CGAffineTransform旋转方向错误 [英] CGAffineTransform rotation goes the wrong way

问题描述

在我的实验中，我得出以下结论：

  YourView.transform = CGAffineTransformMakeRotation（正值）; 
  

将顺时针旋转视图，

  YourView.transform = CGAffineTransformMakeRotation（负值）; 
  

将逆时针旋转视图，

但该文件说：

以弧度表示旋转仿射变换的角度。在iOS中，正值指定逆时针旋转，负值指定顺时针旋转。

那些是否相互矛盾？

解决方案

你的困惑是可以理解的。

事实上，正角度表示从正X轴朝向正Y轴。负角度表示从正X轴向负Y轴的旋转。

原生核心图形坐标系建立在标准解释了这一点。）你可以通过查看当前的变换矩阵来检查这一点（使用 CGContextGetCTM ）。您会发现它的 d 元素中有 -1 ，这意味着Y轴被翻转。

A UIView 还使用翻转坐标系来布置其子视图，这会影响<$ c $的含义c> UIView 转换属性。

In my experiment, i conclude these:

YourView.transform = CGAffineTransformMakeRotation( positive value ); 

will rotate the view clockwise, and

YourView.transform = CGAffineTransformMakeRotation( Negative value ); 

will rotate the view counterclockwise,

But the document says:

The angle, in radians, by which to rotate the affine transform. In iOS, a positive value specifies counterclockwise rotation and a negative value specifies clockwise rotation.

does those contradict with each other?

解决方案

Your confusion is quite understandable.

In truth, a positive angle represents a rotation from the positive X axis toward the positive Y axis. A negative angle represents a rotation from the positive X axis toward the negative Y axis.

The "native" Core Graphics coordinate system is modeled after the standard Cartesian coordinate system, in which the Y axis increases upward on the page. In this system, a positive angle represents a counter-clockwise rotation:

So if you create your own CGContext (for example, by using CGBitmapContextCreate or CGPDFContextCreate), rotations will work as you expect.

However, computer systems have historically used a coordinate system in which the Y axis increases downward on the page. In a flipped coordinate system like this, a positive angle represents a clockwise rotation:

Notice that in both coordinate systems, a positive angle rotates from the positive X axis toward the positive Y axis.

It turns out that UIKit flips the coordinate system of the graphics contexts that it creates for you. This includes the graphics context it sets up before sending you drawRect: and the graphics context it sets up in UIGraphicsBeginImageContext. (The Quartz 2D Programming Guide explains this.) You can check this by looking at the current transform matrix (using CGContextGetCTM). You will find that it has a -1 in its d element, meaning that the Y axis is flipped.

A UIView also uses a flipped coordinate system for laying out its subviews, which affects the meaning of the UIView transform property.

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