Make sure not to display an image that belongs to a different post

The key change here is to store the image subscription in the MediaView’s disposeBag rather than the configuration’s, so that it is properly disposed of in the MediaView’s prepareForReuse().

For good measure, also adding MainActor on prepareForReuse and actual cancellation of all subscriptions before releasing them.

Fixes #1374 [BUG] Posts can load the wrong images

MastodonSDK/Sources/MastodonUI/View/Content/MediaView.swift

@@ -127,21 +127,25 @@ extension MediaView {
         layoutAlt()
     }
 
-    private func bindImage(configuration: Configuration, info: Configuration.ImageInfo) {        
+    private func bindImage(configuration: Configuration, info: Configuration.ImageInfo) {
+        let subscribedConfigurationIdentifier = ObjectIdentifier(configuration) // this shouldn't be necessary now, but allows a check in debug mode. https://github.com/mastodon/mastodon-ios/issues/1374
         Publishers.CombineLatest(
             configuration.$previewImage,
             configuration.$blurhashImage
         )
         .receive(on: DispatchQueue.main)
         .sink { [weak self] previewImage, blurhashImage in
             guard let self = self else { return }
-
+            guard let currentConfiguration = self.configuration, ObjectIdentifier(currentConfiguration) == subscribedConfigurationIdentifier else {
+                assert(false, "\(self) attempt to load an image that belongs to a configuration no longer associated with this MediaView.")
+                return
+            }
             let image = configuration.isReveal ?
                 (previewImage ?? blurhashImage ?? MediaView.placeholderImage) :
                 (blurhashImage ?? MediaView.placeholderImage)
             self.imageView.image = image
         }
-        .store(in: &configuration.disposeBag)
+        .store(in: &_disposeBag)
 
         bindAlt(configuration: configuration, altDescription: info.altDescription)
     }
@@ -220,7 +224,11 @@ extension MediaView {
         overlayViewController.view.pinToParent()
     }
 
+    @MainActor
     public func prepareForReuse() {
+        for cancellable in _disposeBag {
+            cancellable.cancel()
+        }
         _disposeBag.removeAll()
 
         // reset appearance