iOS 12系统版本以后，原本用UIAlertController去遍历图层获取UILabel的方法失效，原因是UILabel的图层位置改变了

iOS 12 之前：

UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"更新提示！" message:tips preferredStyle:UIAlertControllerStyleAlert];
    UIView *subView1 = alert.view.subviews[0];
    UIView *subView2 = subView1.subviews[0];
    UIView *subView3 = subView2.subviews[0];
    UIView *subView4 = subView3.subviews[0];
    UIView *subView5 = subView4.subviews[0];
//分别拿到title 和 message 可以分别设置他们的对齐属性
    UILabel *title = subView5.subviews[0];
    UILabel *message = subView5.subviews[1];
    message.textAlignment = NSTextAlignmentLeft;

iOS 12 之后：

UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"更新提示！" message:tips preferredStyle:UIAlertControllerStyleAlert];
    UIView *subView1 = alert.view.subviews[0];
    UIView *subView2 = subView1.subviews[0];
    UIView *subView3 = subView2.subviews[0];
    UIView *subView4 = subView3.subviews[0];
    UIView *subView5 = subView4.subviews[0];
//分别拿到title 和 message 可以分别设置他们的对齐属性
    NSArray * viewArray = subView5.subviews;
    int index = [self getIndex];
    UILabel *title = viewArray[index + 0];
    UILabel *message = viewArray[index + 1];
    message.textAlignment = NSTextAlignmentLeft;

- (int) getIndex{
    NSString* phoneVersion = [[UIDevice currentDevice] systemVersion];
    int index;
    if ([phoneVersion doubleValue] >= 12.0) {
        index = 1;
    } else {
        index = 0;
    }
    return index;
}