在 Grand Central Dispatch(GCD) 佇列中執行任務

Version = 3.0

要在排程佇列上執行任務，請使用 sync，async 和 after 方法。

要以非同步方式將任務分派到佇列：

let queue = DispatchQueue(label: "myQueueName")

queue.async {
    //do something
    
    DispatchQueue.main.async {
        //this will be called in main thread
        //any UI updates should be placed here
    }
}
// ... code here will execute immediately, before the task finished

要同步將任務分派到佇列：

queue.sync {
    // Do some task
}
// ... code here will not execute until the task is finished

要在一定秒數後將任務分派到佇列：

queue.asyncAfter(deadline: .now() + 3) {
    //this will be executed in a background-thread after 3 seconds
}
// ... code here will execute immediately, before the task finished

注意： 應在主執行緒上呼叫使用者介面的任何更新！ 確保你將使用者更新的程式碼放在 DispatchQueue.main.async { ... } 中

Version = 2.0

佇列型別：

let mainQueue = dispatch_get_main_queue()
let highQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)
let backgroundQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0)

要以非同步方式將任務分派到佇列：

dispatch_async(queue) {
    // Your code run run asynchronously. Code is queued and executed 
    // at some point in the future.
}
// Code after the async block will execute immediately

要同步將任務分派到佇列：

dispatch_sync(queue) {
    // Your sync code
}
// Code after the sync block will wait until the sync task finished

要在一段時間間隔後排程任務（使用 NSEC_PER_SEC 將秒轉換為納秒）：

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(2.5 * Double(NSEC_PER_SEC))), dispatch_get_main_queue()) {
    // Code to be performed in 2.5 seconds here
}

要非同步執行任務而不是更新 UI：

dispatch_async(queue) {
    // Your time consuming code here
    dispatch_async(dispatch_get_main_queue()) {
        // Update the UI code 
    }
}

注意： 應在主執行緒上呼叫使用者介面的任何更新！ 確保你將使用者介面更新的程式碼放在 dispatch_async(dispatch_get_main_queue()) { ... } 中