UICollectionView滚动到特定单元格

Question

i try to create a UICollectionView who should open at a specific cell.

My approach is to store all cells:

func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) ->
        UICollectionViewCell {
  // my code
  cells.append(cell)
  return cell
}

then find the desired cell in viewDidAppear :

let result:[MyCell] = cells.filter{ $0.myProperty!.uuid == selectedUUID }

Unfortunately the desired cell can't be found within the cells array, because not all cells are stored yet.

Am i going in the right direction or do i need a different solution?

EDIT: removed misleading information

Answer 1

Don't work with the cell themselves.

Your cells are displaying data, right? They must. That data (could be numbers, or text, or an image, anything) should be a custom object.

For example, you're displaying a list of animals.

So you have the Animal class, with different properties ( Size , weight , age ). Again, all an example. Maybe you're displaying a list of students which have a FirstName, Lastname, and so on. Lets say we're using the Animal .

You have an array of Animal objects, you can have 1, or 4, or 234234, it doesn't matter. The array can hold as many animals as you want. But you only have one array.

Your collection view (or tableview) needs to create a cell for each row. How many rows?! Easy, the other methods tells us there are a specific number of rows per section. Let's say we only have one section.

How do you define rows? You give him a number. What number? The number of animals in your array. For example, 5. Or, much better, myAnimalsArray.Count . Now if you have 20 animals, you'll have 20 cells. If you have 3 animals, you'll have 3 cells.

Okay, now to the cells, you know how many you have, and the cellForRow will therefore create X cells, X being the count of your array.

You'll probably have a simple cell for testing, like

cell.title = animal.name

and then return the cell. It's all pseudo code here, but you get the idea.

Now to your actual question

I've written all that because, according to your other comments in the other answers, you seem pretty confused.

You have your animal array (the data), and no cells yet. You want to start on the animal that has a name starting with E, for example. Well, after you've all the animals on the array, you need to find the one that starts with E (or any other filter you might like). You find it with a searching method. Once its done, you will know the index of the animal in the array. It's the array item number 23, it's the Elephant.

Create an NSIndexPath object with section 0 and index 23 (because your filter found it's the 23rd item, again, i'm just using examples), and pass that indexPath object to the scroll method of the collectionview .

And you're good ;)

Do that in the ViewDidLayoutSubviews or viewDidAppear . (first one is better). But you must do it after your array of animals is loaded, otherwise you won't have any data to filter.

Once you've called the method in the link above, your collectionview will scroll, and if you've done it soon enough, it will load at that index and not at index zero.

Answer 2

i have solved this problem through this code hope it will helpful for you:

[self.collectionview setContentOffset:CGPointMake(0,self.collectionview.contentSize.height-self.collectionview.frame.size.height/2) animated:YES];

set above only parameter to contentsize:CGSizemake(x,y);

by adjusting your required parameter you can scroll to specific cell. just replace above parameter inplace of c and y in contentsize:CGSizemake(x,y); method.

and you have to adjust parameter.

Answer 3

For your case, you need to filter data array instead of filtering cells array, this way you can get exact index.

Based on the filteredIndex create indexpath using below method,

NSIndexPath(forItem: filteredIndex, inSection: yourSectionIndex)

This way you can scroll to particular cell.

Answer 4

Try adding a call to collectionView.reloadData() before scrollToItemAtIndexPath. This will ensure all required cells are created.

Answer 5

UICollectionView's cell is reused .
For example, if view can only show 3 cells once: A, B , C; then ur cells maybe like [A, B, C, A, B, C, A, B, ...] ;

each time, u set the property of cell, uuid will be reset to a new value. so u cant filter out what u want. Hitendra Hckr is right, u should "filter data array instead of filtering cells array".

Or u can " alloc " new cell instead of " dequeueReusableCellWithReuseIdentifier:forIndexPath: ". (Not recommend)