ios - 同步调度不起作用？

Question

也许我不理解这个概念，但第二段代码不应该在第一段代码完成之前等待吗？

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
        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)
});

有什么我想念的吗？

输出是:

"WE ARE OUT HERE"
"WE ARE IN HERE"

Answer 1

它会等到第一个完成执行。系统让您感到困惑的地方在于您对 myRef.observeSingleEventOfType(, withBlock:

的使用

99% 请求执行 block 的 API 会将其操作推送到单独的线程，然后在完成时执行该 block 。 observeSingleEventOfType 可能在将其执行安排到操作队列后不久返回，导致您的 dispatch_sync 在 block 执行之前退出。

相反，您可能正在寻找更像这样的东西:

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
        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)
        });
    })
});