ios - 正确使用 dispatch_async (GCD) 调用

Question

我正在尝试从服务器加载一组图像并使用返回的图像更新 UI 并通过淡入淡出动画显示它。它将永远重复。

代码片段:

override func viewDidLoad(animated: Bool) {
        super.viewDidLoad(animated)
        self.loadImages()
}


func loadImages(){

        var urlImage:UIImage?

       //self.array_images contains preloaded images (not UIImage) objects which i get from     different api call.
        if imageCount >= self.array_images!.count{
            imageCount = 0
        }

            var img = self.array_images[imageCount]
            var url = NSURL(string: img.url!)
            var data = NSData(contentsOfURL: url!)
            if (data != nil){
                urlImage = UIImage(data: data!)

                UIView.transitionWithView(self.imageView_Promotion, duration: 1.2, options: UIViewAnimationOptions.TransitionCrossDissolve, animations: {
                    println("Begin Animation")
                    self.imageView_Promotion.image = realImage

                    }, completion:{ finished in
                        println("Completed")
                        self.imageCount++
                        self.loadImages() //another issue: it calls the function more than couple of times
                })

            }
        }else{
            var image:UIImage = UIImage(named: "test_Image.jpg")!
            imageView_Promotion.image = image
        }

上面那个挂了UI。我已尝试在 dispatch_async 中调用 loadImages 并在 dispatch_main 队列中调用动画，但问题仍然存在。

let priority = DISPATCH_QUEUE_PRIORITY_BACKGROUND
dispatch_async(dispatch_get_global_queue(priority, 0)) {
 self.loadImages()
}

 dispatch_async(dispatch_get_main_queue(),{
//UIView animation
 })

处理这个问题的正确方法是什么。

谢谢

Answer 1

线程安全问题

基本上，UIKit API 不是线程安全的。这意味着 UIKit API 应该只从主线程(主队列)调用。但是，实际上，也有一些异常(exception)。例如，UIImage +data: 是线程安全的 API。

您的代码包含一些不安全的调用。

UIView.transitionWithView(self.imageView_Promotion, duration: 1.2, options: UIViewAnimationOptions.TransitionCrossDissolve, animations: {

我不认为 UIView.transitionWithView 是线程安全的。

var image:UIImage = UIImage(named: "test_Image.jpg")!

UIImage +named: 不是线程安全的，它使用全局缓存等等。根据UIImage init(named:) Discussion .

You can not assume that this method is thread safe.

阻塞主线程

如果线程安全问题得到解决，它会阻塞主线程。

您的代码调用 loadImages 方法，该方法从 UIView.transitionWithView 的完成 block 中从服务器下载图像或从文件系统加载图像。 UIView.transitionWithView 应该从主线程调用，所以完成 block 也将从主线程调用。这意味着在主线程上下载或加载图像。它阻塞了主线程。

示例代码

因此，loadImages 应该像下面这样。

Swift Playground 代码:

import UIKit
import XCPlayground

func loadImage(file:String) -> UIImage {
    let path = NSBundle.mainBundle().pathForResource(file, ofType: "")
    let data = NSData(contentsOfFile: path!)
    let image = UIImage(data: data!)
    return image!
}

var index = 0
let files = ["image0.png", "image1.png", "image2.png"]
let placementImage = loadImage(files[0])
let view = UIImageView(image:placementImage)

//加载图片函数

func loadImages() {
    let priority = DISPATCH_QUEUE_PRIORITY_BACKGROUND
    dispatch_async(dispatch_get_global_queue(priority, 0)) {
        /*
         * This block will be invoked on the background thread
         * Load an image on the background thread
         * Don't use non-background-thread-safe APIs like UIImage +named:
         */
        if index >= files.count {
            index = 0
        }
        var file = files[index++]
        let image = loadImage(file)

        dispatch_async(dispatch_get_main_queue()) {
            /*
             * This block will be invoked on the main thread
             * It is safe to call any UIKit APIs
             */
            UIView.transitionWithView(view, duration: 1.2, options: UIViewAnimationOptions.TransitionCrossDissolve, animations: {
                /*
                 * This block will be invoked on the main thread
                 * It is safe to call any UIKit APIs
                 */
                view.image = image
            }, completion:{ finished in
                /*
                 * This block will be invoked on the main thread
                 * It is safe to call any UIKit APIs
                 */
                loadImages()
            })
        }
    }
}

//从主线程调用loadImages()

XCPShowView("image", view)
loadImages()
XCPSetExecutionShouldContinueIndefinitely()