Question

just like the path in the picture

(new users aren’t allowed to post images, sorry i can only give the link)

http://cdn.dropmark.com/30180/3bd0f0fc6ee77c1b6f0e05e3ea14c821f8b48a82/questiong1-1.jpg

Thanks a lot for any help

Answer 1

Please look at my answer for this question about "rotating a label around an arbitrary point" for a more detailed explanation about using the an anchor point when rotating (ignore the keyframe animation answers since they are more complicated and not the "right tool for this job" even though the answers says so).

In short: set the anchor point to the point (in the unit-coordinate system of your views layer) to the point outside of your view and just apply a normal rotation animation.

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EDIT: One thing to remember

The frame of your layer (which is also the frame of your view) is calculated using the position, bounds and anchor point. Changing the anchorPoint will change where your view appears on screen. You can counter this by re-setting the frame after changing the anchor point (this will set the position for you). Otherwise you can set the position to the point you are rotating to yourself. The Layer Geometry and Transforms (linked to in the other question) also mentions this.

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The code that I provided in that answer (generalized to a UIView and Core Animation instead of UIView-animations):

#define DEGREES_TO_RADIANS(angle) (angle/180.0*M_PI)

- (void)rotateView:(UIView *)view 
       aroundPoint:(CGPoint)rotationPoint 
          duration:(NSTimeInterval)duration 
           degrees:(CGFloat)degrees {

    CABasicAnimation *rotationAnimation = [CABasicAnimation animationWithKeyPath:@"transform"];
    [rotationAnimation setDuration:duration];
    // Additional animation configurations here...
    
    // The anchor point is expressed in the unit coordinate
    // system ((0,0) to (1,1)) of the label. Therefore the 
    // x and y difference must be divided by the width and 
    // height of the view (divide x difference by width and 
    // y difference by height).
    CGPoint anchorPoint = CGPointMake((rotationPoint.x - CGRectGetMinX(view.frame))/CGRectGetWidth(view.bounds),
                                      (rotationPoint.y - CGRectGetMinY(view.frame))/CGRectGetHeight(view.bounds));

    [[view layer] setAnchorPoint:anchorPoint];
    [[view layer] setPosition:rotationPoint]; // change the position here to keep the frame
    CATransform3D rotationTransform = CATransform3DMakeRotation(DEGREES_TO_RADIANS(degrees), 0, 0, 1);
    [rotationAnimation setToValue:[NSValue valueWithCATransform3D:rotationTransform]];
    
    // Add the animation to the views layer
    [[view layer] addAnimation:rotationAnimation
                        forKey:@"rotateAroundAnchorPoint"]  
}