It is quite hard to tell so I upload an image to show my problem: http://i42.tinypic.com/2eezamo.jpg

Basically in drawRect, I will draw the line from touchesMoved as finger touches and I will call "needsDisplayInRect" for redraw. But I found that the first line is done, the second line will clear the rect part, so some previouse drawing is gone.

Here is my implementation:

enter code here

-(void) drawRect:(CGRect)rect{
//[super drawRect: rect];
CGContextRef context = UIGraphicsGetCurrentContext();
[self drawSquiggle:squiggle at:rect inContext:context];
}

- (void)drawSquiggle:(Squiggle *)squiggle at:(CGRect) rect inContext:(CGContextRef)context
{   

CGContextSetBlendMode(context, kCGBlendModeMultiply);
UIColor *squiggleColor = squiggle.strokeColor; // get squiggle's color
CGColorRef colorRef = [squiggleColor CGColor]; // get the CGColor
CGContextSetStrokeColorWithColor(context, colorRef);        

NSMutableArray *points = [squiggle points]; // get points from squiggle
// retrieve the NSValue object and store the value in firstPoint
CGPoint firstPoint; // declare a CGPoint
[[points objectAtIndex:0] getValue:&firstPoint];
// move to the point
CGContextMoveToPoint(context, firstPoint.x, firstPoint.y);
// draw a line from each point to the next in order

for (int i = 1; i < [points count]; i++)
{
    NSValue *value = [points objectAtIndex:i]; // get the next value
    CGPoint point; // declare a new point
    [value getValue:&point]; // store the value in point

    // draw a line to the new point
    CGContextAddLineToPoint(context, point.x, point.y);
} // end for
CGContextStrokePath(context); 
}