SOLVED: It is not about UITableViewDiffableDataSource. The problem was accessing realm from 2 different thread sequentially and not getting consistent result
One of the thread was main thread and scrolling somehow kept main thread busy and triggered race condition
Everything works if user not scrolling tableview when update is happening.
If user is scrolling or just have finger on the tableview, no animations happening on update and differences does not show up.
I am not getting any error in the console
Data update code is like below:
var snapshot = tableViewDataSource.snapshot()
snapshot.deleteAllItems()
snapshot.appendSections([.conversation])
snapshot.appendItems(conversationList, toSection: .conversation)
tableViewDataSource.apply(snapshot)
Is this somehow an expected behavior?
In my experience, you have to be cognizant of when UI updates occur. UI updates always occur on the main thread.
When the user is actively scrolling, I believe this blocks the main thread, so your diffable data source is likely updating, but your app cannot update the UI until the user releases his/her finger from the display.
Related
I am facing the crash
Modifications to the layout engine must not be performed from a background thread after it has been accessed from the main thread.
I referred to the crash and added the line self.Indicator.stopAnimating() in my code. It runs fine for one or two tests, and it shows the same crash again.
All the UI handling must be done on the main thread instead of the background thread. That's what the crash says.
So move your UI specific code to DispatchQueue.main.async, i.e.
DispatchQueue.main.async {
self?.Indicator.stopAnimating()
}
That stands for all the UI specific changes. Move them to main thread.
Unrelated: use camel-casing for variable names. It must be indicator instead of Indicator.
I have a large application that uses a number of third-party libraries and am now seeing the following error in the logs: "This application is modifying the autolayout engine from a background thread after the engine was accessed from the main thread. This can lead to engine corruption and weird crashes."
From what I have found here on SO, this is probably being caused by a UI element being changed on a background thread, which is triggering the autolayout. Unfortunately, we have so many UI elements being changed by so many different moving parts that I do not know the best way to find the culprit.
Can anyone tell me how can I found out what exactly is triggering the autolayout change from a background thread?
Sometimes it's not a completion block, but anything that is performed in the background.
In my case it was notifications that where sent. I implemented iCloud in my app, which means that when data was edited or added on another device, the other app(s) receive an iCloud update/notification, which triggered an update of the UITableView.
The code to update the UITableView was not done in the MainThread. Adding the code as shown in the comments fixes it.
It seems that the api replaceCurrentItemWithPlayerItem: will stuck the main thread for some seconds, I understand that replacing the item need the information of the new item which might take some time to preload. But questions come up that why replaceCurrentItemWithPlayerItem: with a nil item object would also stuck the main thread?? It happens to me that sometimes it take more than 5 seconds to replace a nil playerItem.
I wonder what can I do to avoid the issue. Thanks for any advices!
I came across a similar blocking UI thread issue when I used UICollectionView to display and preview video in local photo library via ALAssetLibrary.
The scroll on switching videos is not smoothly,so I guess some method block UI thread.Then I use Core Animation of Instruments to analyze what exactly occupy the UI thread.In Time Profiler I found out that replaceCurrentItemWithPlayerItem need about 30ms to execute in main thread,which is more than 16ms (1000/60(fps)) result in choppy scrolling.
To solve the problem,first I tried that put replaceCurrentItemWithPlayerItem in to background thread using GCD,but It not work.I'm not sure if it is because the Cocoa itself need update UI when call replaceCurrentItemWithPlayerItem,which means the UI thread is still block.Finally I made it work by putting replaceCurrentItemWithPlayerItem at the scrolling end (the delegate func scrollViewDidEndDecelerating(scrollView: UIScrollView)).Now the scrolling is smoothly,yep!
Therefore,my advice is obvious : Using Instruments to analyse what exactly occupy the UI thread
I have an iOS application that has an NSTimer which fires every 5 seconds. This then posts a notification, telling several controllers that they must now recalculate some data and then update their UI.
When this all happens on the main thread, scrollviews can become jittery as the data is processed.
Using GCD, I have wrapped the code called when a notification is posted:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// Code here
dispatch_async(dispatch_get_main_queue(), ^(){
// Reload tableviews and UI
};
};
Unfortunately, this leads in certain cases to several threads accessing the same instance variables. This causes my application to crash as sometimes it ends up mutating an array being enumerated in another thread.
Wrapping the code in a synchronised block prevents this.
When the user scrolls the tableview, several methods are called to calculate the height etc, on the main thread. The code within the background thread is working on the code needed by the main thread. As such, this can usually further cause crashes
I have tried setting the variables to 'atomic', but this doesn't solve the issue. Ideally, I don't want to have the main thread waiting on the background thread, but I am not sure how to best resolve this issue given that they need the same data?
This is pretty classic multithreaded programming issues. There are a number of ways to solve it with basic locks (#synchronized blocks), reader/writer locks, etc but the problem is often that you can't control when the user is going to scroll or take other action. If you #synchronize, you have to do it anywhere that data is touched, including your UITableView data source methods. That can lead to stuttering if the background processing happens to be in the middle of something.**
Personally, I would use an immutable snapshot mechanism.
Have the background thread produce the results, then include just the data the UI needs to display in the notification data as an immutable snapshot (copy). That way the background thread never modifies the data the UI is currently reading for display. How you would implement this is highly dependent on how much data you are talking about and the form it takes, but the safe way would be to have copies of your classes with readonly properties. Alternatively, you can use a "frozen" flag. Make a copy, then set frozen = YES on the copy. The UI thread will only ever see "frozen" or readonly objects coming from the background thread.
The benefit is the UI never causes the background thread to stall and there are no locks required. The downside is increased memory usage, though if the amount of data is large you can use copy-on-write mechanisms to allow the background thread and UI thread to share the data, even though logically the UI thread has a completely separate copy.
** Note: In most applications, you don't have this sort of continuous background processing going on so those apps can use simpler mechanisms. This is typically a form of message passing where the background thread finishes its task and "passes the message" to the UI thread (passes the results). At that point the background work is finished so there is no concurrent modification happening.
I have a UITextField, which checks a password and then my app loads data from a remote server. While this is happening I would like a progress view to display the progress of the download. My issue is that the UITextField seems to lock up the display until it returns and therefore I cannot update the progress view on the main thread. I also would like an answer without GCD or any other kind of threading as I am using core data and that would probably overcomplicate the app. Is there a way to have the UITextField not lock up the view so that I can update my progressView on the main thread?
If your app is loading data from a remote server, then you will have to use multi-threading(GCD, etc). Otherwise it just locks up the main thread till the download is finished which makes your app unresponsive.
To keep it simple, use GCD to fetch data(raw NSData) and give it to the main thread. Do all your processing on the main thread(core data, etc) as usual.
EDIT: One more thing, it is not the textfield locking up your UI, it is the download. So I don't think you can do anything other than multi-threading to help you here.