Continuously Redrawing a Path with Updated Data - macos

Question

I am developing an audio visualizer MacOS app, and I want to use Quartz/CoreGraphics to render the time-varying spectrum coordinated with the playing audio. My Renderer code is:
import Cocoa
class Renderer: NSView {
override func draw(_ dirtyRect: NSRect) {
super.draw(dirtyRect)
NSColor.white.setFill()
bounds.fill()
guard let context = NSGraphicsContext.current?.cgContext else {return}
var x : CGFloat = 0.0
var y : CGFloat = 0.0
context.beginPath()
context.move(to: CGPoint(x: x, y: y))
for bin in 0 ..< 300 {
x = CGFloat(bin)
y = CGFloat(Global.spectrum[bin])
context.addLine(to: CGPoint(x: x, y: y))
}
context.setStrokeColor(CGColor( red: 1, green: 0, blue: 0, alpha: 1))
context.setLineWidth(1.0)
context.strokePath()
self.setNeedsDisplay(dirtyRect)
}
}
This draws the path once - using the initial all-zeroes values of the spectrum[] array - and then continues to draw that same all-zeroes line indefinitely. It does not update using the new values in the spectrum[] array. I used a print() statement to verify that the values themselves are being updated, but the draw function does not redraw the path using the updated spectrum values. What am I doing wrong?

Answer 1

The following demo shows how to update an NSView with random numbers created by a timer in a separate class to hopefully mimic your project. It may be run in Xcode by setting up a Swift project for MacOS, copy/pasting the source code into a new file called 'main.swift', and deleting the AppDelegate supplied by Apple. A draw function similar to what you posted is used.
import Cocoa
var view : NSView!
var data = [Int]()
public extension Array where Element == Int {
static func generateRandom(size: Int) -> [Int] {
guard size > 0 else {
return [Int]()
}
return Array(0..<size).shuffled()
}
}
class DataManager: NSObject {
var timer:Timer!
#objc func fireTimer() {
data = Array.generateRandom(size:500)
view.needsDisplay = true
}
func startTimer(){
timer = Timer.scheduledTimer(timeInterval: 2.0, target: self, selector: #selector(fireTimer), userInfo: nil, repeats: true)
}
func stopTimer() {
timer?.invalidate()
}
}
let dataMgr = DataManager()
class View: NSView {
override func draw(_ rect: NSRect) {
super.draw(rect)
NSColor.white.setFill()
bounds.fill()
guard let gc = NSGraphicsContext.current?.cgContext else {return}
var xOld : CGFloat = 0.0
var yOld : CGFloat = 0.0
var xNew : CGFloat = 0.0
var yNew : CGFloat = 0.0
var counter : Int = 0
gc.beginPath()
gc.move(to: CGPoint(x: xOld, y: yOld))
for i in 0 ..< data.count {
xNew = CGFloat(counter)
yNew = CGFloat(data[i])
gc.addLine(to: CGPoint(x: xNew, y: yNew))
xOld = xNew;
yOld = yNew;
counter = counter + 1
}
gc.setStrokeColor(CGColor( red: 1, green: 0, blue: 0, alpha: 1))
gc.setLineWidth(1.0)
gc.strokePath()
}
}
class ApplicationDelegate: NSObject, NSApplicationDelegate {
var window: NSWindow!
#objc func myStartAction(_ sender:AnyObject ) {
dataMgr.startTimer()
}
#objc func myStopAction(_ sender:AnyObject ) {
dataMgr.stopTimer()
}
func buildMenu() {
let mainMenu = NSMenu()
NSApp.mainMenu = mainMenu
// **** App menu **** //
let appMenuItem = NSMenuItem()
mainMenu.addItem(appMenuItem)
let appMenu = NSMenu()
appMenuItem.submenu = appMenu
appMenu.addItem(withTitle: "Quit", action:#selector(NSApplication.terminate), keyEquivalent: "q")
}
func buildWnd() {
data = Array.generateRandom(size: 500)
let _wndW : CGFloat = 800
let _wndH : CGFloat = 600
window = NSWindow(contentRect: NSMakeRect( 0, 0, _wndW, _wndH ), styleMask:[.titled, .closable, .miniaturizable, .resizable], backing: .buffered, defer: false)
window.center()
window.title = "Swift Test Window"
window.makeKeyAndOrderFront(window)
// **** Start Button **** //
let startBtn = NSButton (frame:NSMakeRect( 30, 20, 95, 30 ))
startBtn.bezelStyle = .rounded
startBtn.title = "Start"
startBtn.action = #selector(self.myStartAction(_:))
window.contentView!.addSubview (startBtn)
// **** Stop Button **** //
let stopBtn = NSButton (frame:NSMakeRect( 230, 20, 95, 30 ))
stopBtn.bezelStyle = .rounded
stopBtn.title = "Stop"
stopBtn.action = #selector(self.myStopAction(_:))
window.contentView!.addSubview (stopBtn)
// **** Custom view **** //
view = View( frame:NSMakeRect(20, 60, _wndW - 40, _wndH - 80))
view.autoresizingMask = [.width, .height]
window.contentView!.addSubview (view)
// **** Quit btn **** //
let quitBtn = NSButton (frame:NSMakeRect( _wndW - 50, 10, 40, 40 ))
quitBtn.bezelStyle = .circular
quitBtn.autoresizingMask = [.minXMargin,.maxYMargin]
quitBtn.title = "Q"
quitBtn.action = #selector(NSApplication.terminate)
window.contentView!.addSubview(quitBtn)
}
func applicationDidFinishLaunching(_ notification: Notification) {
buildMenu()
buildWnd()
}
func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
return true
}
}
let applicationDelegate = ApplicationDelegate()
// **** main.swift **** //
let application = NSApplication.shared
application.setActivationPolicy(NSApplication.ActivationPolicy.regular)
application.delegate = applicationDelegate
application.activate(ignoringOtherApps:true)
application.run()