I have a UICollectionView with several cells – there are always some of them outside the viewport, so there are cells being reused.
When an event occurs I want to take a specific cell and move it to the front of the UICollectionView. In order to make moveItemAtIndexPath() work I need to know the current indexPath of the Cell that I want to move.
When I initialize the Cells in the CollectionViewDelegate with the cellForItemAtIndexPath method I save its indexPath into a variable of the Object that is the model of the Cell (not it’s actual Object, just the model). Then I look at this variable when I want to move the cell.
This is working fine as long as the cell was visible once – it seems like it only initiated then, even though it is part of the CollectionViewData all the time.
How can I make sure my CollectionViewCell has an indexPath, even when it is not visible or has not been visible yet?
If you know what data (cell) you want to present at the front (I assume top) of your UICollectionView, why don't you just update your dataSource and reload your UICollectionView?
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I have a TableView whose cells content need to be updated when it's TableViewRowAction is excecuted. More precisely I want to update a UILabel inside the cell. Setting a new text to this label inside the action is no problem but when scrolling down/up and the cell gets reloaded, then this label inside the cell has the same text as before changing it.
I know that the cells get reused and that's why I want to know how to "avoid" this effect and how to update the content of the cells properly. I have already tried to call TableView.ReloadData() which seems to solve the problem but the cells appear in a completly different order which doesn't look very nice.
The most straightforward approach is to update the data that your UITableView is drawing from (i.e. the array of data that you're populating each cell from has been updated to reflect your text change), then reload the specific cell of the UITableView:
let indexPath = IndexPath(row: 0, section: 0)
tableView.reloadRows(at: [indexPath], with: .automatic)
As you said table cells are reused , this means you have to keep a model array for the whole table and make all the changes to it , then reload the table more importantly to set the content of the cell index in cellForRowAt
To achieve this you must create an Array that acts as a dataSource for your labels. When you are changing the text for a specific label, you must update the array. You can get the exact update location from the property indexPath of the method cellForRowAt. Every single time you scroll up and down, cellForRowAt method is called. Just use something like
label.text = array[indexPath.row]
I called cellForItemAtIndexPath: in the didSelectItemAtIndexPath: method. The result was that didSelectItemAtIndexPath: was only called one time per cell. So I can tap the first time on a cell, then I can tap on another cell, but if I tap on the first cell again the didSelectItemAtIndexPath is not called anymore (but all other "new" cells).
Now I'm not calling cellForItemAtIndexPath: anymore and now didSelectItemAtIndexPath: works as expected. But why I'm not allowed to do this? Any reasons for this?
The collection view is caching instances of cells for you and you dequeue them when you create the cell at an index path. The collection view is making some assumptions about what's going on when you do this, so if you start making multiple requests for cells at the same index path strange things start to happen. Sometimes this results in visual issues, sometimes interaction issues.
If you want to get the cell at a specified index path you should ask the collection view directly instead of using your own delegate method, this will give you the existing cell on screen rather than a new cell.
I have a UITableView, and I want to change text color of the selected row. But I see every other 15 row got affected along with the one I clicked. This is tested under the master-detail sample project.
if let indexPath = self.tableView.indexPathForSelectedRow(){
self.tableView.cellForRowAtIndexPath(indexPath)?.textLabel?.textColor = UIColor.redColor()
}
I checked cellForRowAtIndexPath value in debug session, it seems returning only one object, how come other cells got affected too?
Cells are reused - almost certainly you are not resetting your cells to a base state in prepareForReuse and configuring them correctly on each call to cellForRowAtIndexPath.
When you look at a table view that can display some number of cells at once, typically there will only exist one more cell than can be shown. When a cell is moved off the screen it is placed in a pool of cells for reuse. Just before a cell moves onto the screen it is configured by you, in cellForRowAtIndexPath. If you have configured something in the cell and you do not configure that explicitly every time you return a cell from cellForRowAtIndexPath then that configuration persists in the cell that is in the reuse pool. The function prepareForReuse is also called before each cell is reused - if you have subclassed UITableViewCell then you can implement this function to return the cell to a base configuration, so that settings like text color do not unexpectedly affect multiple cells.
This approach makes it possible to scroll through an entire table view smoothly with a minimum amount of memory used. It is an expensive operation to create and destroy cells every time one disappears and a new one is required.
The simplest fix is to always set the text color in cellForRowAtIndexPath - either to the base color or to the special color you want in some cells.
If I create a UICollectionViewCell subclass like:
class StudentCell: UICollectionViewCell {
var student: Student?
}
And in my controller I implement UICollectionView's didSelectItemAtIndexPath and set the variable:
func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {
if let studentCell = collectionView.cellForItemAtIndexPath(indexPath) as? StudentCell {
studentCell.student = self.someStudent
}
}
When I click on the cell it should set student, but if the cell is scrolled off screen it seems like the whole cell might get wiped and when it comes back on screen it would need to rebuild itself in cellForItemAtIndexPath.
The problem I have is that I have a UICollectionView with cells that do not scroll off the screen that I'm storing data in. Because they don't leave the screen I assume they should not get wiped, but one of the cells does not seem to keep it's variable, which makes me think maybe it's getting wiped and I may need to move the state outside of the cells to a dictionary or something and in didSelectItemAtIndexPath instead of setting the variable in the cell I'd set the dictionary. Whenever I need to access the data instead of asking the cell for it I'd look it up in the dictionary.
But either way, I was wondering if it's possible (or a bad idea) to set it in the cell or not.
Yes, cells in both UICollectionView and UITableView can (will) be reused at the systems discretion and should not be used to store state information, only display information. Specifically, both views will reuse cells when they are scrolled off-screen, but there's no guarantee this is the only time they'll be reused. The usual way to handle this is to define some kind of cell data object which stores the data for each cell (visible and not) and refresh the cell view from that as needed/requested.
Tables display their data in cells. A cell is related to a row but it’s not exactly the same. A cell is a view that shows a row of data that happens to be visible at that moment. If your table can show 10 rows at a time on the screen, then it only has10 cells, even though there may be hundreds of rows with actual data. Whenever a row scrolls off the screen and becomes invisible, its cell will be re-used for a new row that scrolls into the screen.
As the title says, i'm trying to put one UITableView inside a custom UICollectionViewCell, i made it so it's perfectly displayed, but now i need the touches inside the Cell that contains the Table to call the didSelectRowForIndexPath method.
I have a .xib for this custom UICollectionViewCell, inside it i have the UITableView, and i have it's delegate and datasource correctly set.
Now, if i run this, the CollectionView cells are loaded with their proper data (this is, each cell with a table containing the proper rows), but if i try to select one row from one table, the CollectionViewCell is selected instead of the TableViewCell (this is expected, but not what i need).
I'd like to know if is there a way for the TableView row to get selected, even if it's inside a CollectionView cell.
Thank you and sorry for my english.