Simple render FPS counter for SwiftUI (NOT GPU FPS)

FPSCounter.swift

import SwiftUI
import Combine

class FPSCounter: ObservableObject {
    @Published var fps: Double = 0.0

    private var timer: Timer?
    private var lastUpdateTime: Date = Date()
    private var frameCount: Int = 0
    
    // How often to update the published FPS value (e.g., every 0.5 seconds)
    private let fpsUpdateInterval: TimeInterval = 0.5
    // How frequently the internal timer should try to fire (e.g., aiming for 120Hz)
    private let internalTimerInterval: TimeInterval = 1.0 / 120.0 

    init() {}

    func start() {
        guard timer == nil else { return }

        lastUpdateTime = Date()
        frameCount = 0
        
        // This timer will fire frequently to count "frames"
        timer = Timer.scheduledTimer(withTimeInterval: internalTimerInterval, repeats: true) { [weak self] _ in
            self?.tick()
        }
        // Ensure the timer runs on high-priority run loops if needed, e.g., for UI updates
        // RunLoop.current.add(timer!, forMode: .common)
    }

    private func tick() {
        frameCount += 1
        let now = Date()
        let elapsedTime = now.timeIntervalSince(lastUpdateTime)

        if elapsedTime >= fpsUpdateInterval {
            DispatchQueue.main.async {
                self.fps = Double(self.frameCount) / elapsedTime
                self.frameCount = 0
                self.lastUpdateTime = now
            }
        }
    }

    func stop() {
        timer?.invalidate()
        timer = nil
        // Optionally reset FPS to 0 when stopped
        // DispatchQueue.main.async {
        //     self.fps = 0.0
        // }
    }

    deinit {
        stop()
    }
} 

FPSView.swift

import SwiftUI

struct FPSView: View {
    @StateObject private var fpsCounter = FPSCounter()

    var body: some View {
        Text("FPS: \(fpsCounter.fps, specifier: "%.1f")")
            .font(.caption)
            .foregroundColor(.white)
            .padding(4)
            .background(Color.black.opacity(0.5))
            .clipShape(RoundedRectangle(cornerRadius: 5))
            .onAppear {
                fpsCounter.start()
            }
            .onDisappear {
                fpsCounter.stop()
            }
    }
}

struct FPSView_Previews: PreviewProvider {
    static var previews: some View {
        FPSView()
            .padding()
            .previewLayout(.sizeThatFits)
    }
}