Ios 如何使用仿射变换应用围绕原点的旋转

我想用仿射变换将矩形旋转45度。我知道我必须移动上下文，使矩形位于原点，应用旋转，然后向后移动。我想完全理解它是如何工作的，所以我从一个小测试开始，围绕原点（父视图坐标系中的0,0）旋转它，而不是向后移动。这是转换到原点后的视图（本例中为绿色矩形）：

然后我旋转10度，它看起来像这样：

- (void)drawRect:(CGRect)rect {

    CGContextRef context = UIGraphicsGetCurrentContext();


    UIColor * redColor = [UIColor colorWithRed:1.0 green:0.0 blue:0.0 alpha:1.0];
    float x = 150;
    float y = 150;
    float w = 120;
    float h = 40;

    CGRect r = CGRectMake(x, y, w, h);
    CGContextSetFillColorWithColor(context, redColor.CGColor);
    CGContextFillRect(context, r);

    CGContextSaveGState(context);


    NSLog(NSStringFromCGRect(self.frame));

    CGAffineTransform transform = CGAffineTransformIdentity;

    float rad = DEGREES_TO_RADIANS(10);

    CGFloat rotation = rad;

    //rotate
    transform = CGAffineTransformRotate(transform, rotation);
    transform = CGAffineTransformTranslate(transform, -x, -y);

    CGContextConcatCTM(context, transform);

    r = CGRectMake(x, y, w, h);
    CGContextSetFillColorWithColor(context, [UIColor greenColor].CGColor);
    CGContextFillRect(context, r);

    CGContextRestoreGState(context);
}

我希望它使用原点作为轴心，但这看起来原点也被平移了，所以当我应用旋转时，它看起来就像我将其应用于旧位置一样，只是平移到了其他点。我错过了什么

这是代码

- (void)drawRect:(CGRect)rect {

    CGContextRef context = UIGraphicsGetCurrentContext();


    UIColor * redColor = [UIColor colorWithRed:1.0 green:0.0 blue:0.0 alpha:1.0];
    float x = 150;
    float y = 150;
    float w = 120;
    float h = 40;

    CGRect r = CGRectMake(x, y, w, h);
    CGContextSetFillColorWithColor(context, redColor.CGColor);
    CGContextFillRect(context, r);

    CGContextSaveGState(context);


    NSLog(NSStringFromCGRect(self.frame));

    //move to the origin
    CGAffineTransform transform = CGAffineTransformMakeTranslation(-x, -y);
    float rad = DEGREES_TO_RADIANS(10); //with 45 it's outside of the view

    CGFloat rotation = rad;

    //rotate
    transform = CGAffineTransformRotate(transform, rotation);

    CGContextConcatCTM(context, transform);

    r = CGRectMake(x, y, w, h);
    CGContextSetFillColorWithColor(context, [UIColor greenColor].CGColor);
    CGContextFillRect(context, r);

    CGContextRestoreGState(context);
    }

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我发现了问题。变换以相反的顺序应用，因此它将首先旋转，然后进行平移

如果我这样改变它：

- (void)drawRect:(CGRect)rect {

    CGContextRef context = UIGraphicsGetCurrentContext();


    UIColor * redColor = [UIColor colorWithRed:1.0 green:0.0 blue:0.0 alpha:1.0];
    float x = 150;
    float y = 150;
    float w = 120;
    float h = 40;

    CGRect r = CGRectMake(x, y, w, h);
    CGContextSetFillColorWithColor(context, redColor.CGColor);
    CGContextFillRect(context, r);

    CGContextSaveGState(context);


    NSLog(NSStringFromCGRect(self.frame));

    CGAffineTransform transform = CGAffineTransformIdentity;

    float rad = DEGREES_TO_RADIANS(10);

    CGFloat rotation = rad;

    //rotate
    transform = CGAffineTransformRotate(transform, rotation);
    transform = CGAffineTransformTranslate(transform, -x, -y);

    CGContextConcatCTM(context, transform);

    r = CGRectMake(x, y, w, h);
    CGContextSetFillColorWithColor(context, [UIColor greenColor].CGColor);
    CGContextFillRect(context, r);

    CGContextRestoreGState(context);
}

然后它就起作用了：

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为什么投票被否决？