Screenshot of weird behavior
The screenshot tells is quite well. I have a tableview with dynamic custom cells. I added a println for one of the contents of the cell to check, if the labels are set. I can see in the debug log, that each cell has its content. Still, on the device there are empty cells at random, which means, the row, where no content appears, is changing a lot. Even just scrolling up and down makes the second row disappear, but the third row is filled. Scrolling again turns this around again. If I close the app and start it again, every row is filled correctly.
Here is the code for the cell generation:
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
// Return a count picker cell
if countPickerTableRow == indexPath.row {
...
}
// Return a normal wish list entry cell
else {
let article = wishListEntries[indexPath.row]!
let cell = tableView.dequeueReusableCellWithIdentifier("ArticleCell", forIndexPath: indexPath) as! WOSArticleCell
// Correct the order in case a count picker cell was inserted
var row = indexPath.row
if countPickerTableRow != -1 && indexPath.row > countPickerTableRow {
row--
}
cell.setThePreviewImage(UIImage(data: article.thumbnail))
cell.setArticleName(article.name)
cell.setArticleDescription(article.text)
cell.setArticleNumber(article.number)
cell.setArticleCount(article.count as Int)
cell.setOrderInTable(row)
cell.setTableViewController(self)
cell.setNeedsDisplay()
cell.setInputAccessoryView(numberToolbar) // do it for every relevant textfield if there are more than one
println(String(indexPath.row) + " " + cell.nameLabel.text!)
return cell
}
}
In the custom cell class there is nothing special. Just a few outlets to the labels.
Here is a screen of the storyboard:
Storyboard
Can anyone please help me finding out what is going on here? I can't find the reason why the debug log can output the contents of a cell, but the device is not able to render them.
You should change the logic of your code. If the PickerCell comes up just call reloadData() and reload everything in the tableview. If the amount of rows you have is small this won’t be an issue and it’s not an expensive operation as you are not doing any heavy calculating during display.
If you need to update only a single cell because of changes you made in the PickerCell then you should be calling reloadRowsAtIndexPaths:withRowAnimation: with the indexPath of the cell to be updated.
Your issue is with your subclass WOSArticleCell. Have you implemented prepareForUse()? If you have, are you setting any properties to nil?
UITableViewCell Class Reference
Discussion
If a UITableViewCell object is reusable—that is, it has a reuse
identifier—this method is invoked just before the object is returned
from the UITableView method dequeueReusableCellWithIdentifier:. For
performance reasons, you should only reset attributes of the cell that
are not related to content, for example, alpha, editing, and selection
state. The table view's delegate in tableView:cellForRowAtIndexPath:
should always reset all content when reusing a cell. If the cell
object does not have an associated reuse identifier, this method is
not called. If you override this method, you must be sure to invoke
the superclass implementation.
Related
let x = X(name: "x")
blocks.append(newBlock)
let indexPath = IndexPath(row: blocks.count - 1, section: 0)
collectionView.insertItems(at: [indexPath])
let aCell = collectionView.cellForItem(at: indexPath) as! CollectionViewCell
The above code is in a function that runs when a button is pressed in the view controller to present a new item in the collection view. As collectionView.insertItems(at: [indexPath])adds a new item at the specific index, I don't understand why let aCell = collectionView.cellForItem(at: indexPath) as! CollectionViewCellwould return a nil value. The specific error is "Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value". I only get this error when the cell is outside of the screen. Meaning that if I add an item when there are few cells on the screen or if I scroll to the bottom if there are many cells and add a new item, it'll work. However, If there are many items and I do not scroll down, it'll have that error
The documentation for cellForItem(at:) says this:
this method returns nil if the cell isn't visible or if indexPath is out of range
You're getting a nil value because your cell is outside of the screen, and the force-cast is causing a crash. You should avoid force-casting in general.
Update
I think you might be unfamiliar with the way tables and collection views work. They don't store a cell for every row/item that you have, instead they create a pool of cells and reuse them.
When the cell goes out of bounds, the table/collection view stops associating it with the IndexPath and, as the user scrolls, this old cell is used again for the new IndexPath that needs to be displayed.
In general, you should configure your cell completely in collectionView(_:cellForItemAt:), and only use cellForItem(at:) to update a visible cell when the content that should be displayed on it changes.
There are 2 similar methods. UICollectionView implements the method cellForItem(at:) and the UICollectionViewDataSource protocol defines the method collectionView(_:cellForItemAt:) They do different things.
