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iosuitableviewuiprogressview

Avoid UITableViewCell updating content when scrolled

I've found some similar questions already on SO, but nothing that seems to address this specific problem.

I'm using a UITableView with around 25 dynamic cells. Each cells contains a hidden UIProgressView. When the user taps on the download button within the cell to download that item, the UIProgressView is displayed and it indicates the progress of the download.

I've achieved this by assigning a tag to each cell which is equivalent to its corresponding downloadItemID. Each instance of MyCell has its own progressBar property.

This works fine as long as the table view is not scrolled during the download. It works also when the user is downloading multiple items at the same time. The code looks like this:

UIProgressView *theProgressBar;
for (MyCell *cell in self.tableView.visibleCells) 
{
    if (cell.tag == downloadItemID) theProgressBar = cell.progressBar;
}

float progressPercentage = (float)completedResources / expectedResources;
[theProgressBar setProgress:progressPercentage animated:YES];

The problem is that when the user scrolls the table view, the cell and progress view are transferred to another cell. It's simple enough to reset and hide the progress view for the new cell, but when the original/downloading cell is scrolled back into view, no progress is visible.

I've tried caching active progress bars into a dictionary and reallocating them back to the original cell in cellForRowAtIndexPath, but this is giving me the same result: no visible progress after scrolling the cell off and on the screen. Even if I can get them to show up, I'm doubtful I can get this to work seamlessly by rolling my own caching method.

What I need is to keep cells in memory. But can I work around dequeueReusableCellWithIdentifier? This whole problem has arisen because I had to switch to a dynamic system of allocating the cells, but it is too dynamic. Can I create the cells dynamically, but keep them in memory all the time once created, or at least keep the ones that are currently downloading?

(I understand the reasons why cell reuse is the preferred way to go with table views.)

Solution

  • You are working against the framework. As vadian says in the comment, you need to separate the logic and state information from the cells and put them elsewhere.

    Here is one way of doing it.

    • Create a class to hold each download state, like a download ongoing flag, download progress, title, URL, etc.
    • In your view controller, create and add all your download objects to an array when the view controller is created. The first object corresponds to the first row in the table.
    • Whenever you dequeue a cell, populate it with data from the array. The NSIndexPath row property serves as the index into the array.
    • All your updates (download progress) updates the download objects in the array, then update the cell content using the updated object. You can use UITableView cellForRowAtIndexPath to get the cell for a specific array index, if you get nil there is no need to update it.