Why does custom NSView redraw when window is resized or moved (drag) to an secondary display (monitor)?

Question

I recently started adding graphic charts to a MacOS application I wrote. The first chart is great and I added a second chart where I ran into a fundamental way the code operates that I did not/do not understand. The only reason I ran into this is because in my second chart I used random to draw some bubbles inside confined areas. It works nicely but when I resized the view window a little bit the chart redrew everything and since I was using random the first set of bubbles remained in the screen and the second set were added to the screen. So I made a simple example to illustrate.

class GraphView: NSView {
    var dcnt = 0

    override func draw(_ dirtyRect: NSRect) {
        super.draw(dirtyRect)
        dcnt+=1
        print(dcnt)
        let path = NSBezierPath(rect: self.bounds)
        var rec = NSRect()
        var circ = NSBezierPath()

        NSColor.lightGray.setFill()
        path.fill()

        rec = NSRect(x:Int.random(in: 100...900), y:Int.random(in: 70...560), width:50, height:50)
        NSColor.red.setFill()
        circ = NSBezierPath(ovalIn: rec)
        circ.fill()
        circ.stroke()
    }
}

This is run from a simple ViewController that calls the GraphView. I added a counter to print out and see what is happening. Also, if I drag the chart/view window over to a second monitor, it also forces a redraw.

How do I stop this from happening. I want to draw the chart and that is it. No clicking, dragging, resizing (I have disabled resizing the window) should redraw anything.

Here is an example of what happens when I drag the chart to a secondary monitor...

Answer 1

Why does custom NSView redraw when window is resized or moved (drag) to an secondary display (monitor)?

A window resize causes view resizes. The secondary display may have a different resolution.

How do I stop this from happening.

You can't, draw is called multiple times.

I want to draw the chart and that is it.

Create a NSImage of the chart and display it in a NSimageView.

the first set of bubbles remained in the screen and the second set were added to the screen

Calculate the coordinates or rect of the circle once when the view is created so draw will always draw the same circle.