信号量在执行后面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了它。

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")

输出始终为：

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

为什么它不等到我的信号量完成后再执行其余的代码呢？

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)
});

既然我把两个同步线程放在同一个队列中，这难道不应该工作吗？

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）在您的块中发出信号量，以及（3）等待块外的信号量：

注意：这将在等待时阻塞它正在运行的任何线程，因此如果它在主线程上，它将暂停用户交互——这不是一个好主意。您最好阅读Apple的并发编程指南来理解其余的概念。

var semaphore = dispatch_semaphore_create(0)
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_signal(semaphore)
}
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)
println("WE MADE IT OUT OF THERE")