使用 CoreGraphics 进行绘图

Question

>我正在使用 CoreGraphics 来实现自由手绘，这对我来说工作正常，现在我想为此图纸实现撤消功能，以便用户可以清除他的最后一笔。

这是我的绘图方法，它适用于UITouchesBegin和UITouchesMoved。

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{
    UITouch *touch  = [touches anyObject];
    previousPoint2  = previousPoint1;
    previousPoint1  = [touch previousLocationInView:self];
    currentPoint    = [touch locationInView:self];

    // calculate mid point
    CGPoint mid1    = midPoint(previousPoint1, previousPoint2);
    CGPoint mid2    = midPoint(currentPoint, previousPoint1);
    CGMutablePathRef path = CGPathCreateMutable();
    CGPathMoveToPoint(path, NULL, mid1.x, mid1.y);
    CGPathAddQuadCurveToPoint(path, NULL, previousPoint1.x, previousPoint1.y, mid2.x, mid2.y);
    CGRect bounds = CGPathGetBoundingBox(path);
    CGPathRelease(path);
    drawBox = bounds;
    //Pad our values so the bounding box respects our line width
    drawBox.origin.x        -= self.lineWidth * 2;
    drawBox.origin.y        -= self.lineWidth * 2;
    drawBox.size.width      += self.lineWidth * 4;
    drawBox.size.height     += self.lineWidth * 4;
    UIGraphicsBeginImageContext(drawBox.size);
    [self.layer renderInContext:UIGraphicsGetCurrentContext()];
    curImage = UIGraphicsGetImageFromCurrentImageContext();
    [curImage retain];
    UIGraphicsEndImageContext();
    [self setNeedsDisplayInRect:drawBox];
}
-(void)drawRect:(CGRect)rect {
    [curImage drawAtPoint:CGPointMake(0, 0)];
    CGPoint mid1 = midPoint(previousPoint1, previousPoint2);
    CGPoint mid2 = midPoint(currentPoint, previousPoint1);
    context = UIGraphicsGetCurrentContext();
    [self.layer renderInContext:context];
    CGContextMoveToPoint(context, mid1.x, mid1.y);
    // Use QuadCurve is the key
    CGContextAddQuadCurveToPoint(context, previousPoint1.x, previousPoint1.y, mid2.x, mid2.y);
    CGContextSetLineCap(context, kCGLineCapRound);
    CGContextSetLineWidth(context, self.lineWidth);
    CGContextSetStrokeColorWithColor(context, self.lineColor.CGColor);
    CGContextStrokePath(context);
    [super drawRect:rect];
}

Answer 1

在我看来，

有两种方法可以实现这一点

1. 您可以在 NSArray 中保存路径并在调用该方法时绘制所有循环drawRect然后在撤消时删除最后一个对象，将其添加到缓冲区数组并重新绘制所有数组。

2. 您可以获得一个离线缓冲区画布，您可以在绘制点时创建图像，每次绘制时都会更新它。在这里，您还需要创建一个点数组，但每次都不要重绘。撤消时，只需删除最后一个对象并在数组中绘制点时创建新的缓冲区画布。