Is there a way to request multiple distinct resources in parallel using URLSession.shared.dataTask

有些话、适合烂在心里 提交于 2019-12-04 18:33:12

You ask:

Is there a way to request multiple distinct resources in parallel using URLSession.shared.dataTask

By default, it does perform requests in parallel.

Let’s step back for a second: In your prior question, you were asking how to implement a Kingfisher-like UIImageView extension. In my answer, I mentioned using objc_getAssociatedObject and objc_setAssociatedObject to achieve that. But in your question here, you’ve taken that associated object logic and put it in your DataRequest object.

Your thought process, to pull the asynchronous image retrieval logic out of the UIImageView is a good idea: You may want to request images for buttons. You might a general “fetch image asynchronously” routine, completely separate from any UIKit objects. So abstracting the network layer code out of the extension is an excellent idea.

But the whole idea behind asynchronous image retrieval UIImageView/UIButton extensions is that we want a UIKit control where not only can it perform asynchronous requests, but that if the cell with the control is reused, that it will cancel the prior asynchronous request (if any) before starting the next one. That way, if we scroll quickly down to images 80 through 99, the requests for cells 0 through 79 will be canceled, and the visible images won’t get backlogged behind all these old image requests.

But to achieve that, that means that the control needs some way to keep track of the prior request for that reused cell somehow. And because we can’t add stored properties in a UIImageView extension, that’s why we use the objc_getAssociatedObject and objc_setAssociatedObject pattern. But that has to be in the image view.

Unfortunately, in your code above, the associated object is in your DataRequest object. First, as I’ve tried to outline, the whole idea is that the image view must keep track of the prior request for that control. Putting this “keep track of the prior request” inside the DataRequest object defeats that purpose. Second, it’s worth noting that you don’t need associated objects in your own types, like DataRequest. You’d just have a stored property. You only need to go through this associated object silliness when extending another type, such as UIImageView.

Below, is a quick example that I whipped together showing a UIImageView extension for asynchronous image retrieval. Note, this doesn’t have the abstraction of the network code out of the extension, but do note that the associated object logic to keep track of the prior request must remain with the extension.

private var taskKey: Void?

extension UIImageView {
    private static let imageProcessingQueue = DispatchQueue(label: Bundle.main.bundleIdentifier! + ".imageprocessing", attributes: .concurrent)

    private var savedTask: URLSessionTask? {
        get { return objc_getAssociatedObject(self, &taskKey) as? URLSessionTask }
        set { objc_setAssociatedObject(self, &taskKey, newValue, .OBJC_ASSOCIATION_RETAIN) }
    }

    /// Set image asynchronously.
    ///
    /// - Parameters:
    ///   - url: `URL` for image resource.
    ///   - placeholder: `UIImage` of placeholder image. If not supplied, `image` will be set to `nil` while request is underway.
    ///   - shouldResize: Whether the image should be scaled to the size of the image view. Defaults to `true`.

    func setImage(_ url: URL, placeholder: UIImage? = nil, shouldResize: Bool = true) {
        savedTask?.cancel()
        savedTask = nil

        image = placeholder
        if let image = ImageCache.shared[url] {
            DispatchQueue.main.async {
                UIView.transition(with: self, duration: 0.1, options: .transitionCrossDissolve, animations: {
                    self.image = image
                }, completion: nil)
            }
            return
        }

        var task: URLSessionTask!
        let size = bounds.size * UIScreen.main.scale
        task = URLSession.shared.dataTask(with: url) { [weak self] data, response, error in
            guard
                error == nil,
                let httpResponse = response as? HTTPURLResponse,
                (200..<300) ~= httpResponse.statusCode,
                let data = data
            else {
                return
            }

            UIImageView.imageProcessingQueue.async { [weak self] in
                var image = UIImage(data: data)
                if shouldResize {
                    image = image?.scaledAspectFit(to: size)
                }

