Cancel downloads when selecting new model.

I had to make Downloader explicitly cancellable because it waits forever
for the semaphore to toggle, so Task cancellation does not work here.
Cancellation is therefore exposed through PipelineLoader.

Diffusion-macOS/ControlsView.swift

@@ -61,6 +61,7 @@ struct ControlsView: View {
     // TODO: refactor download with similar code in Loading.swift (iOS)
     @State private var stateSubscriber: Cancellable?
     @State private var pipelineState: PipelineState = .downloading(0)
+    @State private var pipelineLoader: PipelineLoader? = nil
 
     // TODO: make this computed, and observable, and easy to read
     @State private var mustShowSafetyCheckerDisclaimer = false
@@ -73,13 +74,16 @@ struct ControlsView: View {
         print("Loading model \(model)")
         Settings.shared.currentModel = model
         
+        pipelineLoader?.cancel()
         pipelineState = .downloading(0)
         Task.init {
             let loader = PipelineLoader(model: model)
+            self.pipelineLoader = loader
             stateSubscriber = loader.statePublisher.sink { state in
                 DispatchQueue.main.async {
                     switch state {
                     case .downloading(let progress):
+                        print("\(loader.model.modelVersion): \(progress)")
                         pipelineState = .downloading(progress)
                     case .uncompressing:
                         pipelineState = .uncompressing

Diffusion/Downloader.swift

@@ -23,19 +23,21 @@ class Downloader: NSObject, ObservableObject {
     private(set) lazy var downloadState: CurrentValueSubject<DownloadState, Never> = CurrentValueSubject(.notStarted)
     private var stateSubscriber: Cancellable?
     
+    private var urlSession: URLSession? = nil
+    
     init(from url: URL, to destination: URL) {
         self.destination = destination
         super.init()
         
         // .background allows downloads to proceed in the background
         let config = URLSessionConfiguration.background(withIdentifier: "net.pcuenca.diffusion.download")
-        let urlSession = URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue())
+        urlSession = URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue())
         downloadState.value = .downloading(0)
-        urlSession.getAllTasks { tasks in
+        urlSession?.getAllTasks { tasks in
             // If there's an existing pending background task, let it proceed, otherwise start a new one.
             // TODO: check URL when we support downloading more models.
             if tasks.first == nil {
-                urlSession.downloadTask(with: url).resume()
+                self.urlSession?.downloadTask(with: url).resume()
             }
         }
     }
@@ -59,6 +61,10 @@ class Downloader: NSObject, ObservableObject {
         default:                  throw("Should never happen, lol")
         }
     }
+    
+    func cancel() {
+        urlSession?.invalidateAndCancel()
+    }
 }
 
 extension Downloader: URLSessionDelegate, URLSessionDownloadDelegate {

Diffusion/Pipeline/PipelineLoader.swift

@@ -43,7 +43,8 @@ class PipelineLoader {
         }
     }
     private(set) lazy var statePublisher: CurrentValueSubject<PipelinePreparationPhase, Never> = CurrentValueSubject(state)
-        
+    private(set) var downloader: Downloader? = nil
+
     func setInitialState() {
         if ready {
             state = .readyOnDisk
@@ -63,6 +64,10 @@ extension PipelineLoader {
     }
 }
 
+extension PipelineLoader {
+    func cancel() { downloader?.cancel() }
+}
+
 extension PipelineLoader {
     var url: URL {
         return model.bestURL
@@ -107,9 +112,9 @@ extension PipelineLoader {
         if ready || downloaded { return downloadedURL }
         
         let downloader = Downloader(from: url, to: downloadedURL)
+        self.downloader = downloader
         downloadSubscriber = downloader.downloadState.sink { state in
             if case .downloading(let progress) = state {
-                print(progress)
                 self.state = .downloading(progress)
             }
         }