Classes/RootViewController.m

/*
     File: RootViewController.m 
 Abstract: Controller for the main table view of the LazyTable sample.
    This table view controller works off the AppDelege's data model.
    produce a three-stage lazy load:
    1. No data (i.e. an empty table)
    2. Text-only data from the model's RSS feed
    3. Images loaded over the network asynchronously
 
    This process allows for asynchronous loading of the table to keep the UI responsive.
    Stage 3 is managed by the AppRecord corresponding to each row/cell.
 
    Images are scaled to the desired height.
    If rapid scrolling is in progress, downloads do not begin until scrolling has ended.
  
  Version: 1.5 
  
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 */
 
#import "RootViewController.h"
#import "AppRecord.h"
#import "IconDownloader.h"
 
#define kCustomRowCount 7
 
static NSString *CellIdentifier = @"LazyTableCell";
static NSString *PlaceholderCellIdentifier = @"PlaceholderCell";
 
@interface MyTableViewCell : UITableViewCell
@end
@implementation MyTableViewCell
- (instancetype)initWithStyle:(UITableViewCellStyle)style reuseIdentifier:(NSString *)reuseIdentifier
{
    // ignore the style argument and force the creation with style UITableViewCellStyleSubtitle
    return [super initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:reuseIdentifier];
}
@end
 
 
#pragma mark -
 
@interface RootViewController () <UIScrollViewDelegate>
// the set of IconDownloader objects for each app
@property (nonatomic, strong) NSMutableDictionary *imageDownloadsInProgress;
@end
 
#pragma mark -
 
@implementation RootViewController
 
// -------------------------------------------------------------------------------
//  viewDidLoad
// -------------------------------------------------------------------------------
- (void)viewDidLoad
{
    [super viewDidLoad];
    
    [self.tableView registerClass:[MyTableViewCell class] forCellReuseIdentifier:CellIdentifier];
    [self.tableView registerClass:[MyTableViewCell class] forCellReuseIdentifier:PlaceholderCellIdentifier];
    
    self.imageDownloadsInProgress = [NSMutableDictionary dictionary];
}
 
// -------------------------------------------------------------------------------
//  terminateAllDownloads
// -------------------------------------------------------------------------------
- (void)terminateAllDownloads
{
    // terminate all pending download connections
    NSArray *allDownloads = [self.imageDownloadsInProgress allValues];
    [allDownloads makeObjectsPerformSelector:@selector(cancelDownload)];
    
    [self.imageDownloadsInProgress removeAllObjects];
}
 
// -------------------------------------------------------------------------------
//  dealloc
//  If this view controller is going away, we need to cancel all outstanding downloads.
// -------------------------------------------------------------------------------
- (void)dealloc
{
    // terminate all pending download connections
    [self terminateAllDownloads];
}
 
// -------------------------------------------------------------------------------
//  didReceiveMemoryWarning
// -------------------------------------------------------------------------------
- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    
    // terminate all pending download connections
    [self terminateAllDownloads];
}
 
 
#pragma mark - UITableViewDataSource
 
// -------------------------------------------------------------------------------
//  tableView:numberOfRowsInSection:
//  Customize the number of rows in the table view.
// -------------------------------------------------------------------------------
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    NSUInteger count = self.entries.count;
    
    // if there's no data yet, return enough rows to fill the screen
    if (count == 0)
    {
        return kCustomRowCount;
    }
    return count;
}
 
// -------------------------------------------------------------------------------
//  tableView:cellForRowAtIndexPath:
// -------------------------------------------------------------------------------
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    MyTableViewCell *cell = nil;
    
    NSUInteger nodeCount = self.entries.count;
    
    if (nodeCount == 0 && indexPath.row == 0)
    {
        // add a placeholder cell while waiting on table data
        cell = [tableView dequeueReusableCellWithIdentifier:PlaceholderCellIdentifier forIndexPath:indexPath];
 
        cell.detailTextLabel.text = @"Loading…";
    }
    else
    {
        cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier forIndexPath:indexPath];
 
        // Leave cells empty if there's no data yet
        if (nodeCount > 0)
        {
            // Set up the cell representing the app
            AppRecord *appRecord = (self.entries)[indexPath.row];
            
            cell.textLabel.text = appRecord.appName;
            cell.detailTextLabel.text = appRecord.artist;
            
            // Only load cached images; defer new downloads until scrolling ends
            if (!appRecord.appIcon)
            {
                if (self.tableView.dragging == NO && self.tableView.decelerating == NO)
                {
                    [self startIconDownload:appRecord forIndexPath:indexPath];
                }
                // if a download is deferred or in progress, return a placeholder image
                cell.imageView.image = [UIImage imageNamed:@"Placeholder.png"];                
            }
            else
            {
               cell.imageView.image = appRecord.appIcon;
            }
        }
    }
    
    return cell;
}
 
 
#pragma mark - Table cell image support
 
// -------------------------------------------------------------------------------
//  startIconDownload:forIndexPath:
// -------------------------------------------------------------------------------
- (void)startIconDownload:(AppRecord *)appRecord forIndexPath:(NSIndexPath *)indexPath
{
    IconDownloader *iconDownloader = (self.imageDownloadsInProgress)[indexPath];
    if (iconDownloader == nil) 
    {
        iconDownloader = [[IconDownloader alloc] init];
        iconDownloader.appRecord = appRecord;
        [iconDownloader setCompletionHandler:^{
            
            UITableViewCell *cell = [self.tableView cellForRowAtIndexPath:indexPath];
            
            // Display the newly loaded image
            cell.imageView.image = appRecord.appIcon;
            
            // Remove the IconDownloader from the in progress list.
            // This will result in it being deallocated.
            [self.imageDownloadsInProgress removeObjectForKey:indexPath];
            
        }];
        (self.imageDownloadsInProgress)[indexPath] = iconDownloader;
        [iconDownloader startDownload];  
    }
}
 
// -------------------------------------------------------------------------------
//  loadImagesForOnscreenRows
//  This method is used in case the user scrolled into a set of cells that don't
//  have their app icons yet.
// -------------------------------------------------------------------------------
- (void)loadImagesForOnscreenRows
{
    if (self.entries.count > 0)
    {
        NSArray *visiblePaths = [self.tableView indexPathsForVisibleRows];
        for (NSIndexPath *indexPath in visiblePaths)
        {
            AppRecord *appRecord = (self.entries)[indexPath.row];
            
            if (!appRecord.appIcon)
            // Avoid the app icon download if the app already has an icon
            {
                [self startIconDownload:appRecord forIndexPath:indexPath];
            }
        }
    }
}
 
 
#pragma mark - UIScrollViewDelegate
 
// -------------------------------------------------------------------------------
//  scrollViewDidEndDragging:willDecelerate:
//  Load images for all onscreen rows when scrolling is finished.
// -------------------------------------------------------------------------------
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
    if (!decelerate)
    {
        [self loadImagesForOnscreenRows];
    }
}
 
// -------------------------------------------------------------------------------
//  scrollViewDidEndDecelerating:scrollView
//  When scrolling stops, proceed to load the app icons that are on screen.
// -------------------------------------------------------------------------------
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    [self loadImagesForOnscreenRows];
}
 
@end