Iphone 使用实时预览的核心图形绘制矩形

我正在创建一个简单的绘图应用程序，希望获得一些基于用户触摸事件绘制矩形的帮助。我能够使用核心图形从触摸开始点到触摸结束点绘制一个矩形。触发touchesEnded事件后，将显示此矩形

然而，我希望能够以“现场”的方式做到这一点。这意味着，当用户拖动手指时，矩形将更新并显示。这可能吗？任何帮助都将不胜感激，谢谢

更新：

我能够通过使用以下代码来实现这一点。对于小图像，它工作得非常好，但是对于大图像，它的速度非常慢。我意识到我的解决方案效率极低，我想知道是否有人能提出更好的解决方案。谢谢

    - (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();
    }

---

您现在正在做的是为对

drawSquareFrom:to:

的每次调用创建一个成对的

CGImage

和

cgitmapcontext

，并每次处理它们。相反，创建一个成对的

CGImage

和

cgitmapcontext

，供每次调用重用

-(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];
}

您可以像这样创建一对CGImage和CGContext，尽管有一些。。。填补。所有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设置为新创建的映像。在

drawSquareFrom:to:

中，我假设setNeedsDisplay足以绘制所做的更改，但每次包装相同的CGImage时，可能需要分配一个新的UIImage。

只需使用两个上下文即可。 在其中，你只需“预览”绘制矩形，你可以在触摸移动时清除它。

---

在触摸屏上，您最终在原始上下文上绘制，然后在图像上绘制。

与其绘制位图图像，不如创建由CALayer对象组成的图像？（即Illustrator vs Photoshop）在这种情况下，您可以轻松地将带有矩形路径的CAShapeLayer临时添加到文档中。。用户完成拖动后将其提交到文档。在任何情况下，您都可以使用CAShapeLayer预览您的矩形，该CAShapeLayer在文档视图中的当前图像上具有矩形路径。