                ImageCache.shared[url] = image

                DispatchQueue.main.async {
                    guard
                        let self = self,
                        let savedTask = self.savedTask,
                        savedTask.taskIdentifier == task.taskIdentifier
                    else {
                        return
                    }
                    self.savedTask = nil

                    UIView.transition(with: self, duration: 0.1, options: .transitionCrossDissolve, animations: {
                        self.image = image
                    }, completion: nil)
                }
            }
        }
        task.resume()
        savedTask = task
    }
}

class ImageCache {
    static let shared = ImageCache()

    private let cache = NSCache<NSURL, UIImage>()
    private var observer: NSObjectProtocol?

    init() {
        observer = NotificationCenter.default.addObserver(forName: UIApplication.didReceiveMemoryWarningNotification, object: nil, queue: nil) { [weak self] _ in
            self?.cache.removeAllObjects()
        }
    }

    deinit {
        NotificationCenter.default.removeObserver(observer!)
    }

    subscript(url: URL) -> UIImage? {
        get {
            return cache.object(forKey: url as NSURL)
        }

        set {
            if let data = newValue {
                cache.setObject(data, forKey: url as NSURL)
            } else {
                cache.removeObject(forKey: url as NSURL)
            }
        }
    }
}

And this is my resizing routine:

extension UIImage {

    /// Resize the image to be the required size, stretching it as needed.
    ///
    /// - parameter newSize:      The new size of the image.
    /// - parameter contentMode:  The `UIView.ContentMode` to be applied when resizing image.
    ///                           Either `.scaleToFill`, `.scaleAspectFill`, or `.scaleAspectFit`.
    ///
    /// - returns:                Return `UIImage` of resized image.

    func scaled(to newSize: CGSize, contentMode: UIView.ContentMode = .scaleToFill) -> UIImage? {
        switch contentMode {
        case .scaleToFill:
            return filled(to: newSize)

        case .scaleAspectFill, .scaleAspectFit:
            let horizontalRatio = size.width  / newSize.width
            let verticalRatio   = size.height / newSize.height

            let ratio: CGFloat!
            if contentMode == .scaleAspectFill {
                ratio = min(horizontalRatio, verticalRatio)
            } else {
                ratio = max(horizontalRatio, verticalRatio)
            }

            let sizeForAspectScale = CGSize(width: size.width / ratio, height: size.height / ratio)
            let image = filled(to: sizeForAspectScale)
            let doesAspectFitNeedCropping = contentMode == .scaleAspectFit && (newSize.width > sizeForAspectScale.width || newSize.height > sizeForAspectScale.height)
            if contentMode == .scaleAspectFill || doesAspectFitNeedCropping {
                let subRect = CGRect(
                    x: floor((sizeForAspectScale.width - newSize.width) / 2.0),
                    y: floor((sizeForAspectScale.height - newSize.height) / 2.0),
                    width: newSize.width,
                    height: newSize.height)
                return image?.cropped(to: subRect)
            }
            return image

        default:
            return nil
        }
    }

    /// Resize the image to be the required size, stretching it as needed.
    ///
    /// - parameter newSize:   The new size of the image.
    ///
    /// - returns:             Resized `UIImage` of resized image.

    func filled(to newSize: CGSize) -> UIImage? {
        let format = UIGraphicsImageRendererFormat()
        format.opaque = false
        format.scale = scale

        return UIGraphicsImageRenderer(size: newSize, format: format).image { _ in
            draw(in: CGRect(origin: .zero, size: newSize))
        }
    }

    /// Crop the image to be the required size.
    ///
    /// - parameter bounds:    The bounds to which the new image should be cropped.
    ///
    /// - returns:             Cropped `UIImage`.

    func cropped(to bounds: CGRect) -> UIImage? {
        // if bounds is entirely within image, do simple CGImage `cropping` ...

        if CGRect(origin: .zero, size: size).contains(bounds) {
            return cgImage?.cropping(to: bounds * scale).flatMap {
                UIImage(cgImage: $0, scale: scale, orientation: imageOrientation)
            }
        }

