I am new in iOS development. Currently I am reading this tutorial http://www.appcoda.com/how-to-handle-row-selection-in-uitableview/ . I am facing problem when I am reading this line
UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
cell.accessoryType = UITableViewCellAccessoryCheckmark;
I know object in objective-c is created by following way
classname *objecname = [[classname alloc]init];
My confusion point is here UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
How cell object is created here? Please tell details.
Based on the tutorial you linked to, I'm assuming you are dealing with selecting a row. When you select a row, you have access to the NSIndexPath of that row, which contains two parts:
The section of the cell
The row of the cell
With that information, let's break down the confusing code: UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
UITableViewCell *cell
This part declares your variable. You've chosen to name it cell. You could just as easily have named it theCell, like so:
UITableViewCell *theCell
The * means that you're declaring a pointer, which is just a reference to an actual object, or the table view cell.
[tableView cellForRowAtIndexPath:indexPath]
The tableView refers to the UITableView that was just selected. UITableView has a method called cellForRowAtIndexPath, and what that method does is retrieve the cell at the specified section and row of the UITableView. In your case, it retrieves the row that you've just selected and stores the reference to it in your cell variable.
When declaring a new object, yes, it would take on the following syntax:
classname *objecname = [[classname alloc] init];
The key word here is new. When dealing with selecting rows in a UITableView, you don't want to create a new cell because you can't select a cell that doesn't exist. You want to get the cell that the user has just selected.
I assume you have this code in your didSelectRowAtIndexPath: method, is that right?
If so, you're tapping on a cell that is on the screen, so the cell object already exists. Where and how was it created? Inside cellForRowAtIndexPath:, which you have already written.
So when you make this call
UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
it just gives you the cell object to work with, but it doesn't create a new one.
The cell is not created with that line. That line gets a reference to a cell that is already in the table view at that indexPath.
Related
I used the code below to get an invisible UITableViewCell
NSIndexPath* path = [NSIndexPath indexPathForRow:row inSection:0];
UITableViewCell *cell = [listTableview cellForRowAtIndexPath:path];
path returns something, but cell returns nil.
It looks like the code above only can get the visible UITableViewCell.
Your comment welcome
The docs clearly say it needs to be visible.
An object representing a cell of the table, or nil if the cell is not
visible or indexPath is out of range.
Also, you are initializing the index just like how you would intialize any other variable, which has nothing to do with it having to be a valid indexPath.
I am asking very basic doubt belongs to Tableview, I created tableview programmatically without Storyboard/Xib.
The tableView numberOfRowsInSection it will return 14, the cell view fully dynamic and each cell height will different from one another.
My questaion is in tableview delegate method
static NSString *cellIdentifier = #"Cell";
UITableViewCell *cell=(UITableViewCell *)[tableview dequeueReusableCellWithIdentifier:cellIdentifer];
if (cell == nil )
{
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifer];
// how many time entering this loop
}
// ( adding subview to cell view).
cell==nil means need to enter loop right. Depending on what parameter cell object become nil? how many times it will enter, is it once? not at all.
When I checked, it entered 6 times.
if I use cellIdentifier, it will enter 14 times because Identifier different and every time it will create space for cell, its right because each time name will different and while scrolling it will reused.
NSString *cellIdentifer= [NSString stringWithFormat:#"%ld,%ld",(long)indexPath.section,(long)indexPath.row];
which basis it will enter 6 times. why not one's or 14 times. Please suggest what I did wrong. Because if used #"Cell" identifier, while scrolling repeatedly view will overlap. If I used second one cell view object will not overlaps & looks like perfect, but device memory size will increase
ref by https://developer.apple.com/library/ios/documentation/UIKit/Reference/UITableViewCell_Class/index.html
If I right got your question you need register your class in viewDidLoad method or loadView where you created a tableview like that [tableView registerClass:[UITableViewCell class] forCellReuseIdentifier:CellIdentifier]; for right reuse in tableview
Code : 1
static NSString *cellIdentifier = #"Cell";
UITableViewCell *cell=(UITableViewCell *)[tableview dequeueReusableCellWithIdentifier:cellIdentifer];
if (cell == nil ){
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifer];
// how many time entering this loop
}
In above code the loop will execute as number of visible rows in your tableview at first time. After that it will reuse cell as the cellIdentifier is same Cell and you need to update cell data as per indexpath.
it means if your table display 6 rows then it will execute for 6 times. Change rowheight and you can check.
