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。

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@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。

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 public func activityViewControllerLinkMetadata(_ activityViewController: UIActivityViewController) -> LPLinkMetadata? {
    let metadata = LPLinkMetadata()
    metadata.title = "My title" // Preview Title
    metadata.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。

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extension UIImage {
    func adjustedForShareSheetPreviewIconProvider() -> UIImage {
        let replaceTransparencyWithColor = UIColor.black // change as required
        let minimumSize: CGFloat = 40.0  // points

        let format = UIGraphicsImageRendererFormat.init()
        format.opaque = true
        format.scale = self.scale

        let imageWidth = self.size.width
        let imageHeight = self.size.height
        let imageSmallestDimension = max(imageWidth, imageHeight)
        let deviceScale = UIScreen.main.scale
        let resizeFactor = minimumSize * deviceScale  / (imageSmallestDimension * self.scale)

        let size = resizeFactor > 1.0
            ? CGSize(width: imageWidth * resizeFactor, height: imageHeight * resizeFactor)
            : self.size

        return UIGraphicsImageRenderer(size: size, format: format).image { context in
            let size = context.format.bounds.size
            replaceTransparencyWithColor.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

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func removingAlpha() -> UIImage {
        let format = UIGraphicsImageRendererFormat()
        format.opaque = true // removes Alpha Channel
        format.scale = scale // keeps original image scale
        return UIGraphicsImageRenderer(size: size, format: format).image { _ in
            let size = context.format.bounds.size
        	UIColor.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