UIActivityIndicatorView提供轻型视图，会显示一个标准的旋转进度轮，只要添加到程序中，在合适的地方start和stop即可。start的时候可以吸附在当前视图中，stop的时候就会移除
简答的效果:
实现方式如下:
self.activityIndicatorView=[[UIActivityIndicatorView alloc]initWithFrame:CGRectMake(0, 0, 100, 100)];
self.activityIndicatorView.center=self.view.center;
[self.activityIndicatorView setActivityIndicatorViewStyle:UIActivityIndicatorViewStyleGray];
[self.activityIndicatorView setActivityIndicatorViewStyle:UIActivityIndicatorViewStyleWhiteLarge];
[self.activityIndicatorView setBackgroundColor:[UIColor lightGrayColor]];
[self.view addSubview:self.activityIndicatorView];
[self.activityIndicatorView startAnimating];
代码很简单，设置frame这个不用讲，关于style的设置，有三种白色和灰色，这两种的尺寸都是20*20，白色大图是37*37:
typedef NS_ENUM(NSInteger, UIActivityIndicatorViewStyle) {
UIActivityIndicatorViewStyleWhiteLarge,
UIActivityIndicatorViewStyleWhite,
UIActivityIndicatorViewStyleGray,
};
关闭动画:
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5* NSEC_PER_SEC)), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),^{
[self.activityIndicatorView stopAnimating];
});
