# \[iOS] GCD And Operation Queue

## Dispatch Queues

(1.) 建立一個DispatchQueue

```swift
 let queue = DispatchQueue(label: "com.frost.queue")
```

(2.) Sync

      在Sync block完成前, 主執行緒會先暫停

```swift
  func syncQueue() {
    let queue = DispatchQueue(label: "com.frost.queue")
    
    queue.sync {
      for i in 0...5 {
        print("\(i)")
      }
    }
    
    for i in 100...105 {
      print("\(i)")
    }
  }
  
  -------------------------------
  Output:
  1
  2
  3
  4
  5
  101
  102
  103
  104
  105
  
```

(3.) Async

在Sync block執行時, 主執行會繼續執行

```swift
 func syncQueue() {
    let queue = DispatchQueue(label: "com.frost.queue")
    
    queue.async {
      for i in 0...5 {
        print("\(i)")
      }
    }
    
    for i in 100...105 {
      print("\(i)")
    }
  }
  
  -------------------------------
  Output:
  1
  2
  101
  102
  3
  4
  103
  104
  5
  105
```

(3.) Quality Of Service (QoS)  優先級

   優先級越高, 越會先被執行.

    由高順位至低順位分別為:

* userInteractive
* userInitiated
* default
* utility
* background
* unspecified

 ex:

```swift
let queue1 = DispatchQueue(label: "com.frost.queue1", qos: DispatchQoS.userInitiated)
let queue2 = DispatchQueue(label: "com.frost.queue2", qos: DispatchQoS.unspecified)

queue1.async {
    for i in 0...5 {
     print("\(i)")
    }
}

queue2.async {
    for i in 100...105 {
     print("\(i)")
    }
}

 -------------------------------
  Output:
  1
  2
  101
  3
  4
  5
  102
  103
  104
  105
```

(4.) Concurrent Queues

Concurrent Queues會同步執行任務, 不再依序排隊執行 

ex: 非Concurrent

```swift
let queue = DispatchQueue(label: "com.frost.queue1", qos: .default)

queue.async {
    for i in 0...5 {
     print("\(i)")
    }
}

queue.async {
    for i in 100...105 {
     print("\(i)")
    }
}

 -------------------------------
  Output:
  1
  2
  3
  4
  5
  101
  102
  103
  104
  105
```

ex: Concurrent

```swift
let queue = DispatchQueue(label: "com.frost.queue1", qos: .default, attributes: .concurrent)

queue.async {
    for i in 0...5 {
     print("\(i)")
    }
}

queue.async {
    for i in 100...105 {
     print("\(i)")
    }
}

 -------------------------------
  Output:
  1
  101
  2
  102
  3
  103
  4
  104
  5
  105
```

## 延遲執行

```swift
// to run something after 0.1 seconds

DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
    // your code here
}
```

###

## Main Queues

```swift
DispatchQueue.main.async {
        // 更新UI操作
}
```

## Global Queues

```swift
DispatchQueue.global().async {

}
```


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