笔记：载入viewcontroller的几种方式

uistoryboard *storyboard = [uistoryboard storyboardwithname:@"mainstoryboard" bundle:nil];
uiviewcontroller *vc = [storyboard instantiateviewcontrollerwithidentifier:@"identifier"];
[self.navigationcontroller pushviewcontroller:vc animated:yes];


uiviewcontroller *vc = [[uiviewcontroller alloc] initwithnibname:@"nibname" bundle:nil];
[self.navigationcontroller pushviewcontroller:vc animated:yes];

uiviewcontroller *vc = [[uiviewcontroller alloc] init];
[self.navigationcontroller pushviewcontroller:vc animated:yes];