iOS 特定圖片的button的旋轉動畫

近期做的東西中，要為一個有特定圖片的button加入旋轉動畫，Demo代碼例如以下：

#import "ViewController.h"

@interface ViewController () {
    BOOL flag;
}

@property (strong, nonatomic) UIImageView *imageView;

@end

@implementation ViewController
            
- (void)viewDidLoad {
    [super viewDidLoad];
    
    flag = YES;
    
    self.imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 35, 35)];
    UIImage *aImage = [UIImage imageNamed:@"down.png"];
    [_imageView setImage:aImage];
    _imageView.center = self.view.center;
    [self.view addSubview:_imageView];
    
    UIButton *button = [UIButton buttonWithType:UIButtonTypeSystem];
    [button setTitle:@"旋轉" forState:UIControlStateNormal];
    [button addTarget:self action:@selector(rotate:) forControlEvents:UIControlEventTouchUpInside];
    button.frame = CGRectMake(110, 400, 100, 44);
    [self.view addSubview:button];
}

- (void)rotate:(id)sender {
    if (flag) {
        [UIView animateWithDuration:0.5 animations:^{
            _imageView.transform = CGAffineTransformMakeRotation(M_PI);
        } completion:^(BOOL finished) {
            flag = NO;
        }];
    }
    else {
        [UIView animateWithDuration:0.5 animations:^{
            _imageView.transform = CGAffineTransformMakeRotation(0);
        } completion:^(BOOL finished) {
            flag = YES;
        }];
    }
}

@end

執行時真機設備和模擬器的button旋轉動畫中，方向可能有點不同，有點奇怪。

當中一些執行截圖例如以下：

Demo地址：點擊打開鏈接