使用 Core Graphics 从外部存储加载图像不起作用 iOS 13

Question

我正在尝试使用 iOS 13（测试版）中的核心显卡加载位于外部存储器（SD 卡）上的照片。当文件在设备上时，下面的代码可以正常工作。当文件在外部存储上时，它无法返回 nil，我不知道为什么。

我相信我正在使用正确的安全范围。我根据 Providing Access to Directories

从安全范围的文件夹 url 加载了文件 URL

guard folderUrl.startAccessingSecurityScopedResource() else {
    return nil
}
defer { folderUrl.stopAccessingSecurityScopedResource() }

guard let imageSource = CGImageSourceCreateWithURL(imageURL as CFURL, options) else {
    throw Error.failedToOpenImage(message: "Failed to open image at \(imageURL)")
}

Answer 1

所以...对于我自己的项目，我运行遇到了同样的问题，我现在有以下功能可以给我一个缩略图，从优雅和快速到蛮力。

static func thumbnailForImage(at url: URL, completion: (Result<UIImage, Error>) -> Void) {
    let shouldStopAccessing = url.startAccessingSecurityScopedResource()
    defer { if shouldStopAccessing { url.stopAccessingSecurityScopedResource() } }

    let coordinator = NSFileCoordinator()
    var error: NSError?

    coordinator.coordinate(readingItemAt: url, options: .withoutChanges, error: &error) { url in

        var thumbnailImage: UIImage?
        var storedError: NSError?
        var imageSource: CGImageSource?

        print("Strategy 1: Via URL resource key")
        do {
            let resourceKeys = Set([URLResourceKey.thumbnailDictionaryKey])
            let resources = try url.resourceValues(forKeys: resourceKeys)
            if let dict = resources.thumbnailDictionary, let resource = dict[URLThumbnailDictionaryItem.NSThumbnail1024x1024SizeKey] {
                thumbnailImage = resource
            } else {
                throw "No thumbnail dictionary"
            }
        } catch let error {
            storedError = error as NSError
        }

        let options = [kCGImageSourceCreateThumbnailFromImageIfAbsent: true, kCGImageSourceShouldAllowFloat: true, kCGImageSourceCreateThumbnailWithTransform: true]

        if thumbnailImage == nil {
            print("Strategy 2: Via CGImageSourceCreateWithURL")
            imageSource = CGImageSourceCreateWithURL(url as CFURL, options as CFDictionary)
        }

        if thumbnailImage == nil && imageSource == nil {
            print("Strategy 3: Via CGImageSourceCreateWithData")

            let data = try? Data.init(contentsOf: url)
            if let data = data {
                imageSource = CGImageSourceCreateWithData(data as CFData, options as CFDictionary)
            }
        }

        if let imageSource = imageSource, thumbnailImage == nil {
            print("Attempting thumbnail creation from source created in strategy 2 or 3")

            if let image = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, options as CFDictionary) {
                thumbnailImage = UIImage(cgImage: image)
            }
        }

        if let thumbnailImage = thumbnailImage {
            print("Success")
            completion(.success(thumbnailImage))
        } else {
            print("Failure")
            if let error = storedError { completion(.failure(error)) }
            else { completion(.failure("Everything just fails...")) }
        }
    }

    if let error = error { completion(.failure(error)) }
}

基本上它的工作原理是首先尝试通过 URL 资源获取缩略图。这是最快和最好的方法。如果失败，我会尝试 CGImageSourceCreateWithURL。这在大多数情况下都有效，但在远程存储上除外。我怀疑这仍然是一个错误并为此向苹果提交了反馈票。我建议你这样做。最后一次尝试，尝试使用 NSData 读取整个文件并通过 CGImageSourceCreateWithData...

创建图像源

到目前为止，如果是图像文件 I，这似乎大多数时候都会生成缩略图。但是它可能会很慢，必须读取整个文件。

使用 Core Graphics 从外部存储加载图像不起作用 iOS 13

Loading images from external storage using Core Graphics not working iOS 13

core-graphics

nsurl

ios

swift

ios13