The UICollectionView method cellForItem(at:) will return a cell at the specified IndexPath if it is on screen and not out of range. It's meant for fetching cells that are currently visible.
If you call that method when the first 5 cells of your single-section collection view are visible, and ask for the indexPath of the 6th cell, you will get a nil back. That is what's happening in your code. You're trying for force cast nil to CollectionViewCell, so your code crashes. Don't do that. Check to see if you get back a nil, and handle nil gracefully.
If you scroll down so that cell 6 is visible and then ask for cell 6, it will be returned. If it's currently off-screen, it will return nil. As #EmilioPelaez says in his answer (voted), that's how the function works.
The other method collectionView(_:cellForItemAt:) is the method the collection view calls to ask its data source to create and configure cells. That method must always return a cell for any valid IndexPath in your model. You should't call that method directly though.
I'm unit testing on a tableView whether it renders a cell.
And I found that tableView.cellForRow(at:) returns nil, while tableView.dataSource?tableView(tableView:cellForRowAt:) returns the right cell.
Here's my unit test code.
it("renders one option text") {
let indexPath = IndexPath(row: 0, section: 0)
let cell = sut.tableView.dataSource?.tableView(sut.tableView, cellForRowAt: indexPath)
let cell2 = sut.tableView.cellForRow(at: indexPath)
expect(cell?.textLabel?.text).toEventually(equal("A1")) // test suceeded
expect(cell2?.textLabel?.text).toEventually(equal("A1")) // test failed
}
So I'm curious about the difference of the two methods.
Apple's document says that tableView.cellForRow(at:) returns nil if the cell is not visible, so I'v understood that tableView.cellForRow(at:) returns nil when it's under unit testing,
but I'm not sure the time order of the two methods being called and when tableView.cellForRow(at:) get the right value(cell).
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell
This method is used to generate or dequeue the cells as required by tableView. This is not the UITableView member method. Instead, it is a protocol method and another object, which will be a data source, will implement and return the value. So it will always return a cell whether we are unit testing or while debugging the app.
tableView.cellForRow(at:)
This method is not the generator method. It is a member method of UITableView as a utility method for eg. for getting selected row we use tableView.selectedRow. So it is supposed to return cell for any indexPath.
As we know UITableView doesn't create cells equal to rows drawn. Suppose you wanted to draw 100 rows then UITableView only create few extra cells apart from cells which are visible. So if you pass any indexPath which is not among the visible rows then practically that cell doesn't exist. Because tableview is waiting for you to scroll and reuse the unused cells. So whether you are doing unit testing or working on app it will always show nil for cells which are not visible.
tableView.dataSource?tableView(tableView:cellForRowAt:) will always dequeue a new cell. It isn't the one on display unless tableView is the one that called it.
I'm working through a UITableView tutorial and I've become curious about array iteration as I implement the UITableViewDataSource methods. When we call indexPath.row, is this calling a for loop for us behind the scenes? I'm asking because months back when I was learning to use data from a webservice (I've forgotten the exact steps of how I did it precisely) but I believe I needed to iterate over the array in order to present the information in the console.
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
// Create an instance of UITableViewCell, with default appearance
let cell = UITableViewCell.init(style: .value1, reuseIdentifier: "UITableViewCell")
// Set the text on the cell with the description of the item instance that is at the nth index of the allItems array, where n = row this cell will appear in on the tableview
let item = itemStore.allItems[indexPath.row]
cell.textLabel?.text = item.name
cell.detailTextLabel?.text = "$\(item.valueInDollars)"
return cell
}
Am I correct in thinking that the indexPath.row method is iterating over the array for us behind the scenes?
let item = itemStore.allItems[indexPath.row]
No, calling indexPath.row does not iterate through all rows for you. It's just a variable sent to cellForRowAt that you can access and use to create your cell. It contains information about which row in which section the function cellForRowAt was called for.
However, cellForRowAt is actually called every time a cell is going to be visible on your tableVIew. Imagine you have a dataSource with 100 items and it is possible to only see 10 cells at a time. When the tableView gets initially loaded, cellForRowAt will get called 10 times. Then, if you scroll your tableView to show 3 more cells, cellForRowAt will get called 3 more times.
First of all the tutorial seems to be pretty bad. Table view cells are supposed to be reused
let cell = tableView.dequeueReusableCell(withIdentifier: "UITableViewCell", for: indexPath)
The workflow to display table view cells is:
The framework calls numberOfRowsInSection (and also numberOfSections)
For each section/row pair in the range of the visible cells it calls cellForRowAt and passes the index path in the second parameter.
