DispatchQueue.recursiveSync

main.swift

extension DispatchQueue {
    private struct Const {
        static let rSyncKey = DispatchSpecificKey<NSString>()
    }
    var recursiveSyncEnabled: Bool {
        set { self.setSpecific(key: Const.rSyncKey, value: newValue ? (label as NSString) : nil)}
        get { self.getSpecific(key: Const.rSyncKey) != nil }
    }
    func recursiveSync(_ closure: @escaping () -> ()) {
        let specific = DispatchQueue.getSpecific(key: Const.rSyncKey)
        return (specific != nil && specific == self.getSpecific(key: Const.rSyncKey)) ?
            closure() :
            sync(execute: closure)
    }
}

DispatchQueue.main.recursiveSyncEnabled = true

for _ in 1...1000 {
    DispatchQueue.main.recursiveSync {
        print("a")
    }
}

DispatchQueue.global().async {
    for _ in 1...1000 {
        DispatchQueue.main.recursiveSync {
            print("b")
        }
    }
}