Code : 2
NSString *cellIdentifer= [NSString stringWithFormat:#"%ld,%ld",(long)indexPath.section,(long)indexPath.row];
for above code the loop will execute total number of rows you have declared, Because it will create new cell for each indexpath.
If we give same Identifier to all cells, Disappearing cell uses the memory of Appearing cell. But, If we give different Identifier then every cell will have its own memory location and shows data perfectly.
Now suppose we have 1000 or more records to load in Table-view. If we will give different Identifiers, there will be lots of allocations in memory. This is the benefit of re-using cells.
Why 6 times ?
Because, if you give same identifiers, table will re-use cells. Maximum number of cells visible at the moment, are allocated at first. Then on scroll, appearing cell uses a memory location of a disappearing cell (cell dequeuing). So, every time you scroll, new cells are not allocated. Instead, already allocated cells are re-used
Why 14 times ?
Because, every cell has different identifier in this case. Every cell will have its own memory location.
Remember
Add subviews inside cell nil condition. Cell specific content should be assigned outside nil condition. Have a look at following code snippet:
-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
NSString *cellIdentifier = #"MY_CELL";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:cellIdentifier];
/* Everything that is similar in all cells should be defined here
like background colors, label colors, indentation etc. */
}
/* Everything that is row specific should go here
like label text, progress view progress etc. */
return cell;
}
With same identifier:
When table cell is going to disappeared then this cell will be added in stack and these are reusable.
Now when we are going to show a cell then:
If we are using same identifier then controller will check that cell is available in stack with same identifier.
If yes, then we will get a table cell which is already used and UI was already set for this cell. So we need to reset UI before using it.
If not, then it will create new cell and trying to use it.
In your case I think 4-5 table cell is visible at a time so it is creating 6 table cell and reusing those cell.
With different identifier:
Table cell cell will not available in stack at creating cell for different indexpath. So it will create new one. so cell method will be called 14 times.
I'm a little confused to why an identifer (*MyIdentifier) is always required. The code below demonstrates this. I have noticed all tableviews require at least one.
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
static NSString *MyIdentifier = #"MyIdentifier";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier];
if (cell == nil) {
cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:MyIdentifier] autorelease];
}
What is the purpose behind having an identifier? Ive have seen a few tutorials where there is more than one. Also, reading the Apple documentation, I was a little confused about why the following is called:
if (cell == nil) {
cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:MyIdentifier] autorelease];
}
I would like to know why it takes the input of 'MyIdentifier'? Assuming we had more than one identifier, which one should we choose? To be exact, what if we had a Parent, Child and sub-child etc.
Suppose you have several customized UITableViewCells and each of them have different backgound colors. In your storyboard or xib file, you may name the cell with red background color "redCell" and the one with blue "blueCell". Then you can select what kind of cell to add to a particular row using their identifiers. Let's say, you wanna apply red cells to odd rows and blue ones to even rows then you can use the following code to do this:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
UITableViewCell *redCell = [tableView dequeueReusableCellWithIdentifier:#"redCell"];
UITableViewCell *blueCell = [tableView dequeueReusableCellWithIdentifier:#"blueCell"];
if (indexPath.row % 2 == 0) return redCell;
else return blueCell;
}
Without specifying an identifier, the system wouldn't know which kind of cell to pick.
From the docs:
The identifier is a string identifying the cell object to be reused. This parameter must not be nil.
