How to detect the end of loading of UITableView

Question

I want to change the offset of the table when the load is finished and that offset depends on the number of cells loaded on the table    Is it anyway on the SDK to know when a uitableview loading has finished  I see nothing neither on delegate nor on data source protocols   I can t use the count of the data sources because of the loading of the visible cells only

User · Answer

For the chosen answer version in Swift 3:

var isLoadingTableView = true

func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
    if tableData.count > 0 && isLoadingTableView {
        if let indexPathsForVisibleRows = tableView.indexPathsForVisibleRows, let lastIndexPath = indexPathsForVisibleRows.last, lastIndexPath.row == indexPath.row {
            isLoadingTableView = false
            //do something after table is done loading
        }
    }
}

I needed the isLoadingTableView variable because I wanted to make sure the table is done loading before I make a default cell selection. If you don't include this then every time you scroll the table it will invoke your code again.

User · Answer

To know when a table view finishes loading its content  we first need to have a basic understanding of how the views are put on screen  In the life cycle of an app  there are 4 key moments    The app receives an event  touch  timer  block dispatched etc  The app handles the event  it modifies a constraint  starts an animation  changes background etc  The app computes the new view hierarchy The app renders the view hierarchy and displays it  The 2 and 3 times are totally separated  Why   For performance reasons  we don t want to perform all the computations  done at 3  each time a modification is done  So  I think you are facing a case like this   tableView reloadData   tableView visibleCells count    wrong count oO  What   s wrong here  A table view reloads its content lazily  Actually  if you call reloadData multiple times it won   t create performance issues  The table view only recomputes its content size based on its delegate implementation and waits the moment 3 to loads its cells  This time is called a layout pass  Okay  how to get involved in the layout pass  During the layout pass  the app computes all the frames of the view hierarchy  To get involved  you can override the dedicated methods layoutSubviews  updateLayoutConstraints etc in a UIView subclass and the equivalent methods in a view controller subclass  That   s exactly what a table view does  It overrides layoutSubviews and based on your delegate implementation adds or removes cells  It calls cellForRow right before adding and laying out a new cell  willDisplay right after  If you called reloadData or just added the table view to the hierarchy  the tables view adds as many cells as necessary to fill its frame at this key moment  Alright  but now  how to know when a tables view has finished reloading its content  We can rephrase this question  how to know when a table view has finished laying out its subviews      The easiest way is to get into the layout of the table view   class MyTableView  UITableView       func layoutSubviews             super layoutSubviews              the displayed cells are loaded          Note that this method is called many times in the life cycle of the table view  Because of the scroll and the dequeue behavior of the table view  cells are modified  removed and added often  But it works  right after the super layoutSubviews    cells are loaded  This solution is equivalent to wait the willDisplay event of the last index path  This event is called during the execution of layoutSubviews of the table view when a cell is added      Another way is to be notified when the app finishes a layout pass  As described in the documentation  you can use an option of the UIView animate withDuration completion   tableView reloadData   UIView animate withDuration  0           layout done    This solution works but the screen will refresh once between the time the layout is done and the time the block is executed  This is equivalent to the DispatchMain async solution but specified      Alternatively  I would prefer to force the layout of the table view There is a dedicated method to force any view to compute immediately its subview frames layoutIfNeeded  tableView reloadData   table layoutIfNeeded      layout done  Be careful however  doing so will remove the lazy loading used by the system  Calling those methods repeatedly could create performance issues  Make sure that  they won   t be called before the frame of the table view is computed  otherwise the table view will be loaded again and you won   t be notified   I think there is no perfect solution  Subclassing classes could lead to trubles  A layout pass starts from the top and goes to the bottom so it   s not easy to get notified when all the layout is done  And layoutIfNeeded   could create performance issues etc

User · Answer

folex answer is right   But it will fail if the tableView has more than one section displayed at a time   - void  tableView  UITableView   tableView willDisplayCell  UITableViewCell   cell forRowAtIndexPath  NSIndexPath   indexPath      if  indexPath isEqual   NSIndexPath    tableView indexPathsForVisibleRows  lastObject            end of loading

User · Answer

here is how I do it in Swift 3  let threshold  CGFloat   76 0    threshold from bottom of tableView  internal func scrollViewDidScroll   scrollView  UIScrollView         let contentOffset   scrollView contentOffset y     let maximumOffset   scrollView contentSize height - scrollView frame size height       if    isLoadingMore   amp  amp    maximumOffset - contentOffset  lt   threshold            self loadVideosList

User · Answer

If you have multiple sections  here s how to get the last row in the last section  Swift 3    func tableView   tableView  UITableView  willDisplay cell  UITableViewCell  forRowAt indexPath  IndexPath        if let visibleRows   tableView indexPathsForVisibleRows  let lastRow   visibleRows last  row  let lastSection   visibleRows map   0 section   last           if indexPath row    lastRow  amp  amp  indexPath section    lastSection                  Finished loading visible rows

