如何画出新的线条而又不会分散旧的线条？

Question

I want to create a simple tool for drawing. The purpose is to draw a line that follows the accelerometer of the iPhone & iPad, so if the user tilts the device a line will be draw in the direction the device was moved.

I am able to register acceleration and drawing lines. My problem is that as soon as I draw a line the old one disappears. One possible solution would be to save to points already drawn and then re-draw everything, but I would think there are better solutions?

All help is appreciated!

My drawRect is at the moment like this:

- (void)drawRect:(CGRect)rect {
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetLineWidth(context, 20.0);
    CGContextSetStrokeColorWithColor(context, [UIColor yellowColor].CGColor);
    CGContextMoveToPoint(context, fromPoint.x, fromPoint.y);
    CGContextAddLineToPoint(context, toPoint.x, toPoint.y);
    CGContextStrokePath(context);
}

A different method is responsible for refreshing. This method is also called from the uiviewcontroller with certain intervals. Right now it shows a "trail" (or what I should call it) in the direction the device was moved. Not exactly what I am looking for:

- (void)drawNewLine:(CGPoint)to {
    // calculate trail behind current point
    float pointDifferenceX = ((toPoint.x - to.x) * 9);
    float pointDifferenceY = ((toPoint.y - to.y) * 9);

    fromPoint = CGPointMake(toPoint.x + pointDifferenceX, toPoint.y + pointDifferenceY);    
    toPoint = to;

    [self setNeedsDisplay];
}

Answer 1

I can think of two options:

1. Either save all points and redraw the lines whenever the screen needs to be refreshed (as you mentioned)

2. Draw the lines into an off-screen pixelmap and refresh the screen from there

In either case, respect the Hollywood principle: Don't call, you will be called. That means don't just draw to the screen but wait for until drawRect: of your UIView is called. (You can trigger this by calling setNeedsDisplay.)