How to remove labels from after drawing with drawrect() - ios

Question

I am pretty new to drawing programmatically. I've managed to draw a line graph, complete with points, lines, auto-scaling axes, and axis labels. In other screens, I can change characteristics of the graph and when I return to the screen, I refresh it with setNeedsDisplay() in the viewWillAppear function of the containing viewController. The lines are redrawn perfectly when I do this.
The new data that is added in other screens may require rescaling the axes. The problem is that when the graph is redrawn, the number labels on the axes are just added to the graph, without removing the old ones, meaning that some labels may be overwritten, while some old ones just remain there next to the new ones.
I think I see why this happens, in that I am creating a label and adding a subview, but not removing it. I guess I figured that since the lines are erased and redrawn, the labels would be, too. How do I cleanly relabel my axes? Is there a better way to do this? My function for creating the labels is listed below. This function is called by drawRect()
func createXAxisLabels(interval: Float, numIntervals: Int) {
let xstart: CGFloat = marginLeft
let yval: CGFloat = marginTop + graphHeight + 10 // 10 pts below the x-axis
var xLabelVals : [Float] = [0]
var xLabelLocs : [CGFloat] = [] // gives the locations for each label
for i in 0...numIntervals {
xLabelLocs.append(xstart + CGFloat(i) * graphWidth/CGFloat(numIntervals))
xLabelVals.append(Float(i) * interval)
}
if interval >= 60.0 {
xUnits = "Minutes"
xUnitDivider = 60
}
for i in 0...numIntervals {
let label = UILabel(frame: CGRectMake(0, 0, 50.0, 16.0))
label.center = CGPoint(x: xLabelLocs[i], y: yval)
label.textAlignment = NSTextAlignment.Center
if interval < 1.0 {
label.text = "\(Float(i) * interval)"
} else {
label.text = "\(i * Int(interval/xUnitDivider))"
}
label.font = UIFont.systemFontOfSize(14)
label.textColor = graphStructureColor
self.addSubview(label)
}
}

Answer 1

drawRect should just draw the rectangle, not change the view hierarchy. It can be called repeatedly. Those other labels are other views and do their own drawing.
You have a couple of options
Don't use labels -- instead just draw the text onto the rect.
-or-
Add/remove the labels in the view controller.
EDIT (from the comments): The OP provided their solution
I just replaced my code inside the for loop with:
let str : NSString = "\(xLabelVals[i])"
let paraAttrib = NSMutableParagraphStyle()
paraAttrib.alignment = NSTextAlignment.Center
let attributes = [
NSFontAttributeName: UIFont.systemFontOfSize(14),
NSForegroundColorAttributeName: UIColor.whiteColor(),
NSParagraphStyleAttributeName: paraAttrib]
let xLoc : CGFloat = CGFloat(xLabelLocs[i] - 25)
let yLoc : CGFloat = yval - 8.0
str.drawInRect(CGRectMake(xLoc, yLoc, 50, 16), withAttributes: attributes)