I'm trying to set up a particularly large UICollectionView, and I'm embedding it within a UIScrollView (as per this answer to another question on SO) in order to support scrolling in both directions. I've got it up and running, everything is working perfectly, except that performance takes a HUGE hit as the size of the CollectionView grows.
After some hunting around, I was able to figure out the problem: because that SO answer calls for the CollectionView to be resized so that all the cells are visible at once, "dequeuing" isn't a thing anymore. The CollectionView within the ScrollView has dimensions of 2000x2000 (or whatever), so even though the ScrollView is just the size of the screen and only shows 20 cells or so at a time, the CollectionView thinks all cells are visible and needed right now. Thus, every cell in the CollectionView is generated at once, meaning the page takes a ludicrously long time to appear.
Is there a way to communicate to the CollectionView which cells are currently visible through the ScrollView, so it doesn't generate unnecessary cells and dequeues them as normal? Or if not, is there a way to make a CollectionView that scrolls diagonally, but that doesn't have this problem?
Related
I have custom UICollectionViewCell in xib. When ViewController is loaded - awakeFromNib() in cell subclass called 6 times (the screen initially placed 6 cells, ok), BUT when i start to scroll, awakeFromNib() is called six times more, causing lag. But after this "re-load" scrolling begins to work smoothly. This happens only the first time at scroll start. dequeueReusableCell, obviously, is used.With UITableView instead collection the situation is similar.
Feel like the next 6 cells are loaded from the xib again (in the collection of just 100+ cells), ignoring reuse. What could be the problem?
For clarity:
UIColectionView with Vertical scroll direction, two cells in line. When ViewController loaded i see 6 cells on screen and 6 awakeFromNib calls in console.
I start scroll down to 7-8 cells, before their appearance on the screen i see lag and 2 calls of awakeFromNib method.
Similarly for the following two pairs of cells.
After this scroll begins work fine, until cells 99-100+, and awakeFromNib is never invoked.
Added:
OK, here's what I found out:
I have a subView inside a collectionView (header like in UITableView), and, accordingly, collectionViewLayout set sectionInsets top = height of header. If I remove the header, and make the top insets, for example: -180 (about excluding the height of NavigationBar, so the screen initially fit 12 cells and the scroll lag is gone.
Ie, the system needs to load from the xib at all the cells that will be one-time visible on the screen and reuse them in future. (But why not one time load xib and reuse it immediately for all other initially cells?)
And now the question - how to fix it?
Added.2
Working solution - add header in viewDidAppear method of ViewController, but this... hm.. not good.
This is actually a known side effect (or purposeful design?) of UICollectionView & UITableView. Here's a related question (talking about NSTableView, but the concepts/issues are the same):
awakeFromNib method called multiple times
Your collection view initially displays six cells to start with, but it sounds like iOS is allocating six more cells to allow for scrolling left & right. So there are at least 12 (reuseable) cells in memory.
If you're doing a lot of work in awakeFromNib (which is what's contributing to the lag), you might want to try to determine how to optimize things.
You can also find some additional helpful hints in this related question:
UITableViewCell - Best place to set up the cell
This video demonstrates the issue I am having
UITableView screen lag
I am loading a xib file with a fair bit of stackView and UIViews into my tableview cell. Each cell takes up about 80% of the screen so you would see two cells at max.
I am experiencing a small lag when I am scrolling to the third cell. If I scroll back up again and scroll down like what I did, the lag does not appear to be there anymore and the tableView scrolls smoothly for the remaining cell. If I reload the table and repeat the above step, I will get that lag again when third cell is just about to appear.
It "feels" like the program takes longer to load the xib file in cellForRowAtIndexPath for the very first time. And once it is loaded, it could be loaded nice and fast for any other cell. (That is just a feel though)
All my views have static data. I am finding that this small lag decrease as I removed more view from the xib file.
Is this pattern normal?
Note. In the video when I pull to refresh, the table is reloaded. I have reloaded the table just before the video starts. You will see that at the beginning and whenever third cell is just about to appear (or half way through scrolling second cell) there is a small jump in the table. And after that, everything scrolls butter smooth.
Xib file
XibFile screen shot
I am trying to do something like loading up different type of cells with custom height in a uitableview. The tableview cells are subclassed and consists of labels with the respective constraints. Each cell is having a dynamic height.
Now even before my table reloads the data, I am calculating the height that is required for the resizing of the cells and caching it in my model class so that I dont have to calculate the height when the data is rendered on the device.
To calculate height i did use the tutorial from Ray Wenderlich and I am having the right set of heights applies to the objects.
Now the problem comes. Whenever I am dequeueing the cells there is a
kind of a small jerk that gives me an indication that my cell is
dequeued while scrolling.
How can i make these movement smooth so that there is no jerk while scrolling the view ?