User · Answer

I am copying Andrew s code and expanding it to account for the case where you just have 1 row in the table  It s working so far for me   -  void tableView  UITableView   tableView willDisplayCell  UITableViewCell   cell forRowAtIndexPath  NSIndexPath   indexPath      detect when all visible cells have been loaded and displayed    NOTE  iOS7 workaround used - see  http   stackoverflow com questions 4163579 how-to-detect-the-end-of-loading-of-uitableview lq 1 NSArray  visibleRows    tableView indexPathsForVisibleRows   NSIndexPath  lastVisibleCellIndexPath    visibleRows lastObject   BOOL isPreviousCallForPreviousCell   self previousDisplayedIndexPath row   1    lastVisibleCellIndexPath row  BOOL isLastCell    indexPath isEqual lastVisibleCellIndexPath   BOOL isFinishedLoadingTableView   isLastCell  amp  amp    tableView numberOfRowsInSection 0     1    isPreviousCallForPreviousCell    self previousDisplayedIndexPath   indexPath   if  isFinishedLoadingTableView         self hideSpinner         NOTE  I m just using 1 section from Andrew s code  so keep that in mind

User · Answer

Improve to  RichX answer  lastRow can be both  tableView numberOfRowsInSection  0  - 1 or   NSIndexPath    tableView indexPathsForVisibleRows  lastObject   row  So the code will be   - void  tableView  UITableView   tableView willDisplayCell  UITableViewCell   cell forRowAtIndexPath  NSIndexPath   indexPath       if  indexPath row       NSIndexPath    tableView indexPathsForVisibleRows  lastObject   row             end of loading           for example  activityIndicator stopAnimating             UPDATE  Well   htafoya s comment is right  If you want this code to detect end of loading all data from source  it wouldn t  but that s not the original question  This code is for detecting when all cells that are meant to be visible are displayed  willDisplayCell  used here for smoother UI  single cell usually displays fast after willDisplay  call   You could also try it with tableView didEndDisplayingCell

User · Answer

Using private API    objc func tableViewDidFinishReload   tableView  UITableView        print  function      cellsAreLoaded   true     Using public API   -  NSInteger tableView  UITableView   tableView numberOfRowsInSection  NSInteger section          cancel the perform request if there is another section      NSObject cancelPreviousPerformRequestsWithTarget self selector  selector tableViewDidLoadRows   object tableView           create a perform request to call the didLoadRows method on the next event loop       self performSelector  selector tableViewDidLoadRows   withObject tableView afterDelay 0        return  self myDataSource numberOfRowsInSection section         called after the rows in the last section is loaded - void tableViewDidLoadRows  UITableView  tableView      self cellsAreLoaded   YES      A possible better design is to add the visible cells to a set  then when you need to check if the table is loaded you can instead do a for loop around this set  e g   var visibleCells   Set lt UITableViewCell gt     override func tableView   tableView  UITableView  willDisplay cell  UITableViewCell  forRowAt indexPath  IndexPath        visibleCells insert cell     override func tableView   tableView  UITableView  didEndDisplaying cell  UITableViewCell  forRowAt indexPath  IndexPath        visibleCells remove cell        example property you want to show on a cell that you only want to update the cell after the table is loaded  cellForRow also calls configure too for the initial state  var count   5       didSet           for cell in visibleCells               configureCell cell

User · Answer

I know this is answered  I am just adding a recommendation   As per the following documentation  https   www objc io issues 2-concurrency thread-safe-class-design   Fixing timing issues with dispatch async is a bad idea  I suggest we should handle this by adding FLAG or something

User · Answer

Quite accidentally I bumped into this solution    tableView tableFooterView   UIView   tableViewHeight constant   tableView contentSize height   You need to set the footerView before getting the contentSize e g  in viewDidLoad  Btw  setting the footeView lets you delete  unused  separators

User · Answer

In Swift you can do something like this  Following condition will be true every time you reach end of the tableView  func tableView tableView  UITableView  willDisplayCell cell  UITableViewCell  forRowAtIndexPath indexPath  NSIndexPath            if indexPath row 1    postArray count               println  came to last row

User · Answer

Here is how you do it in Swift 3    override func tableView   tableView  UITableView  cellForRowAt indexPath  IndexPath  - gt  UITableViewCell        if indexPath row    0              perform your logic here  for the first row in the table

User · Answer

Objective C   self tableView reloadData    self tableView performBatchUpdates                                   completion   BOOL finished                                          table-view finished reload                                     Swift  self tableView  reloadData   self tableView  performBatchUpdates      - gt  Void in                                 completion     Bool finished  - gt  Void in                                     table-view finished reload

