Animating the drawing of a line programatically using Quartz 2d on IOS

I'm trying to draw an animated growing line using Quartz 2d, by adding points to an existing line, gradually over time. I started drawing a new line, In the drawRect method of a UIView, by obtaining the CGContextRef, setting its draw properties, and moving the first point to (0,0).

CGContextRef context= UIGraphicsGetCurrentContext();
CGContextSetStrokeColorWithColor(context,[UIColor blueColor].CGColor);
CGContextSetLineWidth(context, 2);
CGContextMoveToPoint(context,0,0);

later, in my next drawRect call, i tried extending that line, by again, obtaining the CGContextRef, and adding a new point to it.

GContextRef context= UIGraphicsGetCurrentContext();
CGContextAddLineToPoint(context,x,y);

but it seems that the current CGContextRef doesn't have any record of my previous CGContextMoveToPoint command from the last drawRect call, therefore doesn't have any reference that i already started drawing a line.

Am i doing something wrong here? is there a way refering a开发者_如何学Cn already drawn line?

You basically need to treat each call to drawRect as if it was starting from scratch. Even if you are only asked to update a subrect of the view, you should assume that any state associated with the graphics context, such as drawing position and colours, will have been reset. So in your case, you need to keep track of the start position and redraw the whole line each time.

I think the better approach is to animate some thin UIView. Look my answer here.
If you need more than just horizontal line, rotate that view. I think it's better for the performance.