The height is getting assigned in and does get the value as per the current type of data getting loaded.
estimatedRowForIndexPath
Also I am calling layoutIfNeeded from my cellForAtindexPath
Suggestions are most welcome.
It's very hard to say without seeing your code in cellForRowAtIndexPath, and without seeing your cells and their respective code. Here are some general questions I would investigate:
What is the content of the cells and how complex is the view hierarchy in the cell?
Even though you are supplying the correct estimated height, an autolayout pass still needs to happen, and a complex view hierarchy will take time to resolve
Does the cell contain images?
Images that need to be decompressed from a file (UIImage imageNamed:) can be intensive and cause scrolling issues, check images are not bigger than they need to be. If needed, bump this work onto a background thread.
Are you calling a complex method to configure the cell for display in cellForRowAtIndexPath?
Look at the work actually being done in cellForRowAtIndexPath, is there a complex method being triggered in you cell subclass or view model?
Are you adding and removing views to the cell view hierarchy in cellForRowAtIndexPath?
If views are being added, removed, created, inflated from a xib, constrained etc during the cell config, this could slow things down. Try to do only what is strictly needed. Check if there is any code being run internally in the cell subclass during cellForRowAtIndexPath that could be moved to cells initWith... or awakeFromNib methods (ie code that could just run once when the cell is created, rather than every time the cell is displayed)
Also run the Instruments time profiler, see if that offers any more clues
I have a UITableView that reads information from CoreData via the proper mechanisms (using a FetchedResultsController, etc). This information is either textual, or a URL to a local image to load into the tableview.
Data needs to be populated in the table in a bottom-up fashion (similar to a messaging app). I am targeting iOS 8+, but if I use estimatedHeightForRowAtIndexPath, I get terrible jerkiness on 3+ multi line labels and images. The estimate seems way too far off unless it's a one line UILabel. My hunch is that the cell height is being estimated in a top down manner, such that cell heights are growing from top of cell to bottom of cell. This means that scrolling top to bottom is fine, but bottom to top is not, since the cell is being resized "downward" dynamically as I scroll upward.
I am currently using heightForRowAtIndexPath to calculate cell heights. The problem with this is that it takes a very long time for the view to initially load because cell heights are all calculated at once. I am using cell height caching to store cell height so that once the view has loaded, scrolling is buttery smooth.
So my question is this: how do you use heightForRowAtIndexPath without taking the 3-5 second initial load hit?
And follow up bonus question, is there any way to reliably use estimatedHeightForRowAtIndexPath when you have cells that are vastly different in height? We're talking anywhere from 44px to 300px. From what I've read, I can't use the estimatedHeight calculation at all in this situation.
I've exhausted all of the stackoverflow posts concerning estimatedHeight/heightForRowAtIndexPath and I'm now starting to look at the same posts more than once. So I'm stuck.
why woncha stuff a few rows in the table to populate the visible area and after
the viewDidAppear start stuffing older messages on top of the table one or two
at the time with animation none, automatic or whatever.
this way with the postponement of the uitableview population
me thinks you'd get a passable performance.
or you could do it the skype way, postponing population of the table
with older messages until after table bounces off the top edge.
Context:
Building an app that populates a table that takes in data from a asyc json dump.
The cells are of a custom class (I defined). The main label in the cell can be very long.
It is "placed" in storyboard within a prototype cell but customized via code (pretty standard stuff).
Labels are resized in cellForRowAtIndexPath and rows are resized via heightForRowAtIndexPath -- rows are resized by forcing a call to cellForRowAtIndex like Massimo's answer here
So per the question at hand - I've noticed some interesting (bad) things that happen.
First issue: When the table loads, the rows and labels are dynamically resized correctly! Great! However, when I scroll down and then scroll back up, the label heights will be incorrect -- (for example) the first row was correct at loading. Then when I scroll down and then scroll back up to see it again, it will be truncated. Specifically, the row size will be fine but the label height will change and become truncated to 2 lines only. Wondering if this is because I did both storyboard and coding to customize the cell. Anybody see this before?
Second issue: When I scroll down, while the rows are sized correctly (large), the labels are short (truncated.) Wondering if it's some reverse of the above "potential answer".
"potential answer" is that the rows are all calculated and stored "up front" so that scrolling down/then back up doesn't affect it. However, when cells go "out of view" and are dequeued then when they re-viewed (scroll down/then back up) it will rely on the storyboard.(inappropriately?)
All three of your issues are symptomatic of returning the wrong height in heightForRowAtIndexPath. In my data model classes I have a calculateHeight method that I call in heightForRowAtIndexPath. The model also caches the answer so it doesn't have to recalculate it after the first call. The cell class uses the model's calculated height to layout its subviews.
"ANSWERED" by deleting the prototype cell from the storyboard and making them fully in code, the issue went away. The fundamental workings are still not understood (ie. the interactions between storyboard vs. code when cells are put queued and then viewed again)