viewDidAppear 在它实际出现之前调用？

Question

我知道这个问题听起来类似于this，但我已经知道解决方法。这更像是一个分析问题。

上面的链接是UITabBarController特有的问题，但我也观察到UINavigationController的行为，当我pushViewController:... animated:NO时，即。没有动画。

所以我的问题是为什么视图会出现在viewDidAppear 方法之后，而它的名字暗示，并且定义说，该视图已经出现（定义说视图已经添加到窗口中）。

我在方法中标记了一个断点来查看调用堆栈，这里是推送到UINavigationController的日志：

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidApear:animated];
    NSLog("...");              // <<<<<Breakpoint here
}

这是日志：

(gdb) next
Single stepping until exit from function -[UINavigationController navigationTransitionView:didEndTransition:fromView:toView:], which has no line number information.
(gdb) next
Single stepping until exit from function -[UINavigationTransitionView _notifyDelegateTransitionDidStopWithContext:], which has no line number information.
(gdb) next
Single stepping until exit from function -[UINavigationTransitionView _navigationTransitionDidStop], which has no line number information.
(gdb) next
Single stepping until exit from function -[UIViewAnimationState sendDelegateAnimationDidStop:finished:], which has no line number information.
(gdb) next
Single stepping until exit from function +[UIViewAnimationState popAnimationState], which has no line number information.
(gdb) next
Single stepping until exit from function dyld_stub_objc_msgSend, which has no line number information.
(gdb) next
Single stepping until exit from function objc_msgSend, which has no line number information.
(gdb) next
Single stepping until exit from function -[NSObject(NSObject) release], which has no line number information.
(gdb) next
Single stepping until exit from function dyld_stub_objc_msgSend, which has no line number information.
(gdb) next
Single stepping until exit from function objc_msgSend, which has no line number information.
(gdb) next
Single stepping until exit from function -[UIViewAnimationState dealloc], which has no line number information.
(gdb) next
Single stepping until exit from function -[UINavigationTransitionView transition:fromView:toView:], which has no line number information.
(gdb) next
Single stepping until exit from function -[UINavigationTransitionView transition:toView:], which has no line number information.
(gdb) next
Single stepping until exit from function -[UINavigationController _startDeferredTransitionIfNeeded], which has no line number information.
(gdb) next
Single stepping until exit from function -[UILayoutContainerView layoutSubviews], which has no line number information.
(gdb) next
Single stepping until exit from function -[CALayer layoutSublayers], which has no line number information.
(gdb) next
Single stepping until exit from function dyld_stub_objc_msgSend, which has no line number information.
(gdb) next
Single stepping until exit from function objc_msgSend, which has no line number information.
(gdb) next
Single stepping until exit from function -[NSObject(NSObject) release], which has no line number information.
(gdb) next
Single stepping until exit from function CALayerLayoutIfNeeded, which has no line number information.

Answer 1

我发现了以下内容：

• ViewWillAppear：我使用 ViewWillAppear 通常只是为了更新表单上的数据。因此，对于上面的示例，我将使用它来实际将域中的数据加载到表单中。 UIViews 的创建是相当昂贵的，你应该尽可能避免在 ViewWillAppear 方法上这样做，因为当它被调用时，这意味着 iPhone 已经准备好向用户显示 UIView，你在这里做的任何事情将以非常明显的方式影响性能（如动画延迟等）。

• ViewDidAppear：最后，我使用 ViewDidAppear 启动新线程来处理需要很长时间才能执行的事情，例如执行 Web 服务调用以获取上述表单的额外数据。好处是因为视图已经存在并且正在向用户显示，所以您可以在获取数据时向用户显示一个很好的“等待”消息

从 SO 问题中窃取：What is the difference between -viewWillAppear: and -viewDidAppear:?