1import SwiftUI
2import Photos
3
4/// This view gets an NSImage for a PHAsset.
5///
6/// When you get a photo from the Photos library, it may not be available
7/// immediately -- for example, if the image has to be downloaded from
8/// iCloud first.  Downstream views can create an instance of this object,
9/// and then watch the `image` property -- this will be populated with the
10/// appropriate image as it loads.
11///
12/// You can use this class in two ways:
13///
14///   1. Create a new instance for every PHAsset you want to render
15///   2. Create a single instance and update the `asset` property; the `image`
16///      property will be updated shortly after
17///
18/// Note: PhotoKit may return multiple versions of an image, e.g. a low-res
19/// version immediately and a high-res version later.  You can inspect the
20/// `isDegraded` property -- this will tell you if Photos has returned a
21/// low quality image now and expects to return a higher quality image later.
22class PHAssetImage: NSObject, ObservableObject {
23
24    @Published var image = NSImage()
25    @Published var isDegraded = false
26
27    init(_ asset: PHAsset?, size: CGSize, deliveryMode: PHImageRequestOptionsDeliveryMode) {
28        self.size = size
29        self.deliveryMode = deliveryMode
30        self.imageCache = Dictionary()
31        
32        super.init()
33        
34        self.asset = asset
35    }
36    
37    private var _asset: PHAsset?
38    private var size: CGSize
39    private var deliveryMode: PHImageRequestOptionsDeliveryMode
40    
41    // Often we'll be retrieving the same image repeatedly, as the user shuttles
42    // back and forth between a few images they're comparing.  In this case, we
43    // don't want to go back to Photos every time -- so we keep a cache of images
44    // we've retrieved previously.
45    //
46    // In theory the `PHCachingImageManager` does this for us; in practice the app
47    // feels snappier to me with this additional cache.
48    //
49    // TODO: Replace this Dictionary with an LRU cache of some sort; this could
50    // allow the app's memory usage to balloon indefinitely.
51    private var imageCache: Dictionary<PHAsset, NSImage>
52    
53    var asset: PHAsset? {
54        get {
55            self._asset
56        }
57        
58        set {
59            self._asset = newValue
60            self.isDegraded = true
61            regenerateImage()
62        }
63    }
64        
65    private func regenerateImage() {
66        if let thisAsset = asset {
67            if let nsImage = imageCache[thisAsset] {
68                self.image = nsImage
69                self.isDegraded = false
70                return
71            }
72
73            print("regenerating image for \(thisAsset.localIdentifier)")
74            
75            // This implementation is based on code in a Stack Overflow answer
76            // by Francois Nadeau: https://stackoverflow.com/a/48755517/1558022
77            
78            let options = PHImageRequestOptions()
79            
80            options.isSynchronous = false
81            options.deliveryMode = deliveryMode
82            
83            // If i don't set this value, then sometimes I get an error like
84            // this in the `info` variable:
85            //
86            //      Error Domain=PHPhotosErrorDomain Code=3164 "(null)"
87            //
88            // This means that the asset is in the cloud, and by default Photos
89            // isn't allowed to download assets here.  Apple's documentation
90            // suggests adding this option as the fix.
91            //
92            // See https://developer.apple.com/documentation/photokit/phphotoserror/phphotoserrornetworkaccessrequired
93            options.isNetworkAccessAllowed = true
94            
95            PHCachingImageManager.default()
96                .requestImage(
97                    for: thisAsset,
98                    targetSize: size,
99                    contentMode: .aspectFill,
100                    options: options,
101                    resultHandler: { (result, info) -> Void in
102                        if let isDegraded = info?[PHImageResultIsDegradedKey] as? Bool {
103                            self.isDegraded = isDegraded
104                        }
105                        
106                        if let imageResult = result {
107//                            print("got image!")
108                            self.image = imageResult
109                            
110                            if !self.isDegraded {
111                                self.imageCache[thisAsset] = imageResult
112                            }
113                        } else {
114                            self.isDegraded = true
115                            print("Error getting image: \(String(describing: info))")
116                        }
117                    }
118                )
119        }
120    }
121}