ios - 信号量在执行它之后的代码块之前不会等待

Question

我在使用信号量方面还很陌生，所以我不确定自己是否正确使用了它。

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")

输出总是:

"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)
});

既然我将 2 个同步线程放在同一个队列中，这不应该工作吗？

Answer 1

你在这里混淆了几个概念——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 block 中，向信号量发出信号，以及 (3) 在 block 外等待信号量:

注意:这会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 Concurrency Programming Guide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")