It appears that presenting and dismissing a view controller both prompt the presenting view to layout its subviews and/or update its constraints. With a heavy view hierarchy, this is introducing performance issues. Again - this is the existing, currently displayed view. The modal being created and displayed is very light.
This occurs whether I use autolayout (as in my example project) or not.
I have built a demo project that approximates an app I am working on. There is a main parent controller with a horizontally scrolling UIScrollView. Multiple child controllers are added to the parent controller, and their views are added to the scrollview and arranged using NSLayoutConstraints. Each child view has one subview itself, a simple UIView, also arranged with a constraint.
In the navigation bar, there is a button to launch a modal. When presented, the parent controller makes a call to setNeedsLayout on each child view, multiple times. In my demo project, I am overriding setNeedsLayout to log when it is accessed. The same occurs when closing the modal. Open and close the modal a few times and observe the console.
I can see no reason why a new layout is needed, and with more complex views I am finding that hundreds of these calls are firing, with a noticeable performance impact.
Note that when the layout code from ChildView is omitted, setNeedsLayout is not called. I encourage you to comment out the constraints and see the difference in the logging.
Why is this happening? How can I prevent an unnecessary layout pass when presenting and dismissing a modal?
First of all, you are logging setNeedsLayout, which is just a flagging mechanism and does not really incur any work yet. Multiple calls to setNeedsLayout may only trigger a single layout. You should be logging -[UIView layoutSubviews] or -[UIViewController viewDidLayoutSubviews] instead, because these are where the actual heavy-lifting happen.
Second, layout-related methods are meant to be called repeatedly and rapidly during presentations because:
The window needs to rotate all its subviews to respect the presented view controller's preferred interface orientation.
Animations will need to know the initial and final states of your views.
When layouts happen on parent views for whatever reason, all their subviews (which may include views of your view controllers) will of course need to update their layouts too.
If you want to minimize the number of layout passes, you can try give up using presentViewController:animated: and instead use addChildViewController: and animate just the necessary views manually. But even then, you may still trigger the parent controller's layout anyway.
You are doing a very, very, very odd thing: you're maintaining a custom parent view controller with 10 child view controllers all of whose views are in the interface simultaneously. View controllers are not designed for that sort of thing. It is this that is triggering the multiple layoutSubviews calls that you are seeing. It is fine to have multiple child view controllers, but their views should not all be in the hierarchy - especially in your case, where only one such child view is actually visible.
In fact, the interface that you've constructed - a paging scroll view, each of whose "pages" is a view managed by a view controller - is already implemented for you by UIPageViewController, which is far more efficient, as it only actually maintains at most three view controllers at a time: the view controller managing the visible view within the scroll view, and the view controllers managing the views to its right and left. It is also wonderfully convenient and easy to use.
So either:
You should use UIPageViewController, or
You should imitate what UIPageViewController does, removing view controllers' views (and perhaps even releasing the view controllers) when they have scrolled out of sight - as we had to do in the days before UIPageViewController existed - see the Advanced Scroll View Techniques video from WWDC 2011. My Latin "flashcard" app worked this way before UIPageViewController came along; it has thousands of vocabulary cards, each of which is managed by a view controller, but only a maximum of three card view controllers ever exist at any one moment.
(By the way, you should also not be using your own self.childControllers mutable array, as this list is already maintained for you as self.childViewControllers.)
I think layoutSubviews is getting called because the presenting controller's view changes superviews while animating out of the screen once hidden by the presented view.
If you want to avid skip layoutSubviews when the frame hasn't changed, just save a reference to the last frame and if equal return without doing anything. Also there is no need to call setNeedslayout on subviews as the system will trigger it automatically if you resize them.
Anyway, your main problem is your approach:
Only use view controllers if you're going to use them as such (inside a tab bar controller, pushed to a navigation controller, as a window's rootController, presented modally, etc.). If you want to manually add views do not use view controllers and just use custom views! This is a very common error and you can see more details here.
Load views and objects lazily and reuse them. For instance you should only load 1~3 pages of contents and load new ones only when the user scrolls to them. When loading a new one remove one of the old views, or better yet reuse it.
You can separate the logic not only with controllers but also with custom views. Some reasons why you should not use controllers in your particular case:
Controllers won't get retained by a container controller or window as you're manually adding their views.
Controllers won't get orientation, memory, viewDidAppear, etc., events. Again because you're not using them as proper view controllers.
If you properly implemented a custom container controller (which is a lot of work to do properly), then you could use controllers. Otherwise stick to custom views.
Related
For now I have a single login view controller that hide and adds subviews dynamically based on user interaction. Is it better to have multiple UIViews with one view controller, or multiple view controllers?
