avoid costly re-sampling during resizing

and do a low-quality re-scale instead; this is just for illustration for #104

Author: dmrschmidt

Sources/DSWaveformImageViews/SwiftUI/WaveformView.swift

@@ -11,6 +11,8 @@ public struct WaveformView<Content: View>: View {
     private let content: (WaveformShape) -> Content
 
     @State private var samples: [Float] = []
+    @State private var rescaleTimer: Timer?
+    @State private var currentSize: CGSize = .zero
 
     /**
      Creates a new WaveformView which displays a waveform for the audio at `audioURL`.
@@ -39,26 +41,54 @@ public struct WaveformView<Content: View>: View {
     public var body: some View {
         GeometryReader { geometry in
             content(WaveformShape(samples: samples, configuration: configuration, renderer: renderer))
+                .scaleEffect(x: scaleDuringResize(for: geometry), y: 1, anchor: .trailing)
                 .onAppear {
                     guard samples.isEmpty else { return }
                     update(size: geometry.size, url: audioURL, configuration: configuration)
                 }
-                .modifier(OnChange(of: geometry.size, action: { newValue in update(size: newValue, url: audioURL, configuration: configuration) }))
+                .modifier(OnChange(of: geometry.size, action: { newValue in update(size: newValue, url: audioURL, configuration: configuration, delayed: true) }))
                 .modifier(OnChange(of: audioURL, action: { newValue in update(size: geometry.size, url: audioURL, configuration: configuration) }))
                 .modifier(OnChange(of: configuration, action: { newValue in update(size: geometry.size, url: audioURL, configuration: newValue) }))
         }
     }
 
-    private func update(size: CGSize, url: URL, configuration: Waveform.Configuration) {
-        Task(priority: priority) {
-            do {
-                let samplesNeeded = Int(size.width * configuration.scale)
-                let samples = try await WaveformAnalyzer().samples(fromAudioAt: url, count: samplesNeeded)
-                await MainActor.run { self.samples = samples }
-            } catch {
-                assertionFailure(error.localizedDescription)
+    private func update(size: CGSize, url: URL, configuration: Waveform.Configuration, delayed: Bool = false) {
+        rescaleTimer?.invalidate()
+
+        let updateTask: @Sendable (Timer?) -> Void = { _ in
+            Task(priority: .userInitiated) {
+                do {
+                    let samplesNeeded = Int(size.width * configuration.scale)
+                    let samples = try await WaveformAnalyzer().samples(fromAudioAt: url, count: samplesNeeded)
+
+                    await MainActor.run {
+                        self.currentSize = size
+                        self.samples = samples
+                    }
+                } catch {
+                    assertionFailure(error.localizedDescription)
+                }
             }
         }
+
+        if delayed {
+            rescaleTimer = Timer.scheduledTimer(withTimeInterval: 0.05, repeats: false, block: updateTask)
+            RunLoop.main.add(rescaleTimer!, forMode: .common)
+        } else {
+            updateTask(nil)
+        }
+    }
+
+    /*
+     * During resizing, we only visually scale the shape to make it look more seamless,
+     * before we re-calculate the pixel-perfect re-sampled waveform, which is costly.
+     * Due to the complex way we need to render the actual waveform based on samples
+     * available and size to occupy, the re-scaling currently only supports enlarging.
+     * If we resize to a smaller size, the waveform simply overflows.
+     */
+    private func scaleDuringResize(for geometry: GeometryProxy) -> CGFloat {
+        guard currentSize != .zero else { return 1 }
+        return max(geometry.size.width / currentSize.width, 1)
     }
 }