使用UIActivityViewController分享图片，没有preview

以前都是用第三方sdk来分享的，最近使用官方的UIActivityViewController来做分享，结果分享图片的时候preview不了分享的图片。
自定义一个继承UIActivityItemProvider的类。关于分享的内容自定义可以自己实现UIActivityItemSource这个协议。首先看看协议的订阅，当我看到有一个返回thumbimage的接口时候，第一感觉就是它是我需要的。

@protocol UIActivityItemSource <NSObject>@required- (id)activityViewControllerPlaceholderItem:(UIActivityViewController *)activityViewController;	// called to determine data type. only the class of the return type is consulted. it should match what -itemForActivityType: returns later
- (nullable id)activityViewController:(UIActivityViewController *)activityViewController itemForActivityType:(nullable UIActivityType)activityType;	// called to fetch data after an activity is selected. you can return nil.@optional- (NSString *)activityViewController:(UIActivityViewController *)activityViewController subjectForActivityType:(nullable UIActivityType)activityType; // if activity supports a Subject field. iOS 7.0
- (NSString *)activityViewController:(UIActivityViewController *)activityViewController dataTypeIdentifierForActivityType:(nullable UIActivityType)activityType; // UTI for item if it is an NSData. iOS 7.0. will be called with nil activity and then selected activity
- (nullable UIImage *)activityViewController:(UIActivityViewController *)activityViewController thumbnailImageForActivityType:(nullable UIActivityType)activityType suggestedSize:(CGSize)size; // if activity supports preview image. iOS 7.0
- (nullable LPLinkMetadata *)activityViewControllerLinkMetadata:(UIActivityViewController *)activityViewController API_AVAILABLE(ios(13.0)); // called to fetch LinkPresentation metadata for the activity item. iOS 13.0@end

结果测试发现，这个方法再iOS17上根本不触发，应该是iOS13以前就用上面几个，iOS13后就用LPLinkMetadata的方法。LPLinkMetadata中有一个imageProvider，他就是preview的内容。

 public func activityViewControllerLinkMetadata(_ activityViewController: UIActivityViewController) -> LPLinkMetadata? {let metadata = LPLinkMetadata()metadata.title = "My title" // Preview Titlemetadata.originalURL = URL(string: "You sub title here") // Set image metadata.imageProvider = NSItemProvider(object: image)return metadata
}

感觉完成了，但是测试的时候有发现问题了，preview没有铺满那个区域。搜索了一下，发现是图片有alpha透明的区域。对应的处理方法就是去掉透明区域。

extension UIImage {func adjustedForShareSheetPreviewIconProvider() -> UIImage {let replaceTransparencyWithColor = UIColor.black // change as requiredlet minimumSize: CGFloat = 40.0  // pointslet format = UIGraphicsImageRendererFormat.init()format.opaque = trueformat.scale = self.scalelet imageWidth = self.size.widthlet imageHeight = self.size.heightlet imageSmallestDimension = max(imageWidth, imageHeight)let deviceScale = UIScreen.main.scalelet resizeFactor = minimumSize * deviceScale  / (imageSmallestDimension * self.scale)let size = resizeFactor > 1.0? CGSize(width: imageWidth * resizeFactor, height: imageHeight * resizeFactor): self.sizereturn UIGraphicsImageRenderer(size: size, format: format).image { context inlet size = context.format.bounds.sizereplaceTransparencyWithColor.setFill()context.fill(CGRect(x: 0, y: 0, width: size.width, height: size.height))self.draw(in: CGRect(origin: .zero, size: size))}}
}

如果不想处理图片大小的就直接去掉alpha通道

func removingAlpha() -> UIImage {let format = UIGraphicsImageRendererFormat()format.opaque = true // removes Alpha Channelformat.scale = scale // keeps original image scalereturn UIGraphicsImageRenderer(size: size, format: format).image { _ inlet size = context.format.bounds.sizeUIColor.whiteColor.setFill()context.fill(CGRect(x: 0, y: 0, width: size.width, height: size.height))draw(in: CGRect(origin: .zero, size: size))}}

这下就正常了。

Referene:
https://stackoverflow.com/questions/57850483/ios13-share-sheet-how-to-set-preview-thumbnail-when-sharing-uiimage
https://nemecek.be/blog/189/wip-sharing-data-with-uiactivityviewcontroller-tips-tricks