The benefit of having children UIViewControllers would be if you needed to care about the view lifecycle in any of the children. For instance, if in some of your subviews (child views) you needed to know if it appeared to trigger some logic, then it would be good to use children UIViewControllers.
Another reason might be, if you wanted to use view controller transitions within your custom container, as described by Apple in the UIViewController class reference:
https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIViewController_Class/index.html#//apple_ref/occ/instm/UIViewController/transitionFromViewController:toViewController:duration:options:animations:completion:
But, to me it doesn't sound like you would need this, it sounds like you would just show/hide your views, or otherwise animate them.
But if your views don't care about any of that, and just need to render UI elements, I'd say using children UIViewControllers adds additional complexity without much benefit.
This is under the assumption that either way, all your views are living on the single UIViewController's view. If you are pushing and popping view controllers inside a UINavigationController, or presenting ViewControllers modally, you definitely should use UIViewControllers for that.
Also, you definitely shouldn't put all your view code inside that single view controller. You should use subclasses of UIView, or some other mechanism of making sure you separate your components.
For example you might have:
LoginViewController - organizes and hides and shows the individual views
SignInView - shows the sign in form
RegisterView - shows the register form
...etc
Of course its better to have multiple view controllers for many reasons:
Each view controller has its own code/logic/functionality.
Better for memory management. Once you're done from a view controller, system will automatically deallocates it from memory. Using multiple views will be overload in your system when some views are not used.
Better organisation for your project. You will just present/dismiss/push/pop view controllers and pass data between each other. Instead you will deal with horrible logic for hiding and showing UIViews.
Using one view controller will have a massive amount of code which in long term the project will be an impossible mission to manage.
Having multiple view controllers is better in the sense that you would have more ease at coding them individually. Also Navigation will be good which is good for User Experience.
Memory Management will be efficient.
The MVC architecture(Model View Controller) will instead become Massive View Controller which will be a headache while debugging.
Check out this answer for more clarity. I think you are confused between ViewController and View.
Check out the accepted answer of this question in the link. Hope it helps.
About viewController's "viewDidLoad" and "viewWillAppear" methods
All,
I am running into some performance/memory issues when using several ChildViewControllers in my ParentViewController. Here is my situation: I have a ParentViewController with a dynamic number of ChildViewControllers - some times as many as 20. They are contained in a UIScrollView, and are paged through. I'm running into problems when I have several on the page (I'm only loading the first two, then building the others as I swipe), however, having that many within the ParentViewController is starting to cause some crashes due to memory.
The ChildViewController has a lot going on in it, and I'm going through it to make sure it's as efficient as possible, however, I have concerns about this approach working on older devices (as is, I'm getting crashes on the 5S).
It seems that it would help to change the view controllers to just be views, but it'd be a pretty large endeavor as the VC is complex. One suggestion I had was to create a view from the existing view controller's view, and set several delegate methods on the view and interact with the views from the ParentViewController that way. Does any one have any thoughts on the efficiency of that method as a opposed to the current method of using ChildViewControllers?
Another thought I had was to build a custom ContainerViewController and have all the children in there to swipe through, but I wasn't sure if that would give me an advantage over using the children in a UIScrollView.
Any thoughts?
I personally would not advocate the refactoring of your code to use views rather than view controllers. The view controller, itself, is unlikely to be the source of the memory problems, but rather the model objects they keep track of (as well as the assets the view controller's view uses). I think the key is to simply remove the view controllers (and their views) as they scroll off of the screen.
In your scrolling logic, as you're adding child view controllers that scroll into view, you are presumably doing all of the appropriate containment calls:
UIViewController *newChildViewController = ...
[self addChildViewController:newChildViewController];
newChildViewController.view.frame = ...;
[self.scrollView addSubview:newChildViewController.view];
[newChildViewController didMoveToParentViewController:self];
(See WWDC 2011 video Implementing UIViewController Containment for a discussion about why it's important to do these containment calls, namely to keep your view controller hierarchy synchronized with your view hierarchy.)
As the child views scroll out of view, you just do the appropriate containment calls to remove the child controller (and its view):
[childViewControllerToRemove willMoveToParentViewController:nil];
[childViewControllerToRemove.view removeFromSuperview];
[childViewControllerToRemove removeFromParentViewController];
// also remove any other strong references you have to that childViewControllerToRemove
Alternatively, you might want to contemplate using a UIPageViewController which (in iOS 6+) offers scrolling page view (UIPageViewControllerTransitionStyleScroll) for the transitionStyle. This simplifies the amount of custom container code you have to write to handle the view controllers that scroll in and out of view. The UIPageViewController is designed precisely for this situation of scrolling (or paging) through a bunch of different view controllers' views. See the Page View Controllers discussion in the View Controller Catalog for iOS.
I don't think moving them all to UIView's will help. You could achieve this same effect by just adding aChildViewController.view without ever technically adding it as a childView. I would optimize the loading of the views into your UIScrollView. Make sure you only have, say 4 views loaded into memory at any one time. Another option would be to use a horizontal UITableView or UICollectionView so you can reap the memory management features they have build in.
You could turn it into a tableview and let the cell dequeue take over. You would only have a few in memory at a time then.
I am wondering what happens if a single instance of a UIView object gets added as a subview of multiple other views simultaneously.
If UIView:removeFromSubview: gets called then does it get removed from all superviews or just the currently displayed one?
For background:
I have a status-bar like view object that needs to be displayed within several different other views (each other view is managed by its own view controller).
[i.e. a) the user is in one view, b) something happens to make the status-bar-like view appear, c) the user switches to another view d)the status bar is still visible in the new view e) the status bar expires after a time and disappears from site. And so on]
Initially I implemented this by adding/removing it as required as a subview of the window, and this was managed by a singleton.
However due to some complications with some animations I have instead added it as a subview of each of the main view's for each of the view controllers.
Note that there are not multiple copies
When the view needs to be removed I am calling its removeFromSuperview:, and everything is all working perfectly.
However I am wondering what the situation is regarding the removal of the view, is it being fully removed or is there something else I need to do?
For example the view might get added to N view controller's views by calling addSubview as required (it will only get added to each view controller if that view controller actually launches)
However when it is being removed I am only calling removeFromSuperview: for the view of currently loaded view controller, not all view controllers it might have been added to.
Next time I navigate to one of these other view controllers it displays fine without the view being there, even though I didn't explicitly call removeFromSuperView.
As I said everything is working as it is, however at the back of my mind I feel there might be something missing?
Hope this was understandable.
You can only have it added to one view. Documentation is your friend!
(void)addSubview:(UIView *)view:
Views can have only one superview. If view already has a superview and that view is not the receiver, this method removes the previous superview before making the receiver its new superview.
From my point of view, having to add a same view to different parent views (and more important, from different view controllers) is an indication that something is wrong on the design...
However, if you really (really) need so, I had always thought that a view instance could have one and only one parent view... Moreover, you can access it by [myView superview] message, which gives you a UIView instance instead of an array... It may auto remove from its old parent before adding to a new superview?
About the design, what about creating it each time you need a new one and have a singleton to manage their status/logic?
Good luck with that!
When using custom container view controller, I don't quite understand why the presenting view controller needs to specify the from, because being the container class, it should already know what's in the view hierarchy, no?
transitionFromViewController:toViewController:duration:options:animations:completion:
Container view controllers sometimes need to put the views of the contained controllers into specific subviews of their own view. (For example, a SplitViewController reimplementation might have left and right positioning views holding the master and detail controller views respectively.) Providing the fromViewController tells UIViewController where in the view hierarchy the new controller's view should be inserted, and also which specific view should be removed after the animation.
(contrary to another answer, the frames of the views aren't set for you at all. You do that, before the call, and in the animation block. The "Creating Custom Container View Controllers" system guide in the docs has a pretty good example.)
As it happens, actually using transitionFromViewController:... appears to be optional. You can manage your view hierarchy manually, with or without animations, and it works fine. I'm still forming my opinions, but I think I prefer to do it manually, to more easily handle cases where one of the VCs is nil.
This is done this way to allow you to have a view controller that has views with viewControllers in it. The from defines the originating view controller and gives the system the ability to position the animations appropriately.
Imaging you had a view with 4 views in it like tiles. the main view controller can consecutively call this on its "child" view controllers and with the from -> to specification, it won't make the assumption that the caller is the from viewController.
There are two UIViewControllers: MyParentViewController, and MyToolbarViewController. MyParentViewController manages a view which fills most of the screen. MyToolbarViewController is a subview of MyParentViewController, and manages a comparatively small view at a higher z-index than the view managed by its parent, MyParentViewController.
Because MyParentViewController's view is higher up in the view hierarchy, its shouldAutorotateToDeviceOrientation method seems to trump that of MyToolbarViewController's. Whatever it returns is the value that is used. I am not seeing MyToolbarViewController's didAutorotateFromDeviceOrientation get called, however. Is it my responsibility to override MyParentViewController didAutorotateFromDeviceOrientation and call the method on the subview?
In iOS 4.x, Apple defines all the "legal" container controllers (split view, table view, navigation view, and tab view). Those controllers manage all view management methods (viewWill…, viewDid…, shouldAutoRotate…) and forward them appropriately.
If you try to do something similar, none of those methods are forwarded. Currently, if you need to act on these methods, the "top" controller is responsible for that.
[read between the lines for a future answer]