ios - 如何使用完成处理程序方法运行 for 循环？

Question

我需要使用 DJI SDK 从无人机获取一些图像文件，但我完成的方式似乎不是最理想的。我想知道如何才能做到这一点，但在 for 循环内。代码如下:

func getImages(with file: DJIMediaFile?) {
    guard let file = file else { return }
    file.fetchPreview { (error) in
        if error != nil {
            print(String(describing: error?.localizedDescription))
            return
        }

        guard let image = file.preview else { print("No preview"); return }
        self.images.append(image)
        self.imageView.image = image
        self.index += 1

        if self.index < self.mediaList.count {
            self.getImages(with: self.mediaList[self.index])
        } else {
            // [...]
        }

       }
   }
}

熟悉 DJI SDK 和 Swift 的人将不胜感激(也许我应该使用其他 API 方法？)。谢谢!

Answer 1

这主要是一个快速问题，而不是 DJI SDK 问题，更多地与代码审查相关。但这只是简单的

func getNewImages() {
    for file in self.mediaList where file != nil { 
        self.getImages(with: file!) { image, error in //not ideal to force unwarp but i don't know if the where clause supports if let binding, or if file is really nil, you'll have to check yourself in code
            guard let newImage = image else { return }
            self.cachedImages.append(newImage)
        }
    }

    self.setImage(at: 0)
}

func getImages(with file: DJIMediaFile, completion: (UIImage?, Error?)->()) {
    file.fetchPreview { (error) in // is this async? If its' async you may want to use dispatch groups in the for in loop to run the setImage at the correct time
        if let newError = error {
            print(String(describing: newError.localizedDescription))
            completion(nil, error)
            return
        }

        guard let image = file.preview else { 
            print("No preview")
            completion(nil, nil)
            return
        }

        completion(image, nil)
   }
}

func setImage(at row: Int) {
     guard row < self.cachedImages.count - 1 else { return }
     self.imageView.image = self.cachedImages[row]
}

您的递归可能会导致您不希望出现的错误。这有点像您想要做的，但根据我在这里看到的内容应该很容易进行更改。

编辑:您可能还打算替换该文件，因为人们从无人机本身加载图像，在这种情况下，代码更像是这样:

func getNewImages() {
    for file in self.mediaList where file != nil { 
        self.getImages(with: file!) { image, error in //not ideal to force unwarp but I don't know if the where clause supports if let binding, or if file is really nil, you'll have to check yourself in code
            guard let newImage = image else { return }
            self.imageView.image = newImage
        }
    }  
}

func getImages(with file: DJIMediaFile, completion: (UIImage?, Error?)->()) {
    file.fetchPreview { (error) in
        if let newError = error {
            print(String(describing: newError.localizedDescription))
            completion(nil, error)
            return
        }

        guard let image = file.preview else { 
            print("No preview")
            completion(nil, nil)
            return
        }

        completion(image, nil)
   }
}

edit2:另请注意，这不一定是执行此操作的最佳方法，但以前是在 for 循环中执行此操作的方法。我认为更好的方法是在 mediaList 数组本身上使用 map 或 forEach ，如下所示。 (相同的 getImages 方法，没有任何变化)。这也可以在协议(protocol)、扩展等之间进一步细化，但在不了解您的程序的情况下，很难做出更好的建议。

func getNewImages() {
    self.mediaList.forEach { file in
        guard let newFile = file else { return }

        self.getImages(with: newFile) { image, error in
            guard let newImage = image else { return }
            self.imageView.image = newImage
        }
    }  
}