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Struggling with async/await: Fetching an image off the main thread
Hey everyone, I’m learning async/await and trying to fetch an image from a URL off the main thread to avoid overloading it, while updating the UI afterward. Before starting the fetch, I want to show a loading indicator (UI-related work). I’ve implemented this in two different ways using Task and Task.detached, and I have some doubts: Is using Task { @MainActor the better approach? I added @MainActor because, after await, the resumed execution might not return to the Task's original actor. Is this the right way to ensure UI updates are done safely? Does calling fetchImage() on @MainActor force it to run entirely on the main thread? I used an async data fetch function (not explicitly marked with any actor). If I were to use a completion handler instead, would the function run on the main thread? Is using Task.detached overkill here? I tried Task.detached to ensure the fetch runs on a non-main actor. However, it seems to involve unnecessary actor hopping since I still need to hop back to the main actor for UI updates. Is there any scenario where Task.detached would be a better fit? class ViewController : UIViewController{ override func viewDidLoad() { super.viewDidLoad() //MARK: First approch Task{@MainActor in showLoading() let image = try? await fetchImage() //Will the image fetch happen on main thread? updateImageView(image:image) hideLoading() } //MARK: 2nd approch Task{@MainActor in showLoading() let detachedTask = Task.detached{ try await self.fetchImage() } updateImageView(image:try? await detachedTask.value) hideLoading() } } func fetchImage() async throws -> UIImage { let url = URL(string: "https://via.placeholder.com/600x400.png?text=Example+Image")! //Async data function call let (data, response) = try await URLSession.shared.data(from: url) guard let httpResponse = response as? HTTPURLResponse, httpResponse.statusCode == 200 else { throw URLError(.badServerResponse) } guard let image = UIImage(data: data) else { throw URLError(.cannotDecodeContentData) } return image } func showLoading(){ //Show Loader handling } func hideLoading(){ //Hides the loader } func updateImageView(image:UIImage?){ //Image view updated } }
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1.3k
Jan ’25
The requested URL is not permitted to be downloaded until the application is launched error in background assets
Hi , i tried to download image using background assets framework. I tried to use install and update events to test the extension. But the extension always delegate to backgroundDownload(_ :failedWithError :) function with following error Error Domain=BAErrorDomain Code=202 "The requested URL is not permitted to be downloaded until the application is launched." UserInfo={NSLocalizedFailureReason=The requested URL is not permitted to be downloaded until the application is launched.} I used the following url to download the image https://w0.peakpx.com/wallpaper/934/165/HD-wallpaper-avatar-aang-aang-airbender-aire-appa-avatar-legend-leyenda.jpg Why am i getting this error and how to resolve this?
3
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1.2k
Jun ’23
Invalid absolute dimension: inf. The dimension must be a finite value
Hi I tried to use compositional layout, while using stage manager rare the app getting crashed. Please refer below error message Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid absolute dimension: inf. The dimension must be a finite value.' let itemSize = NSCollectionLayoutSize(widthDimension: .absolute(cellWidth),heightDimension: .absolute(cellHeight)) let item = NSCollectionLayoutItem(layoutSize: itemSize) let groupSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1), heightDimension: .absolute(cellHeight)) let group = NSCollectionLayoutGroup.horizontal(layoutSize: groupSize, subitems: [item]) group.interItemSpacing = NSCollectionLayoutSpacing.fixed(cellSpacing) let section = NSCollectionLayoutSection(group: group) section.contentInsets = NSDirectionalEdgeInsets(top: 10,leading: inset, bottom: 0,trailing:inset) section.interGroupSpacing = lineSpacing Why am I getting this error after providing exact height and width?
Topic: UI Frameworks SubTopic: UIKit Tags:
3
0
1.3k
Jul ’23
Exploring VoiceOver Accessibility for UITableView
I’m currently exploring VoiceOver accessibility in iOS and looking for the best way to reduce the number of swipes required to navigate a UITableView. I’ve come across a couple of potential solutions but am unsure which is preferred. Solution 1: Grouping Subviews in Each Cell Combine all subviews inside a UITableViewCell into a single accessibility element. Provide a concise and meaningful accessibilityLabel. Use custom actions (UIAccessibilityCustomAction) or accessibilityActivationPoint to handle interactions on specific elements within the cell. Solution 2: Using UIAccessibilityContainerDataTableCell & UIAccessibilityContainerDataTable Implement UIAccessibilityContainerDataTable for structured table navigation. Make each cell conform to UIAccessibilityContainerDataTableCell, defining its row and column positions. However, I’m finding this approach a bit complex, and I need guidance on properly implementing these protocols. Additionally, in my case, VoiceOver is not navigating to Section 2—I’m not sure why. Questions: Which of these approaches is generally preferred for better VoiceOver navigation? How do I properly implement UIAccessibilityContainerDataTable so that all sections and rows are navigable? Any best practices or alternative recommendations? Would really appreciate any insights or guidance!
3
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681
Mar ’25
How to Receive Callbacks for UIAccessibilityAction Methods Like accessibilityPerformMagicTap()?
I’ve tried implementing the accessibilityPerformMagicTap() method in a specific UIViewController, its view, and even in AppDelegate, but I am not receiving any callbacks. I directly overrode this method in the mentioned areas, but it never gets triggered when performing a magic tap. How can I properly observe and handle the accessibilityPerformMagicTap() action?
3
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469
Mar ’25
accessibilityElements excludes the unlisted subviews – How to Fix?
I have a parent view containing 10 subviews. To control the VoiceOver navigation order, I set only a few elements in accessibilityElements. However, the remaining elements are not being focused or are completely inaccessible. Is this the expected behavior? If I only specify a subset of elements in accessibilityElements, does it exclude the rest? What’s the best way to ensure all elements remain accessible while customising the order?
