简化UIActivityIndicatorView的创建和使用
头文件：
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#import "UIView.h"@implementation UIView (ActivityView)- (void)showActivityViewAtCenter{UIActivityIndicatorView *activityView = [self getActivityViewAtCenter];if (activityView == nil){activityView = [self createActivityViewAtCenter:UIActivityIndicatorViewStyleWhite];}[activityView startAnimating];}- (void)hideActivityViewAtCenter{UIActivityIndicatorView *activityView = [self getActivityViewAtCenter];if (activityView != nil){[activityView stopAnimating];} }- (UIActivityIndicatorView *)createActivityViewAtCenter:(UIActivityIndicatorViewStyle)style{static int size = 30;UIActivityIndicatorView *activityView = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:style];activityView.frame = CGRectMake([UIScreen mainScreen].bounds.size.width / 2 - size / 2, [UIScreen mainScreen].bounds.size.height / 2 - size * 2, size, size);activityView.tag = activityViewTag;[self addSubview:activityView];[activityView release];return activityView;}- (UIActivityIndicatorView *)getActivityViewAtCenter{UIView *view = [self viewWithTag:activityViewTag];if (view != nil && [view isKindOfClass:[UIActivityIndicatorView class]]){return (UIActivityIndicatorView *)view;} else {return nil;}}@end?