        // ... otherwise, manually render whole image, only drawing what we need

        let format = UIGraphicsImageRendererFormat()
        format.opaque = false
        format.scale = scale

        return UIGraphicsImageRenderer(size: bounds.size, format: format).image { _ in
            let origin = CGPoint(x: -bounds.minX, y: -bounds.minY)
            draw(in: CGRect(origin: origin, size: size))
        }
    }

    /// Resize the image to fill the rectange of the specified size, preserving the aspect ratio, trimming if needed.
    ///
    /// - parameter newSize:   The new size of the image.
    ///
    /// - returns:             Return `UIImage` of resized image.

    func scaledAspectFill(to newSize: CGSize) -> UIImage? {
        return scaled(to: newSize, contentMode: .scaleAspectFill)
    }

    /// Resize the image to fit within the required size, preserving the aspect ratio, with no trimming taking place.
    ///
    /// - parameter newSize:   The new size of the image.
    ///
    /// - returns:             Return `UIImage` of resized image.

    func scaledAspectFit(to newSize: CGSize) -> UIImage? {
        return scaled(to: newSize, contentMode: .scaleAspectFit)
    }

    /// Create smaller image from `Data`
    ///
    /// - Parameters:
    ///   - data: The image `Data`.
    ///   - maxSize: The maximum edge size.
    ///   - scale: The scale of the image (defaults to device scale if 0 or omitted.
    /// - Returns: The scaled `UIImage`.

    class func thumbnail(from data: Data, maxSize: CGFloat, scale: CGFloat = 0) -> UIImage? {
        guard let imageSource = CGImageSourceCreateWithData(data as CFData, nil) else {
            return nil
        }

        return thumbnail(from: imageSource, maxSize: maxSize, scale: scale)
    }

    /// Create smaller image from `URL`
    ///
    /// - Parameters:
    ///   - data: The image file URL.
    ///   - maxSize: The maximum edge size.
    ///   - scale: The scale of the image (defaults to device scale if 0 or omitted.
    /// - Returns: The scaled `UIImage`.

    class func thumbnail(from fileURL: URL, maxSize: CGFloat, scale: CGFloat = 0) -> UIImage? {
        guard let imageSource = CGImageSourceCreateWithURL(fileURL as CFURL, nil) else {
            return nil
        }

        return thumbnail(from: imageSource, maxSize: maxSize, scale: scale)
    }

    private class func thumbnail(from imageSource: CGImageSource, maxSize: CGFloat, scale: CGFloat) -> UIImage? {
        let scale = scale == 0 ? UIScreen.main.scale : scale
        let options: [NSString: Any] = [
            kCGImageSourceThumbnailMaxPixelSize: maxSize * scale,
            kCGImageSourceCreateThumbnailFromImageAlways: true
        ]
        if let scaledImage = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, options as CFDictionary) {
            return UIImage(cgImage: scaledImage, scale: scale, orientation: .up)
        }
        return nil
    }

}

extension CGSize {
    static func * (lhs: CGSize, rhs: CGFloat) -> CGSize {
        return CGSize(width: lhs.width * rhs, height: lhs.height * rhs)
    }
}

extension CGPoint {
    static func * (lhs: CGPoint, rhs: CGFloat) -> CGPoint {
        return CGPoint(x: lhs.x * rhs, y: lhs.y * rhs)
    }
}

extension CGRect {
    static func * (lhs: CGRect, rhs: CGFloat) -> CGRect {
        return CGRect(origin: lhs.origin * rhs, size: lhs.size * rhs)
    }
}

That having been said, we really should constrain our concurrent requests to something reasonable (4-6 at a time) so that they don’t try to start until the prior requests are done (or are canceled) to avoid timeouts. The typical solution is wrapping the requests with asynchronous Operation subclasses, add them to an operation queue, and constrain the maxConcurrentOperationCount to whatever value you choose.

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