所有Firebase调用完成后如何重新加载数据？

如果您希望在完成所有firebase调用后收到通知,则可以使用此代码

let ref = FIRDatabase.database().reference()
ref.child("users/mchen/groups").observeSingleEvent(of: .value, with: { snapshot in
    let groupKeys = snapshot.children.flatMap { $0 as? FIRDataSnapshot }.map { $0.key }

    // This group will keep track of the number of blocks still pending 
    let group = DispatchGroup()

    for groupKey in groupKeys {
        group.enter()
        ref.child("groups").child(groupKey).child("name").observeSingleEvent(of: .value, with: { snapshot in
            print("Mary is a member of this group: \(snapshot.value)")
            group.leave()
        })
    }

    // We ask to be notified when every block left the group
    group.notify(queue: .main) {
        print("All callbacks are completed")
    }
})

它是如何工作的？

涉及4个主要说明.

首先,我们创建一个小组DispatchGroup().该值将跟踪待处理块的数量.

let group = DispatchGroup()

然后在开始新的异步调用之前,我们告诉组有一个新的挂起块.

group.enter()

在回调闭包内,我们告诉组一个块已完成其工作.

group.leave()

当进入组的块数变为零时,我们告诉块运行一个闭包.

group.notify(queue: .main) {
    print("All callbacks are completed")
}