UIImageView的实例不会保存一个image file，它显示的是一个UIImage实例。当给UIImageView设置imageNamed后，无法再从UIImageView实例获取fileName。
1.从UIImageView继承一个subclass，设置一个NSString * imageFileName；属性
和setImage方法：
-(void) setImage:(NSString*)fileName
{
imageFileName = fileName;
[super setImage:fileName];
}
2.使用accessibilityIdntifier
通过下面setAccessibilityIdentifier
[_checkImageView.image setAccessibilityIdentifier:@"yixuan"];
NSString *file_name = [_checkImageView.image accessibilityIdentifier] ;
// 判断当前是已选还是未选中状态
if ([_checkImageView.image.accessibilityIdentifier isEqualToString:@"yixuan"]) {
_checkImageView.image = [UIImage imageNamed:@"weixuan"];
_checkImageView.image.accessibilityIdentifier = @"weixuan";
}else {
_checkImageView.image = [UIImage imageNamed:@"yixuan"];
_checkImageView.image.accessibilityIdentifier = @"yixuan";
}
