Question

我正在创建一个简单的绘图应用程序，并希望根据用户的触摸事件绘制矩形。我可以使用Core Graphics从touchesBegan到touchesEnded中的点绘制一个矩形。触发touchesEnded事件后，将显示此矩形。

但是，我希望能够以“现场”的方式做到这一点。意思是，当用户拖动手指时，矩形会更新并显示。这可能吗？非常感谢任何帮助，谢谢！

更新

我可以使用以下代码使其工作。它适用于小图像，但对于大图像来说速度非常慢。我意识到我的解决方案非常低效，并且想知道是否有人可以提出更好的解决方案。感谢。

    - (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
    {
        UITouch *touch = [touches anyObject];
        _firstPoint = [touch locationInView:self.view];
        _firstPoint.y -= 40;
        _lastPoint = _firstPoint;
    }

    - (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
    {
        UITouch *touch = [touches anyObject];
        CGPoint currentPoint = [touch locationInView:self.view];
        currentPoint.y -= 40;

        [self drawSquareFrom:_firstPoint to:currentPoint];

        _lastPoint = currentPoint;
    }

    - (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
    {
        UITouch *touch = [touches anyObject];
        CGPoint currentPoint = [touch locationInView:self.view];
        currentPoint.y -= 40;

        [self drawSquareFrom:_firstPoint to:currentPoint];

        [_modifiedImage release];
        _modifiedImage = [[UIImage alloc] initWithCGImage:_imageView.image.CGImage];
    }

    - (void)drawSquareFrom:(CGPoint)firstPoint to:(CGPoint)lastPoint
    {
        _imageView.image = _modifiedImage;

        CGFloat width  = lastPoint.x - firstPoint.x;
        CGFloat height = lastPoint.y - firstPoint.y;

        _path = [UIBezierPath bezierPathWithRect:CGRectMake(firstPoint.x, firstPoint.y, width, height)];

        UIGraphicsBeginImageContext( _originalImage.size );
        [_imageView.image drawInRect:CGRectMake(0, 0, _originalImage.size.width, _originalImage.size.height)];

        _path.lineWidth = 10;
        [[UIColor redColor] setStroke];
        [[UIColor clearColor] setFill];
        [_path fill];
        [_path stroke];

        _imageView.image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
    }

Answer 1

您现在正在做的是为CGImage的每次调用创建配对的CGBitmapContext和drawSquareFrom:to:并每次处理它们。而是创建一个您为每次调用重复使用的单个配对CGImage和CGBitmapContext。

-(void) drawSquareFrom:(CGPoint)from to:(CGPoint)to {
    CGContextRef context = [self offscreenContext];
    CGRect draw , full = [self offscreenContextBounds];

    draw.origin = from;
    draw.size.width = to.x - from.x;
    draw.size.height = to.y - from.y;
    draw = CGRectStandardize( draw );

    [[UIColor redColor] setStroke];
    [[UIColor clearColor] setFill];
    CGContextClearRect( context , full );
    CGContextFillRect( context , draw );
    CGContextStrokeRectWithWidth( context , draw , 10 );
    [_imageView setNeedsDisplay];
}

你创建一个像这样的配对CGImage和CGContext，虽然有一些......要填写。所有的myX变量都是类成员。

-(CGContextRef) offscreenContext {
    if ( nil == myContext ) {
        size_t width = 400;
        size_t height = 300;
        CFMutableDataRef data = CFDataCreateMutable( NULL , width * height * 4 ); // 4 is bytes per pixel
        CGDataProviderRef provider = CGDataProviderCreateWithCFData( data );
        CGImageRef image = CGImageCreate( width , height , ... , provider , ... );
        CGBitmapContextRef context = CGBitmapContextCreate( CFDataGetMutableBytePtr( data ) , width , height , ... );
        CFRelease( data ); // retained by provider I think
        CGDataProviderRelease( provider ); // retained by image

        myImage = image;
        myContext = context;
        myContextFrame.origin = CGPointZero;
        myContextFrame.size.width = width;
        myContextFrame.size.height = height;
        _imageView.image = [UIImage imageWithCGImage:image];
    }

    return myContext;
}
-(CGRect) offscreenContextBounds {
    return myContextFrame;
}

这会将_imageView.image设置为新创建的图像。在drawSquareFrom:to:中，我假设setNeedsDisplay足以绘制所做的更改，但是每次包装相同的CGImage时，您可能需要分配一个新的UIImage。

Answer 2

只需使用两个上下文。在一个你只需“预览”绘制矩形，你可以在TouchesMoved上清除它。在TouchesEnd上，您最终将绘制原始上下文，然后绘制在图像上。

使用带有实时预览的Core Graphics绘制矩形

2 个答案: