Black background in UIView?

问题:

I followed a tutorial online to draw in a subclassed UIView. The tutorial showed a UIView with a white background, I fixed this by simply changing the super's bg color. The problem is, when touches end, the background does not remain clear. I have no idea. I simply tried setting the fill color to [uicolor clearcolor]; unsuccessfully. Here is the code I am using:

@implementation CachedLIView
{
    UIBezierPath *path;
    UIImage *incrementalImage; // (1)
}

- (id)initWithCoder:(NSCoder *)aDecoder
{
    if (self = [super initWithCoder:aDecoder])
    {
        [self setMultipleTouchEnabled:NO];
        [self setBackgroundColor:[UIColor whiteColor]];
        path = [UIBezierPath bezierPath];
        [path setLineWidth:2.0];
    }
    return self;
}

- (void)drawRect:(CGRect)rect
{
    [incrementalImage drawInRect:rect]; // (3)
    [path stroke];
}


- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    CGPoint p = [touch locationInView:self];
    [path moveToPoint:p];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    CGPoint p = [touch locationInView:self];
    [path addLineToPoint:p];
    [self setNeedsDisplay];
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event // (2)
{
    UITouch *touch = [touches anyObject];
    CGPoint p = [touch locationInView:self];
    [path addLineToPoint:p];
    [self drawBitmap]; // (3)
    [self setNeedsDisplay];
    [path removeAllPoints]; //(4)
}

- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
{
    [self touchesEnded:touches withEvent:event];
}

- (void)drawBitmap // (3)
{
    UIGraphicsBeginImageContextWithOptions(self.bounds.size, NO, 0.0);
    [[UIColor blackColor] setStroke];
    if (!incrementalImage) // first draw; paint background white by ...
    {
        UIBezierPath *rectpath = [UIBezierPath bezierPathWithRect:self.bounds]; // enclosing bitmap by a rectangle defined by another UIBezierPath object
        [[UIColor clearColor] setFill];
        [rectpath fill]; // filling it with white
    }
    [incrementalImage drawAtPoint:CGPointZero];
    [path stroke];
    incrementalImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
}

回答1:

Put the following in your view -(id) initWithFrame method:

self = [super initWithFrame:frame];
if (self) {

    self.backgroundColor = [UIColor clearColor];

}

return self;

That way your view subclass will have a transparent background = no black when drawing using bezier, CGContext… methods or anything else.