feat: add download speed and progress bar UX

Author: simba

Sources/mlx-server/Server.swift

@@ -21,6 +21,61 @@ import MLXVLM
 
 // ── CLI ──────────────────────────────────────────────────────────────────────
 
+final class ProgressTracker {
+    var lastUpdate: TimeInterval = 0
+    var lastBytes: Int64 = 0
+    var speedStr = "0.0 MB/s"
+    var isDone = false
+    
+    func printProgress(_ progress: Progress) {
+        if isDone { return }
+        let now = Date().timeIntervalSince1970
+        
+        let completed = progress.completedUnitCount
+        let total = progress.totalUnitCount
+        let fraction = progress.fractionCompleted
+        
+        if lastUpdate == 0 { lastUpdate = now }
+        let interval = now - lastUpdate
+        
+        if interval >= 0.5 {
+            let diff = Double(completed - lastBytes)
+            let speedMBps = (diff / interval) / 1_048_576.0
+            speedStr = String(format: "%.1f MB/s", speedMBps)
+            
+            lastBytes = completed
+            lastUpdate = now
+        }
+        
+        let pct = Int(fraction * 100)
+        let completedMB = String(format: "%.1f", Double(completed) / 1_048_576)
+        let totalMB = String(format: "%.1f", Double(total) / 1_048_576)
+        
+        let barLength = 20
+        let completedBars = min(barLength, Int(fraction * Double(barLength)))
+        let emptyBars = max(0, barLength - completedBars)
+        
+        var bars = ""
+        if completedBars > 0 {
+            bars += String(repeating: "=", count: completedBars - 1) + ">"
+        } else {
+            bars += ""
+        }
+        bars += String(repeating: " ", count: emptyBars)
+        
+        let pctStr = String(format: "%3d%%", pct)
+        let msg = String(format: "\r[mlx-server] Download: [%@] %@ (%@ MB / %@ MB) | Speed: %@", bars, pctStr, completedMB, totalMB, speedStr)
+        
+        print(msg.padding(toLength: 100, withPad: " ", startingAt: 0), terminator: "")
+        fflush(stdout)
+        
+        if fraction >= 1.0 {
+            print("")
+            isDone = true
+        }
+    }
+}
+
 @main
 struct MLXServer: AsyncParsableCommand {
     static let configuration = CommandConfiguration(
@@ -170,20 +225,19 @@ struct MLXServer: AsyncParsableCommand {
 
         let isVision = self.vision
         let container: ModelContainer
+        let tracker = ProgressTracker()
         if isVision {
             print("[mlx-server] Loading VLM (vision-language model)...")
             container = try await VLMModelFactory.shared.loadContainer(
                 configuration: modelConfig
             ) { progress in
-                let pct = Int(progress.fractionCompleted * 100)
-                print("[mlx-server] Download: \(pct)%")
+                tracker.printProgress(progress)
             }
         } else {
             container = try await LLMModelFactory.shared.loadContainer(
                 configuration: modelConfig
             ) { progress in
-                let pct = Int(progress.fractionCompleted * 100)
-                print("[mlx-server] Download: \(pct)%")
+                tracker.printProgress(progress)
             }
         }