No. The indexPath is just a struct with a section and a row. You use those to directly look up the information from your array. No iteration is happening.
cellForRowAt is only called for the cells that are on screen, so the order they are called depends on which direction you are scrolling.
In fact, the UITableView doesn't even know if you are using an array, or generating the information on the fly. What you do with the indexPath row and section is up to you.
Check the documentation of UITableView and cellForRowAtIndexpath functions.
An index path locating a row in tableView.
IndexPath will be a structure, which helps you to get the specific location from a nested array. In your case of the table view, rows inside the section.
cellForRow will return a cell for particular index path(in the specified section and row)
So indexPath.section will give the section number for which the cell is going to be modified and returned to the data source. And Inside the section, and indexPath.row will be the corresponding row inside that section.
index path documentation
tableView-CellForRowAt: IndexPath
UITableView DataSource Documentation
[update]
If you want to add multiple sections, add another dataSource numberOfSections, and return the number of sections count from it.
I can link to a create a table view tutorial from Apple, but it's a long document, you can skip the starting and read the To display a section in your table view.
If you want, you can use a 2D array for keeping sections and rows. And in numberOfSections method return array.count, and in numberOfRows:inSection, return array[section].count
More examples,
example
example
thanks,
J
I have created a single prototype cell which has two labels (mainLabel and subLabel) and an uiimageview. In the uitableview I'd like to have several cells which reuse the prototype and when needed the subLabel is hidden and the uiimageview is changed with different one or with a uiswitch. The two labels have different text for each cell. Do you have any suggestions/hints in order to do it? possibly in a mvvm architecture?
I'll describe what I am doing:
I have a struct (the Model) with two properties: label and sublabel. This is then instantiate by a viewModel which provides text for each cell, done by a method called getModel(_ indexPath: IndexPath) -> cellModel { ... }. Finally in UIViewController, in tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) { ... } I am calling getModel(), using dequeueReusableCell and setting up each cell.
In getModel() there is a huuuuge switch which I use to know which cell is which
Then in uitableviewcell I have some method that hides sublabel and changes uiimageview.
It kind of works, however I have some issues with while scrolling. For example, sometimes a uiimageview is drawn in another cell, or a subLabel is hidden, even if it is not supposed to. I guess this is due because it is reusing the cell, and I am not resetting it.
Anyway, any suggestions or ideas?
I know this is overkilling...
No need for any pattern. Yes, you can use that single cell design for all cells. Just hide/empty label(s) and image view as you like per cell.
First of all you have to set default value to both the labels and imageview
i.e. (consider a title label, a sub label and a imageview)
lblTitle.isHidden = false
lblSubLabel.isHidden = false
imgViewIcon.image = nil
Then just show labels in specific condition that you want to match and set image in imageview
i.e. (consider your condition to hide sub label)
if needToHide == true {
lblSubLabel.isHidden = true
}
In my application, I have a UITableView which dynamically creates new cells as the user clicks on an "add" button. My cells have several fields that are intended to take user input. However, after creating a fourth cell, the cell contains duplicates of the input added in the first cell. For example, say each cell had a textfield
FirstCell.textfield.text = 0 <--- manually assigned
SecondCell.textfield.text = 1 <--- ..
ThirdCell.textfield.text = 2 <---- ..
FourthCell.textfield.text = 0 <--- automatically assigned
FifthCell.textfield.text = 1 <--- automatically assigned
After some digging, I believe this is due to the cells being dequeued using a reuse identifier and the cells being reused. How can I create multiple cells from the same prototype, but do not automatically hold the manually assigned values from the previous cell?
Update:
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCellWithIdentifier("MyCustomCell", forIndexPath: indexPath) as! CustonUITableViewCellClass
cellB.delegate = self
return cell
}
I tried assigning each cell's UI element in this function according to indexPath.row, but it doesn't seem to be working. It'd be working fine until I start scrolling after adding 4 rows, the cell in the first row would return indexPath.row = 4 and all the UI elements in the first row would be assigned to the value inputted on the fourth row.
As you said, your cells are being reused and that's why you are experiencing this weird behavior. You need to hold on to the assigned values for each cell and clean/set each cell every time it is reused in cellForRowAtIndexPath.
Since you have a textField you could use the delegate to respond to it's text being changed and save that value in an array, then every time a cell is reused look for that value and set it.