量信号，以及 (3) 等待块外的信号量：

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注意：这将阻止它在等待时运行的任何线程，因此如果它在主线程上，它将挂起用户交互\xe2\x80\x94这不是一个好主意。你最好阅读Apple的并发编程指南以了解其余概念。

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var semaphore = dispatch_semaphore_create(0)\ndispatch_async(dispatch_get_global_queue(Int(QOS_CLASS_USER_INITIATED.value), 0)){\n    var myRef = self.userRef.childByAppendingPath(self.userRef.authData.uid)\n    myRef.observeSingleEventOfType(.Value, withBlock: { snapshot in\n        // do some stuff once\n        println("WE ARE IN HERE")   \n        firstName = snapshot.value["firstName"] as String!\n        lastName = snapshot.value["lastName"] as String!\n    })\n    dispatch_semaphore_signal(semaphore)\n}\ndispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)\nprintln("WE MADE IT OUT OF THERE")\n

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