I have an odd problem I was hoping someone could perhaps explain. Searching yields nothing in this case.
If heightForRowAtIndexPath specifies a height which is taller than that of the contents drawn as a result of cellForRowAtIndexPath then upon scrolling, once the view refreshes its contents, it won't draw some of the cell contents that I've rendered (each row height is dynamic). However, if I reduce the row height somewhat, everything will be rendered correctly.
Is there some documentation pertaining to this problem? Should heightForRowAtIndexPath always return the precise row height based on contents? For example, I just made the row heights rather large so I could always see the contents, but alas, cellForRowAtIndexPath wouldn't be called as many times as it should, and general rendering problems ensued.
Apologies for the fuzzy description, but perhaps those who have experienced the same problem could offer some insight.
Cheers.
If your cell content contains dynamic content which affects the height, you have to return different height values from heightForRowAtIndexPath.
Tableview calls heightForRowAtIndexPath many times so don't do any calculations inside heightForRowAtIndexPath, instead create an array which holds the calculated height values and simply return the values from the array.
Related
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 am trying to build a UICollectionView that consists of cells that are of fixed height and varying widths. The cells can either be 100%, 50%, or 25% in width.
I am creating these cells from a custom subclass, and then I am adjusting the size in sizeForIndexPath. My problem is that when reordering cells, I get a lot of weird visual glitches as the cells have their sizes adjusted rapidly as the index path changes.
Basically, I want the size of the cell to depend on the type of cell (or the content in the cell), not the index path of the cell.
What is the best way to handle this? I thought about using multiple classes for the different types of cells, but I can't set the width of the cell with a frame when initializing it. Any ideas?
Update: Here's a link to a video showing the bug I am trying to solve
http://d.pr/v/FTM8+
Try this tutorial. It for a table view but in the conclusion some recomendations for a collection view. I cannot try this tutorial now and garaunted it works for collection view but tomorrow I will do it.
Cheers.
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)
I have a subclass of a UITableViewCell. Its got some additional items on it for a total of 3 labels and up to 2 buttons. When creating the call in cellForRowAtIndexPath I call a method resizeToFitSubviews that resizes labels, and moves buttons around to the right position, it will remove unused buttons, etc. At this point I know the final height of the cell.
I tried to store the height in a NSMutableArray in the UITableViewController so I could return it in the heightForRowAtIndexPath call but for some reason heightForRowAtIndexPath is called before cellForRowAtIndexPath. And it seams its only called once as far as I can tell.
How do I go about knowing the height, when the cell isn't yet created?
I would love to know a better solution to this, but I think after you calculate the height of the cell in cellForRowAtIndexPath you have to let the table know to reload the data in the cell, so then you get to tell it the correct height in heightForRowAtIndexPath. Of course you should make sure that you don't loop yourself by doing it again when you are in cellForRowAtIndexPath after the reload. Sucky, but works.