How can I run a for-loop with completion handler methods? - ios

Question

I have some image files I need to get from a drone using the DJI SDK, but the way I've done it doesn't seem the most ideal. I was wondering how I would be able to do this, but inside of a for-loop instead. Here's the code:
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 {
// [...]
}
}
}
}
Any help would be appreciated by someone who's familiar with the DJI SDK and Swift (perhaps I should have used some other API method?). Thanks!

Answer 1

This is mostly a swift question rather than a DJI SDK issue and more related to code review. But its simply
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]
}
Your recursion may result in errors you don't want. This is kinda what you want to do but it should be straightforward to change based on what I can see here.
Edit: You may also be intending to replace the file as people are loading their images from the drone itself in which case the code is more like this:
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: Also note, this is not necessarily the best way to do this, but previously is how you do it in a for loop. I think a better way would be to use map or a forEach on the mediaList array itself like so. (same getImages method, nothing changes). This could be further refined as well between protocols, extensions and what not but without knowing your program, its hard to do better recommendations.
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
}
}
}