UIActivityIndicatorView居中

activityIndicator = [[UIActivityIndicatorView alloc]initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
	activityIndicator.frame = CGRectMake(0.0, 0.0, 30.0, 30.0);
	//activityIndicator.center = self.view.center;
	activityIndicator.center = CGPointMake(160.0f, 208.0f);
	[self.view addSubview: activityIndicator];