# \[iOS] Asynchronous Operation

iOS的Operation是synchronous的,  無法做到一些asynchronous的動作如排序下載檔案, 這時候就需要自己寫一個asynchronous的Operation.

**FCAsynchronousOperation**:

(1.) 繼承Operation

(2.) override **isAsynchronous**變數, 設為true

(3.)  override **isReady**,  **isExecuting**, **isFinished** 三個變數

       (此例用一個**state**變數結合KVO來實現)

(4.)  override **start**()與**main**() function

(5.) 新增一個finish() function 來定義Asynchronous Operation何時結束

(6.) 繼承**FCAsynchronousOperation**的subclass 需implement main() function

```swift
import UIKit

public class FCAsynchronousOperation: Operation {
    
    @objc private enum State: Int {
        case ready
        case executing
        case finished
    }
    
    /// Concurrent queue for synchronizing access to `state`.
    private let stateQueue = DispatchQueue(label: Bundle.main.bundleIdentifier! + ".operationQueue.state", attributes: .concurrent)
    
    /// Private backing stored property for `state`.
    private var _state: State = .ready
    
    /// The state of the operation
    @objc private dynamic var state: State {
        get { return stateQueue.sync { _state } }
        set { stateQueue.sync(flags: .barrier) { _state = newValue } }
    }
    
    // MARK: - Various `Operation` properties
    open override var isAsynchronous: Bool { return true }
    open override var isReady: Bool { return state == .ready && super.isReady }
    public final override var isExecuting: Bool { return state == .executing }
    public final override var isFinished: Bool { return state == .finished }
    
    // KVN for dependent properties
    open override class func keyPathsForValuesAffectingValue(forKey key: String) -> Set<String> {
        if ["isReady", "isFinished", "isExecuting"].contains(key) {
            return [#keyPath(state)]
        }
        
        return super.keyPathsForValuesAffectingValue(forKey: key)
    }
    
    // Start
    public final override func start() {
        if isCancelled {
            finish()
            return
        }
        
        state = .executing
        main()
    }
    
    /// Subclasses must implement this to perform their work and they must not call `super`. The default implementation of this function throws an exception.
    open override func main() {
        fatalError("Subclasses must implement `main`.")
    }
    
    /// Call this function to finish an operation that is currently executing
    public final func finish() {
        if isExecuting { state = .finished }
    }
}
```


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