UIScrollView Lazy Loading Strategy With a Twist

Question

I have a horizontally scrolling UIScrollview with paging enabled. Each page represents a data feed. Each data feed consumes a fairly large amount of memory including text and images displayed in a UITableView. Since the user can have an unlimited amount of data feeds, I need to lazy load them to prevent maxing out my memory usage. My thought is to keep up to 5 data feeds in memory at any given point, and release anything outside of that range. My initial take is to keep the page in the viewport in memory, and the 2 pages to either side of it. This way when the user scrolls, the next sequential page will always be in memory and will display quickly.

Here is my problem: We also need to support a scenario where a user can skip to a specific data feed, possibly 10 or more pages to the right or left, which throws my entire lazy loading scheme out the window.

Might there be a better strategy to support this scenario?

Answer 1

Yes, what you can do is create an outer scroll view, which houses individual cells akin to a tableview. In this respect, the cells have a content view which you can place your data, let's just assume you know how to do that since it seems you do.

Once you have this architecture in your mind, it becomes fairly clear: You can, knowing the width of your cells, and the size of the screen, some simple math can tell you how many you have on screen, and you can add one to the left or right so you're preloading some data for when the user scrolls.

This will say, give you the ability to have in memory, at most 5 feeds, if 3 are visible, at the start or end of the content view in the scrollview, 4 feeds, regardless of whether or not you have a billion feeds.

One critical part of this is cell reuse. You maintain a couple NSSets, one for recycled cells, and one for visible cells. Add items that have gone off screen to the recycled cells, dequeue items from recycled cells when setting up the cell, as to save additional memory allocations, which can be expensive. Just remember, using this strategy, you are still subjective to the same caveats as a UITableView with respect to cell reuse.

I'm plugging some software I wrote here, so forgive me for that, but I'm doing so as an example of what I'm talking about if you get stuck implementing what I discussed here, it's available here for your perusal.

One final note. To support skipping of cells, it's just simple math again to adjust the point in the scrollview you are at. The action for this can be done with a gesture, though now we're talking a pan gesture recognizer, with some specific properties that will be specific to your application, as an example. Or you can use buttons if you really must. Just ensure you know how to calculate your offsets to your cells, and scroll to that point. You'll be fine.