For performance reasons, a table view’s data source should generally reuse UITableViewCell objects when it assigns cells to rows in its tableView:cellForRowAtIndexPath: method. A table view maintains a queue or list of UITableViewCell objects that the data source has marked for reuse. Call this method from your data source object when asked to provide a new cell for the table view. This method dequeues an existing cell if one is available or creates a new one using the class or nib file you previously registered. If no cell is available for reuse and you did not register a class or nib file, this method returns nil.
Every time you return a cell you have 2 options - you can create new cell and return it, dequeue cell that already exists and reconfigure it.
So the way this works is when you create cell you give it reuse identifier, so when cell goes off screen it can be used instead of creating new cell. After the cell is dequeued though, you might want to change its properties(like text or image)
You can have different reuse identifiers for different cell types(different content views)
Ahoy!
I'm trying to create a reusable UIView (for various reasons) similar to the UITableViewCell implementation used in UITableViewController. I'd like to use the reusable view in a UIScrollView so I know i'm not trying to achieve something that's entirely unattainable.
The default implementation of this is:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
//declare cell identifier
static NSString *cellIdentifier = #"cell_identifier";
//dequeue cell
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];
//check cell is valid
if(cell == nil)
{
//create a new cell
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:cellIdentifier];
}
//
//return cell
return cell;
}
From this, it's worth noting that the cell is dequeued from the UITableView. If the cell is invalid, a new cell is created. My question is, how does this cell then become "queued" for reuse later?
My current attempted implementation looks like this:
- (TestScrollViewCell *)scrollView:(TestScrollView *)_scrollView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
//declare cell identifier
static NSString *cellIdentifier = #"cell_identifier";
//dequeue cell
TestScrollViewCell *cell = (TestScrollViewCell *)[scrollView dequeueReusableCellWithIdentifier:cellIdentifier];
//check cell is valid
if(cell == nil)
{
//create a new cell
cell = [[TestScrollViewCell alloc] initWithFrame:CGRectZero];
}
//
//return cell
return cell;
}
I'm thinking that adding a NSMutableDictionary to my TestScrollView to store the cellIdentifier and the TestScrollViewCell (UIView) and then plucking them back out based on the dictionary key would be a good start but is this really a true implementation of "reusable" cells?
The issue I can see is that I would then be adding the UIView to the ScrollView which is positioned based on the frame. Dequeing a view in this sense wouldn't allow me to then add the view to the scroll view without affecting the first view (by modifying the frame) but surely this is how UITableViewCells work, as well as section headers/footers?
I've been looking at this implementation which seems to be following the same route I was intending on implementing but i'm not 100% sold that this is a true implementation of reusable cells.
Has anyone had any luck with this previously? I'm trying to take Apple's lead on this one but other than UITableViewCell and MKAnnotationView (MapKit) there aren't any accessible implementations of this for me to glean from.
Any help would be greatly appreciated.
It's not just the view, it's the whole UITableViewController you'll need to recreate. The reuse flow goes like this:
dequeueReusableCell gets empty reused cell from some storage, I guess, from NSMutableArray (grab first object from array, then delete it from array and return it). If array is empty, method returns nil. You check for cell value, if it's nil, you create a new instance of cell class. If it's not nil, you fill it with your data.
This goes for every visible cell, that is, every cell that can fit on screen. Any non-visible cells are not initialized. When user scrolls the table, cell that are gone completely off-screen (not a single pixel visible) sent to reuseQueue – all their subviews and values return to default values or just nilled, and then cell gets added to the end of our NSMutableArray that is the queue.
I hope I explained well enough.
EDIT: Oh, and one more thing - you'll need different reuse queues for each reuse identifier.
I have the indexPath of type NSIndexPath*,a reference to the tableView, I want to use these to get a handle on the corresponding cell.How dp I do this?
UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
But note that that method returns nil if corresponding row is not visible at the moment (or indexPath is out of the table's range)