iphone - 为什么 UIBezierPath 比 Core Graphics 路径快？

Question

我在研究绘制路径时发现，至少在某些情况下，UIBezierPath 的性能优于我认为的 Core Graphics 等价物。下面的 -drawRect: 方法创建了两条路径:一条 UIBezierPath 和一条 CGPath。除了位置不同，路径完全相同，但抚摸 CGPath 的时间大约是抚摸 UIBezierPath 的两倍。

- (void)drawRect:(CGRect)rect
{
    CGContextRef ctx = UIGraphicsGetCurrentContext();

    // Create the two paths, cgpath and uipath.
    CGMutablePathRef cgpath = CGPathCreateMutable();
    CGPathMoveToPoint(cgpath, NULL, 0, 100);

    UIBezierPath *uipath = [[UIBezierPath alloc] init];
    [uipath moveToPoint:CGPointMake(0, 200)];

    // Add 200 curve segments to each path.
    int iterations = 200;
    CGFloat cgBaseline = 100;
    CGFloat uiBaseline = 200;
    CGFloat xincrement = self.bounds.size.width / iterations;
    for (CGFloat x1 = 0, x2 = xincrement;
         x2 < self.bounds.size.width;
         x1 = x2, x2 += xincrement)
    {
        CGPathAddCurveToPoint(cgpath, NULL, x1, cgBaseline-50, x2, cgBaseline+50, x2, cgBaseline);
        [uipath addCurveToPoint:CGPointMake(x2, uiBaseline)
                  controlPoint1:CGPointMake(x1, uiBaseline-50)
                  controlPoint2:CGPointMake(x2, uiBaseline+50)];
    }
    [[UIColor blackColor] setStroke];
    CGContextAddPath(ctx, cgpath);

    // Stroke each path.
    [self strokeContext:ctx];
    [self strokeUIBezierPath:uipath];

    [uipath release];
    CGPathRelease(cgpath);
}

- (void)strokeContext:(CGContextRef)context
{
    CGContextStrokePath(context);
}

- (void)strokeUIBezierPath:(UIBezierPath*)path
{
    [path stroke];
}

两条路径都使用 CGContextStrokePath()，所以我创建了单独的方法来描边每条路径，这样我就可以在 Instruments 中看到每条路径使用的时间。以下是典型的结果(调用树倒置)；你可以看到 -strokeContext: 需要 9.5 秒，而 -strokeUIBezierPath: 只需要 5 秒:

Running (Self)      Symbol Name
14638.0ms   88.2%               CGContextStrokePath
9587.0ms   57.8%                 -[QuartzTestView strokeContext:]
5051.0ms   30.4%                 -[UIBezierPath stroke]
5051.0ms   30.4%                  -[QuartzTestView strokeUIBezierPath:]

看起来 UIBezierPath 正在以某种方式优化它创建的路径，或者我正在以一种天真的方式创建 CGPath。我可以做些什么来加快我的 CGPath 绘图？

Answer 1

您是正确的，UIBezierPath 只是 Core Graphics 的 objective-c 包装器，因此性能相当。差异(以及性能差异的原因)是直接绘制 CGPath 时的 CGContext 状态与 UIBezierPath 的设置完全不同。如果您查看 UIBezierPath，它具有以下设置:

• 线宽,
• lineJoinStyle,
• lineCapStyle,
• miterLimit 和
• 平坦度

检查对[path stroke] 的调用(反汇编)时，您会注意到它会在执行CGContextStrokePath 调用之前根据那些先前的值配置当前图形上下文.如果您在绘制 CGPath 之前执行相同的操作，它将执行相同的操作:

- (void)drawRect:(CGRect)rect
{
    CGContextRef ctx = UIGraphicsGetCurrentContext();

    // Create the two paths, cgpath and uipath.
    CGMutablePathRef cgpath = CGPathCreateMutable();
    CGPathMoveToPoint(cgpath, NULL, 0, 100);

    UIBezierPath *uipath = [[UIBezierPath alloc] init];
    [uipath moveToPoint:CGPointMake(0, 200)];

    // Add 200 curve segments to each path.
    int iterations = 80000;
    CGFloat cgBaseline = 100;
    CGFloat uiBaseline = 200;
    CGFloat xincrement = self.bounds.size.width / iterations;
    for (CGFloat x1 = 0, x2 = xincrement;
         x2 < self.bounds.size.width;
         x1 = x2, x2 += xincrement)
    {
        CGPathAddCurveToPoint(cgpath, NULL, x1, cgBaseline-50, x2, cgBaseline+50, x2, cgBaseline);
        [uipath addCurveToPoint:CGPointMake(x2, uiBaseline)
                  controlPoint1:CGPointMake(x1, uiBaseline-50)
                  controlPoint2:CGPointMake(x2, uiBaseline+50)];
    }
    [[UIColor blackColor] setStroke];
    CGContextAddPath(ctx, cgpath);

    // Stroke each path
    CGContextSaveGState(ctx); {
        // configure context the same as uipath
        CGContextSetLineWidth(ctx, uipath.lineWidth);
        CGContextSetLineJoin(ctx, uipath.lineJoinStyle);
        CGContextSetLineCap(ctx, uipath.lineCapStyle);
        CGContextSetMiterLimit(ctx, uipath.miterLimit);
        CGContextSetFlatness(ctx, uipath.flatness);
        [self strokeContext:ctx];
        CGContextRestoreGState(ctx);
    }
    [self strokeUIBezierPath:uipath];

    [uipath release];
    CGPathRelease(cgpath);
}

- (void)strokeContext:(CGContextRef)context
{
    CGContextStrokePath(context);
}

- (void)strokeUIBezierPath:(UIBezierPath*)path
{
    [path stroke];
}

来自 Instruments 的快照: