iOS 特定图片的button的旋转动画

近期做的东西中，要为一个有特定图片的button加入旋转动画，Demo代码例如以下：

#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:@"旋转" 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

执行时真机设备和模拟器的button旋转动画中，方向可能有点不同，有点奇怪。

当中一些执行截图例如以下：

Demo地址：点击打开链接