ios - 如何在 GCD 中添加 NSOperationQueue 等操作之间的依赖关系

Question

下面是使用 NSOperation 队列加载 4 个图像的 4 个操作。这里操作2依赖于操作1，操作3依赖于操作2。

有谁知道我们可以在 GCD 中实现同样的目标吗？

@IBAction func didClickOnStart(sender: AnyObject) {

        queue = NSOperationQueue()
        let operation1 = NSBlockOperation(block: {
            let img1 = Downloader.downloadImageWithURL(imageURLs[0])
            NSOperationQueue.mainQueue().addOperationWithBlock({
                self.imageView1.image = img1
            })
        })

        operation1.completionBlock = {
            print("Operation 1 completed")
        }
        queue.addOperation(operation1)

        let operation2 = NSBlockOperation(block: {
            let img2 = Downloader.downloadImageWithURL(imageURLs[1])
            NSOperationQueue.mainQueue().addOperationWithBlock({
                self.imageView2.image = img2
            })
        })

        operation2.completionBlock = {
            print("Operation 2 completed")
        }
        queue.addOperation(operation2)


        let operation3 = NSBlockOperation(block: {
            let img3 = Downloader.downloadImageWithURL(imageURLs[2])
            NSOperationQueue.mainQueue().addOperationWithBlock({
                self.imageView3.image = img3
            })
        })

        operation3.completionBlock = {
            print("Operation 3 completed")
        }
        queue.addOperation(operation3)

        let operation4 = NSBlockOperation(block: {
            let img4 = Downloader.downloadImageWithURL(imageURLs[3])
            NSOperationQueue.mainQueue().addOperationWithBlock({
                self.imageView4.image = img4
            })
        })

        operation4.completionBlock = {
            print("Operation 4 completed")
        }
        queue.addOperation(operation4)
    }

这里我们可以添加操作之间的依赖关系

operation2.addDependency(operation1)
operation3.addDependency(operation2)

Answer 1

你不能(或者只是不这样做，因为它比使用 OperationQueue 和 Operation 子类要工作得多)。