Drawing circles in custom UIView - ios

Question

I have a custom UIView with drawRect:
-(void)drawRect:(CGRect)rect{
CGContextRef context = UIGraphicsGetCurrentContext();
[self drawControlPointsWithCoordsX:point.x andY:point.y forRect:CGRectMake(0, 0, 320, 600) andContext:context];
NSLog(#"Values: %f and %f", point.x, point.y);
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
[self startTimer];
});
}
[self startTimer]; is the initiation method. In this method I start a timer which produces some pairs of ordinates. It is called only once. The coordinates come in this form:
Values: 276.711670 and 117.279999
and keep changing all the time. All the values are logged. In the method which is repeated by the timer every 0.1s I call:
[self drawRect:CGRectMake(0, 0, 320, 600)];
The log works, and the coordinates keep changing, and are correct, but only two or three dots are plotted (instead of more or less 400) and they are in the wrong place.
This is the code to draw the points:
- (void)drawControlPointsWithCoordsX:(int)x andY:(int)y forRect:(CGRect)rect andContext:(CGContextRef)contextRef
{
UIGraphicsPushContext(contextRef);
CGContextSetLineWidth(contextRef, 2.0);
CGContextSetRGBFillColor(contextRef, 0, 0, 1.0, 1.0);
CGContextSetRGBStrokeColor(contextRef, 0, 0, 1.0, 1.0);
CGRect circlePoint = (CGRectMake(x,y, 5.0, 5.0));
CGContextFillEllipseInRect(contextRef, circlePoint);
}

Answer 1

First, you should not be manually calling drawRect:, because it gets called automatically when the view is told to be redrawn. So if you choose to redraw it manually like this, the behavior is probably undefined because what the current graphics context is might be different or not valid. The following is taken from Apple's documentation on the drawRect: method:
This method is called when a view is first displayed or when an event occurs that invalidates a visible part of the view. You should never call this method directly yourself. To invalidate part of your view, and thus cause that portion to be redrawn, call the setNeedsDisplay or setNeedsDisplayInRect: method instead.
Also you should probably not be starting a timer running from within drawRect:. You should probably be starting your timer from somewhere else, like somewhere in your code where you first display this view. Also each time you redraw the view, it is clearing out what was previously there, not overlaying it. So if you expect this to build on what was previously drawn, you must keep track of every point that you draw and redraw all of them up to the current point. You could accomplish this in other ways that wouldn't involve redrawing what you've already drawn, but it would be more complicated.