What is a good way to animate UIViews inside UITableViewCell after a callback?

Question

We couldn't find a way to animate UIViews inside a UITableCell as an action from a callback.

Suppose in a scenario where you click on a button on the UITableViewCell and it fires off an asynchronous action to download a picture. Suppose further that when the picture is downloaded we want a UIView in the cell to animate the picture to give user a visual feedback that something new is about to be presented.

We couldn't find a way to track down the UIVIew to invoke beginAnimation on because the original cell that the user clicked on might now be used for another row due to the nature of cells being reused when you scroll up and down in the table. In other words we can't keep a pointer to that UITableViewCell. We need to find another way to target the cell and animate it if that row is visible and don't animate if the row is scrolled out of range.

Answer 1

Keep the cell object different from the object being animated so the cell holds a UIView. When the animation callback occurs check to make sure that the UIView still exists and, if it does, animate the changes.

When the cell object gets bumped off the screen and recycled, release the UIView that would have been animated and create a new one. When the animation callback occurs it will have nothing to do because the UIView no longer exists.

A modification of the above is to keep some sort of object in the UIView that your callback can check to see if the animation is still appropriate. This could be some sort of unique identifier for the picture being downloaded. If the identifier changes, no animation is needed. If it matches, do the animation.

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EDIT:

- (UITableViewCell *)tableView:(UITableView *)tableView
         cellForRowAtIndexPath:(NSIndexPath *)indexPath {

    static NSString *MyIdentifier = @"MyTableCell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle
                                       reuseIdentifier:MyIdentifier] autorelease];
    } else {
        UIView *oldViewToAnimate = [cell.contentView viewWithTag:1];
        [oldViewToAnimate removeFromSuperview];
    }

    UIView *viewToAnimate = [[UIView alloc] initWithFrame:CGRectZero]; //replace with appropriate frame
    viewToAnimate.tag = 1;
    [cell.contentView addSubview:viewToAnimate];

    return cell;
}

When you spawn your download process you pass in [cell.contentView viewWithTag:1]. When the download is done, it will update the appropriate view. If the table cell was reused the view will no longer have a superview and will not update the wrong cell.

There are things you can do to make this more efficient but this is the basic idea. If you have a custom UITableViewCell than this will probably look a bit different.

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EDIT 2:

To reuse the viewToAnimate objects to make sure that they get updated if their parent cells were recycled, do something like the following:

- (UITableViewCell *)tableView:(UITableView *)tableView
         cellForRowAtIndexPath:(NSIndexPath *)indexPath {

    static NSString *MyIdentifier = @"MyTableCell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle
                                       reuseIdentifier:MyIdentifier] autorelease];
    } else {
        UIView *oldViewToAnimate = [cell.contentView viewWithTag:1];
        [oldViewToAnimate removeFromSuperview];
    }

    UIView *viewToAnimate = [self viewToAnimateForIndexPath:indexPath];
    viewToAnimate.tag = 1;
    [cell.contentView addSubview:viewToAnimate];

    return cell;
}

viewToAnimateForIndexPath will need to:

• Check to see if a viewToAnimate has been created for this indexPath
• Create a viewToAnimate if there isn't one
• Save a reference to the view that can be looked up by indexPath
• Return the viewToAnimate so the table cell can use it

I don't know enough about your data structure to do this for you. Once the download process completes it can call this same method to get the view and animate it.