Question

I am using the TableViewUpdates example from WWDC #2010. Basically Apple creates collapsable and expandable TableViews by clicking on the section header. The data for the TableView gets created in viewWillAppear like so:

- (void)viewWillAppear:(BOOL)animated {

    [super viewWillAppear:animated]; 

    /*
     Check whether the section info array has been created, and if so whether the section count still matches the current section count. In general, you need to keep the section info synchronized with the rows and section. If you support editing in the table view, you need to appropriately update the section info during editing operations.
     */
    if ((self.sectionInfoArray == nil) || ([self.sectionInfoArray count] != [self numberOfSectionsInTableView:self.tableView])) {

        // For each play, set up a corresponding SectionInfo object to contain the default height for each row.
        NSMutableArray *infoArray = [[NSMutableArray alloc] init];

        for (Play *play in self.plays) {

            SectionInfo *sectionInfo = [[SectionInfo alloc] init];          
            sectionInfo.play = play;
            sectionInfo.open = NO;

            NSNumber *defaultRowHeight = [NSNumber numberWithInteger:DEFAULT_ROW_HEIGHT];
            NSInteger countOfQuotations = [[sectionInfo.play quotations] count];
            for (NSInteger i = 0; i < countOfQuotations; i++) {
                [sectionInfo insertObject:defaultRowHeight inRowHeightsAtIndex:i];
            }

            [infoArray addObject:sectionInfo];
            [sectionInfo release];
        }

        self.sectionInfoArray = infoArray;
        [infoArray release];
    }

}

I’ve noticed for my case, where I have a lot of data, this is an expensive operation. I’d like to cache the data. The data gets created each time since it’s in viewWillAppear. Because I’m using a UINavigationController to push this view onto the stack, if I put it into viewDidLoad, when I move away from this view and go back to home, I have to recreate the view again, viewDidLoad will run again, and it’ll be slow again.

I haven’t cached data before and was wondering what a good way to do it would be? Right now all of the data for the row headers and rows are in a database. So when this view gets pushed onto the stack, I grab the data, and create the table. I didn’t know what a good mechanism would be to create the table and somehow cache the view or something to make it load faster on subsequent pushes of the viewController. Thanks.

Answer 1

The code you are showing is constructing the data source for the table view, and is not part of the view itself, per se. Wouldn’t it meet your needs to execute this code in the view controller initializer and/or whenever your data source requires an update?

You can draw the parallel with NSFetchedResultsController and its delegate methods. These are executed apart form the view handling methods, with the fetched results controller being an ivar or (very often) a property of your view controller. Then, for example, once a fetched results controller has done its fetch, it can manage changes on a case-by-case basis, coordinated with the table view and its controller, or you can purposely refetch entirely. The view can appear and disappear completely independent of the data source maintenace.