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In iOS7 0x the solution is a bit different   Here is what I came up with       -  void tableView  UITableView   tableView        willDisplayCell  UITableViewCell   cell      forRowAtIndexPath  NSIndexPath   indexPath       BOOL isFinishedLoadingTableView    self isFinishedLoadingTableView tableView                                                                indexPath indexPath       if  isFinishedLoadingTableView            NSLog   end loading             -  BOOL isFinishedLoadingTableView  UITableView   tableView                           indexPath  NSIndexPath   indexPath          The reason we cannot just look for the last row is because         in iOS7 0x the last row is updated before        looping through all the visible rows in ascending order         including the last row again  Strange but true      NSArray   visibleRows    tableView indexPathsForVisibleRows        did verify sorted ascending via logging     NSIndexPath  lastVisibleCellIndexPath    visibleRows lastObject          For tableviews with multiple sections this will be more complicated      BOOL isPreviousCallForPreviousCell                 self previousDisplayedIndexPath row   1    lastVisibleCellIndexPath row      BOOL isLastCell    indexPath isEqual lastVisibleCellIndexPath       BOOL isFinishedLoadingTableView   isLastCell  amp  amp  isPreviousCallForPreviousCell      self previousDisplayedIndexPath   indexPath      return isFinishedLoadingTableView

User · Answer

The best approach that I know is Eric s answer at  Get notified when UITableView has finished asking for data   Update  To make it work I have to put these calls in -tableView cellForRowAtIndexPath    tableView beginUpdates    tableView endUpdates

User · Answer

Swift 2 solution      willDisplay function override func tableView tableView  UITableView  willDisplayCell cell  UITableViewCell  forRowAtIndexPath indexPath  NSIndexPath        let lastRowIndex   tableView numberOfRowsInSection 0      if indexPath row    lastRowIndex - 1           fetchNewDataFromServer               data fetcher function func fetchNewDataFromServer         if  loading  amp  amp   allDataFetched               call beginUpdates before multiple rows insert operation         tableView beginUpdates              for loop            insertRowsAtIndexPaths         tableView endUpdates

User · Answer

Here is what I would do    In your base class  can be rootVC BaseVc etc    A  Write a Protocol to send the  DidFinishReloading  callback    protocol ReloadComplition  lt NSObject gt   required -  void didEndReloading  UITableView   tableView   end   B  Write a generic method to reload the table view   - void reloadTableView  UITableView   tableView withOwner  UIViewController   aViewController   In the base class method implementation  call reloadData followed by delegateMethod with delay   - void reloadTableView  UITableView   tableView withOwner  UIViewController   aViewController        NSOperationQueue mainQueue  addOperationWithBlock             tableView reloadData           if aViewController  amp  amp   aViewController respondsToSelector  selector didEndReloading                   aViewController performSelector  selector didEndReloading   withObject tableView afterDelay 0                        Confirm to the reload completion protocol in all the view controllers where you need the callback   - void didEndReloading  UITableView   tableView        do your stuff       Reference  https   discussions apple com thread 2598339 start 0 amp tstart 0

User · Answer

Are you looking for total number of items that will be displayed in the table or total of items currently visible  Either way   I believe that the  viewDidLoad  method executes after all the datasource methods are called   However  this will only work on the first load of the data if you are using a single alloc ViewController

User · Answer

I always use this very simple solution   - void  tableView  UITableView   tableView willDisplayCell  UITableViewCell   cell forRowAtIndexPath  NSIndexPath   indexPath       if  indexPath row     lastRow             end of loading           for example  activityIndicator stopAnimating

User · Answer

Here s another option that seems to work for me  In the viewForFooter delegate method check if it s the final section and add your code there  This approach came to mind after realizing that willDisplayCell doesn t account for footers if you have them   -  UIView   tableView  UITableView   tableView viewForFooterInSection  NSInteger section         Perform some final layout updates   if  section      tableView numberOfSections  - 1          self tableViewWillFinishLoading tableView             Return nil  or whatever view you were going to return for the footer   return nil     -  CGFloat tableView  UITableView   tableView heightForFooterInSection  NSInteger section        Return 0  or the height for your footer view   return 0 0     -  void tableViewWillFinishLoading  UITableView   tableView     NSLog   finished loading        I find this approach works best if you are looking to find the end loading for the entire UITableView  and not simply the visible cells  Depending on your needs you may only want the visible cells  in which case folex s answer is a good route

User · Answer

Swift 3  amp  4  amp  5 version   func tableView   tableView  UITableView  willDisplay cell  UITableViewCell  forRowAt indexPath  IndexPath        if let lastVisibleIndexPath   tableView indexPathsForVisibleRows  last           if indexPath    lastVisibleIndexPath                  do here

[ios] How to detect the end of loading of UITableView

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