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156
Apr ’25
Best Way to Navigate to the Top Element Using VoiceOver
I’m currently focused on an element at the bottom of the screen. What is the proper way to quickly navigate to the top element? By default, there’s a four-finger single tap to move to the first element, but should I use the Rotor action instead to focus on the element I need? For example, in the Contacts app while adding a new contact, if I enter a value in a field at the bottom, there’s no quick way to directly save the contact. I have to manually navigate all the way to the top to tap the Done button, which feels a bit inconvenient. Is there a better way to handle this using VoiceOver?
2
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341
Mar ’25
accessibilityActivationPoint Not Working When Set Directly on UITableViewCell
I’m trying to set the accessibilityActivationPoint directly on a UITableViewCell so that VoiceOver activate on a specific button inside the cell. However, this approach doesn’t seem to work. Instead, when I override the accessibilityActivationPoint property inside the UITableViewCell subclass and return the desired point, it works as expected. Why doesn’t setting accessibilityActivationPoint directly on the cell work, but overriding it inside the cell does? Is there a recommended approach for handling this scenario? The following approach works, override var accessibilityActivationPoint: CGPoint { get { return convert(toggleSwitch.center, to: nil) } set{ super.accessibilityActivationPoint = newValue } } but setting accessibility point directly not works private func configureAccessibility() { isAccessibilityElement = true accessibilityLabel = titleLabel.text accessibilityTraits = .toggleButton accessibilityActivationPoint = self.convert(toggleSwitch.center, to: self) accessibilityValue = toggleSwitch.accessibilityValue }
2
0
128
Apr ’25
VoiceOver Text Recognition Announcing Hidden Labels
I have a UIImageView as the background of a custom UIView subclass. The image itself does not contain any text. On top of this image view, I have added two UILabels. To improve accessibility, I converted the entire view into a single accessibility element and set a proper accessibilityLabel. Additionally, I disabled accessibility for the UIImageView and the labels by setting isAccessibilityElement = false. However, when VoiceOver's Accessibility Recognition's Text Recognition feature is enabled, VoiceOver still detects and announces the text inside the UILabels at the end after reading my custom accessibility properties. This text should not be announced. It seems that VoiceOver treats the UILabel content as part of the UIImageView. Additionally, when using the Explore Image rotor action, the entire subview is recognized as a single image. Is this the expected behavior? If so, is there a way to disable VoiceOver’s text recognition for this view while keeping custom accessibility intact? class BackgroundLabelView: UIView { private let backgroundImageView = UIImageView() private let backgroundImageView2 = UIImageView() private let titleLabel = UILabel() private let subtitleLabel = UILabel() override init(frame: CGRect) { super.init(frame: frame) setupView() } required init?(coder: NSCoder) { super.init(coder: coder) setupView() configureAceesibility() } private func configureAceesibility() { backgroundImageView.isAccessibilityElement = false backgroundImageView2.isAccessibilityElement = false titleLabel.isAccessibilityElement = false subtitleLabel.isAccessibilityElement = false isAccessibilityElement = true accessibilityTraits = .button } func configure(backgroundImage: UIImage?, title: String, subtitle: String) { backgroundImageView.image = backgroundImage titleLabel.text = title subtitleLabel.text = subtitle accessibilityLabel = "Holiday Offer ," + title + "," + subtitle } private func setupView() { backgroundImageView2.contentMode = .scaleAspectFill backgroundImageView2.clipsToBounds = true backgroundImageView2.translatesAutoresizingMaskIntoConstraints = false backgroundImageView2.image = UIImage(resource: .bannerfestival) addSubview(backgroundImageView2) backgroundImageView.contentMode = .scaleAspectFit backgroundImageView.clipsToBounds = true backgroundImageView.translatesAutoresizingMaskIntoConstraints = false addSubview(backgroundImageView) titleLabel.font = UIFont.systemFont(ofSize: 18, weight: .bold) titleLabel.textColor = .white titleLabel.translatesAutoresizingMaskIntoConstraints = false titleLabel.numberOfLines = 0 addSubview(titleLabel) subtitleLabel.font = UIFont.systemFont(ofSize: 14, weight: .regular) subtitleLabel.textColor = .white.withAlphaComponent(0.8) subtitleLabel.translatesAutoresizingMaskIntoConstraints = false subtitleLabel.numberOfLines = 0 addSubview(subtitleLabel) NSLayoutConstraint.activate([ backgroundImageView2.leadingAnchor.constraint(equalTo: leadingAnchor), backgroundImageView2.trailingAnchor.constraint(equalTo: trailingAnchor), backgroundImageView2.heightAnchor.constraint(equalToConstant: 200), backgroundImageView.centerYAnchor.constraint(equalTo: centerYAnchor), backgroundImageView.topAnchor.constraint(equalTo: topAnchor), backgroundImageView.leadingAnchor.constraint(greaterThanOrEqualTo: leadingAnchor), backgroundImageView.trailingAnchor.constraint(equalTo: trailingAnchor), backgroundImageView.bottomAnchor.constraint(equalTo: bottomAnchor), titleLabel.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 16), titleLabel.trailingAnchor.constraint(lessThanOrEqualTo: centerXAnchor), titleLabel.bottomAnchor.constraint(equalTo: centerYAnchor, constant: -4), subtitleLabel.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 16), subtitleLabel.trailingAnchor.constraint(lessThanOrEqualTo: centerXAnchor), subtitleLabel.topAnchor.constraint(equalTo: centerYAnchor, constant: 4) ]) } override func layoutSubviews() { super.layoutSubviews() backgroundImageView.layer.cornerRadius = layer.cornerRadius } }
2
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127
Apr ’25