开发当中NSAlert，在删除消息，退出登录等场景中会用到。代码如下：
NSString *message = @"这里是提示的信息";
NSAlert *alert = [NSAlert new];
[alert addButtonWithTitle:@"确定"];
[alert addButtonWithTitle:@"取消"];
[alert setMessageText:message];
[alert setAlertStyle:NSCriticalAlertStyle];
[alert beginSheetModalForWindow:[self.view window] completionHandler:^(NSModalResponse returnCode) {
if(returnCode == NSAlertFirstButtonReturn){
//确定
}else if(returnCode == NSAlertSecondButtonReturn){
//取消
}
}];
