objective-c - 对路径中的变换参数使用 NULL 和 CGAffineTransformIdenity 之间的区别

Question

以下各项之间是否有任何差异，特别是在性能方面:

方法 1 - 使用 NULL 转换:

- (CGPathRef)createPathForRect:(CGRect)rect
{
    CGMutablePathRef path = CGPathCreateMutable();
    CGPathMoveToPoint(path, NULL, rect.size.width / 2, rect.size.height - 1);
    CGPathAddLineToPoint(path, NULL, (rect.size.width / 2) - 20, rect.size.height - 22);
    CGPathAddLineToPoint(path, NULL, 0, rect.size.height - 22);
    CGPathAddLineToPoint(path, NULL, 0, 0);
    CGPathAddLineToPoint(path, NULL, rect.size.width - 1, 0);
    CGPathAddLineToPoint(path, NULL, rect.size.width - 1, rect.size.height - 22);
    CGPathAddLineToPoint(path, NULL, (rect.size.width / 2) + 20, rect.size.height - 22);
    CGPathCloseSubpath(path);
    return path;
}

方法 2 - 使用身份转换:

- (CGPathRef)createPathForRect:(CGRect)rect
{
    CGAffineTransform transform = CGAffineTransformIdentity;
    CGMutablePathRef path = CGPathCreateMutable();
    CGPathMoveToPoint(path, &transform, rect.size.width / 2, rect.size.height - 1);
    CGPathAddLineToPoint(path, &transform, (rect.size.width / 2) - 20, rect.size.height - 22);
    CGPathAddLineToPoint(path, &transform, 0, rect.size.height - 22);
    CGPathAddLineToPoint(path, &transform, 0, 0);
    CGPathAddLineToPoint(path, &transform, rect.size.width - 1, 0);
    CGPathAddLineToPoint(path, &transform, rect.size.width - 1, rect.size.height - 22);
    CGPathAddLineToPoint(path, &transform, (rect.size.width / 2) + 20, rect.size.height - 22);
    CGPathCloseSubpath(path);
    return path;
}

我猜测它们是完全相同的，但想确认这一点。

Answer 1

如果您查看诸如CGPathMoveToPoint之类的函数的文档，它会这样说:

m
A pointer to an affine transformation matrix, or NULL if no transformation is needed. If specified, Quartz applies the transformation to the point before changing the path.

由于 CGAffineTransformIdentity 本质上不是转换，因为它是身份，因此这两段代码实际上是相同的。