UIActivityIndicatorView用法

1、基本用法：

    UIActivityIndicatorView * activityIndicatorView = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
    activityIndicatorView.bounds = CGRectMake(0, 0, 50, 50);
    activityIndicatorView.center = self.view.center;
    [self.view addSubview:activityIndicatorView];
    [activityIndicatorView startAnimating];
    
    activityIndicatorView.hidesWhenStopped = YES;//动画停止的时候隐藏
//    [activityIndicatorView stopAnimating];//停止动画

activityIndicatorView.activityIndicatorViewStyle = UIActivityIndicatorViewStyleGray

activityIndicatorView.activityIndicatorViewStyle = UIActivityIndicatorViewStyleWhite