引用navigationController对象
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@property(nonatomic,readonly,retain) UINavigationController *navigationController; // If this view controller has been pushed onto a navigation controller, return it.?
发现在控制器中就直接有这个对象属性了：
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[self.navigationController?pushViewController: printView animated:TRUE];
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[self.navigationController presentModalViewController:self.worldCitiesListNavigationController animated:YES];
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不知道的东西太多了。
