[](https://travis-ci.org/Norsez Orankijanan/BDUIViewUpdateQueue)
A singleton class facilitates queuing on updating UITableView, UICollectionView, and other UIView based classes.
This is an open-source-library-slash-UITableView/UICollectionView tutorial on updating the view asynchronously.
UITableView (and UICollectionView) expects you to have all your data ready for populating your cells before drawing them on the view. In this case, when you insert, delete, -batchUpdate…, or -reloadData, your datasource state is synced up with your view state. Everything is great.
Unfortunately, we don't always have our data ready before drawing the views. Sometimes we want to start with our minimal amount of data, then download more over time so that the app appears to be fast and ready. So you asynchronously update your UITableView. However, when the UITableView gets updated repeatedly in a short interval, which typically happens in asynchronous updates, you run the risk of missing the sync between your datasource and your UITableView state. This often causes crashes with NSInternalInconsistencyException or endupdates EXC_BAD_ACCESS errors. BDUIViewUpdateQueue helps you queue up these updates in order to avoid these problems.
BDUIViewUpdateQueue uses Grand Central Dispatch (dispatch_queue_t) to lock your view to a serial queue, then execute your block in the main queue. That way datasource will always sync up with the state of your UITableView or UICollectionView. Other views will also work.
There are a few variations of lock and update BDUIViewUpdateQueue provides:
-updateView:block:adds input blocks into a serial queue associated with the input view, then executes the blocks in sequence in the main queue-updateView:block:delay:delays the input time interval before adding inputs blocks into a serial queue associated with the input view, then executes the blocks in sequeuence in the main queueupdateView:block:waitUntil:adds input blocks into a serial queue associated with the input view, periodically executes the waitUntil block until it returns true, then executes the corresponding block in the main queue
BDUIViewUpdateQueue is a singleton class. So use +shared to get an instance.
[BDUIViewUpdateQueue shared];
BDUIViewUpdateQueue requires a UIView as an input in order to associate a serial queue with it. This means that we can use BDUIViewUpdateQueue to queue updates for as many UIViews as you want. BDUIViewUpdateQueue keeps track of dispatch_queue_t structs for you.
Inside the input block, that's where you want to update your datasource and your view. Like so…
[[BDUIViewUpdateQueue shared] updateView:self.tableView block:^{
self.items = @[@"Hello"];
[self.tableView insertIndexPaths@[[NSIndexPath indexPathWithRow:0 section:0 withRowAnimation:UITableViewRowAnimationBottom]];
}];
###Pitfalls
- Updating based on
NSIndexPathis risky. A common pattern for upatingUITableViewlike the above example is actually a pitfall especially when the your datasource items can be rearranged by another thread or changed entirely. Therefore, it's safer to update by re-finding the correspondingNSIndexPathbefore the view update, somehow. For example:
Person *john = [_items objectAtIndex:indexPath.row];
[[BDUIViewUpdateQueue shared] updateView:self.tableView block:^{
//do some work
//[self.tableView reloadRowsAtIndexPaths:@[indexPath] withRowAnimation:0]; //Using an indexPath from outside this block is risky. let's not do this.
//do this instead.
NSUInteger indexToUpdate = [_items indexOfObject:john];
if (indexToUpdate != NSNotFound){
[self.tableView reloadRowsAtIndexPaths:@[[NSIndexPath indexPathForRow:indexToUpdate section:0]] withRowAnimation:0];
}
}];
- Limit on animation items
UICollectionView, in particular, will throw anendAnimationexception even when you serially queue your update nicely if you call insert/delete/update items faster than its limit (31 animations).
The example probably deserves some explanation:
The example app, as seen in the video above, is a typcial case of using UITableView to display data rows. First, rows are loaded with some placeholder text. The thumbnail is then downloaded asynchronously. More JSON info is also downloaded asynchronously. The results get populated back into each row and the row gets reloaded by the UITableView.
In NSLog printout, you'll see how the row gets updated asynchronously. If you comment out all calls to BDUIViewUpdateQueue and rerun the app, you'll see how this app works without properly syncing, which is not pretty.
To run the example project, clone the repo, and run pod install from the Example directory first.
- iOS 6.0 (Should be okay down to iOS 4.0)
BDUIViewUpdateQueue is available through CocoaPods. To install it, simply add the following line to your Podfile:
pod "BDUIViewUpdateQueue"
Norsez Orankijanan, http://about.me/norsez
BDUIViewUpdateQueue is available under the MIT license. See the LICENSE file for more info.