我正在尝试将 Lazy Tableview 加载到情节提要的自定义单元格中，但表格视图看起来只是空白：
这是源代码：
延迟加载表.m
#import "CellData.h"
#import "LazyLoadTableView.h"
#import "CustomCell.h"
#import "ParseOperation.h"
#import "ImageDownloader.h"
#define kCustomRowHeight 157
#define kCustomRowCount 1
static NSString *const xmlDataUrl =
@"http://itunes.apple.com/us/rss/topfreeapplications/limit=300/xml";
@interface LazyLoadTableView ()
@end
@implementation LazyLoadTableView
@synthesize tableElements;
@synthesize queue;
@synthesize connection;
@synthesize xmlData;
@synthesize tView;
@synthesize imageDownloadsInProgress;
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
if (self) {
// Custom initialization
self.tableElements=[[NSMutableArray alloc] init];
self.imageDownloadsInProgress = [NSMutableDictionary dictionary];
}
return self;
}
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view.
self.title=@"Menu";
NSURLRequest *urlRequest = [NSURLRequest requestWithURL:[NSURL URLWithString:xmlDataUrl]];
self.connection = [[NSURLConnection alloc] initWithRequest:urlRequest delegate:self];
// Test the validity of the connection object.
NSAssert(self.connection != nil, @"Failure to create URL connection.");
// show in the status bar that network activity is starting
[UIApplication sharedApplication].networkActivityIndicatorVisible = YES;
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
int count = [tableElements count];
// ff there's no data yet, return enough rows to fill the screen
if (count == 0)
{
return kCustomRowCount;
}
return count;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
// customize the appearance of table view cells
static NSString *placeholderCellIdentifier = @"PlaceholderCell";
// add a placeholder cell while waiting on table data
int nodeCount = [self.tableElements count];
if (nodeCount == 0 && indexPath.row == 0)
{
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:placeholderCellIdentifier];
if (cell == nil)
{
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle
reuseIdentifier:placeholderCellIdentifier];
cell.detailTextLabel.textAlignment = NSTextAlignmentCenter ;
cell.selectionStyle = UITableViewCellSelectionStyleNone;
}
cell.detailTextLabel.text = @"Loading…";
return cell;
}
static NSString *CustomCellIdentifier = @"CustomCellIdentifier";
CustomCell *cell = (CustomCell *)[tableView dequeueReusableCellWithIdentifier:CustomCellIdentifier];
if (cell == nil) {
NSArray *nib = [[NSBundle mainBundle] loadNibNamed:@"CustomCell" owner:self options:nil];
for (id oneObject in nib) if ([oneObject isKindOfClass:[CustomCell class]])
cell = (CustomCell *) [nib objectAtIndex:0];
}
// Leave cells empty if there's no data yet
if (nodeCount > 0)
{
// Set up the cell...
CellData *cellData = (self.tableElements)[indexPath.row];
cell.name.text = cellData.name;
// Only load cached images; defer new downloads until scrolling ends
if (!cellData.icon)
{
if (self.tView.dragging == NO && self.tView.decelerating == NO)
{
[self startIconDownload:cellData forIndexPath:indexPath];
}
// if a download is deferred or in progress, return a placeholder image
cell.itemImage.image = [UIImage imageNamed:@"Placeholder.png"];
}
else
{
cell.itemImage.image = cellData.icon;
}
}
return cell;
}
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
return kCustomRowHeight;
}
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
self.xmlData = [NSMutableData data]; // start off with new data
}
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
[self.xmlData appendData:data]; // append incoming data
}
#pragma mark -
#pragma mark NSURLConnection delegate methods
- (void)handleError:(NSError *)error
{
NSString *errorMessage = [error localizedDescription];
UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Cannot Show Data"
message:errorMessage
delegate:nil
cancelButtonTitle:@"OK"
otherButtonTitles:nil];
[alertView show];
}
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
[UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
if ([error code] == kCFURLErrorNotConnectedToInternet)
{
// if we can identify the error, we can present a more precise message to the user.
NSDictionary *userInfo = @{NSLocalizedDescriptionKey: @"No Connection Error"};
NSError *noConnectionError = [NSError errorWithDomain:NSCocoaErrorDomain
code:kCFURLErrorNotConnectedToInternet
userInfo:userInfo];
[self handleError:noConnectionError];
}
else
{
// otherwise handle the error generically
[self handleError:error];
}
self.connection = nil; // release our connection
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
self.connection = nil; // release our connection
[UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
// create the queue to run our ParseOperation
self.queue = [[NSOperationQueue alloc] init];
ParseOperation *operation = [[ParseOperation alloc] initWithData:self.xmlData delegate:self];
[queue addOperation:operation]; // this will start the "ParseOperation"
self.xmlData = nil;
}
- (void)didFinishParsing:(NSArray *)cellDataList
{
[self performSelectorOnMainThread:@selector(handleLoadedApps:) withObject:cellDataList waitUntilDone:NO];
self.queue = nil; // we are finished with the queue and our ParseOperation
}
- (void)parseErrorOccurred:(NSError *)error
{
[self performSelectorOnMainThread:@selector(handleError:) withObject:error waitUntilDone:NO];
}
- (void)handleLoadedApps:(NSArray *)loadedCellData
{
[self.tableElements addObjectsFromArray:loadedCellData];
// tell our table view to reload its data, now that parsing has completed
[self.tView reloadData];
}
// Override to allow orientations other than the default portrait orientation.
- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation {
// Return YES for supported orientations
return YES;
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
- (void)startIconDownload:(CellData *)cellData forIndexPath:(NSIndexPath *)indexPath
{
ImageDownloader *imageDownloader = imageDownloadsInProgress[indexPath];
if (imageDownloader == nil)
{
imageDownloader = [[ImageDownloader alloc] init];
imageDownloader.cellData = cellData;
imageDownloader.indexPathInTableView = indexPath;
imageDownloader.delegate = self;
imageDownloadsInProgress[indexPath] = imageDownloader;
[imageDownloader startDownload];
}
}
// 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.tableElements count] > 0)
{
NSArray *visiblePaths = [self.tView indexPathsForVisibleRows];
for (NSIndexPath *indexPath in visiblePaths)
{
CellData *cellData = (self.tableElements)[indexPath.row];
if (!cellData.icon) // avoid the app icon download if the app already has an icon
{
[self startIconDownload:cellData forIndexPath:indexPath];
}
}
}
}
// called by our ImageDownloader when an icon is ready to be displayed
- (void)imageDidLoad:(NSIndexPath *)indexPath
{
ImageDownloader *imageDownloader = imageDownloadsInProgress[indexPath];
if (imageDownloader != nil)
{
CustomCell *cell = (CustomCell *)[self.tView cellForRowAtIndexPath:imageDownloader.indexPathInTableView];
// Display the newly loaded image
cell.itemImage.image = imageDownloader.cellData.icon;
}
}
#pragma mark -
#pragma mark Deferred image loading (UIScrollViewDelegate)
// Load images for all onscreen rows when scrolling is finished
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
if (!decelerate)
{
[self loadImagesForOnscreenRows];
}
}
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
[self loadImagesForOnscreenRows];
}
@end
CustomCell.m
#import "CustomCell.h"
@implementation CustomCell
@synthesize itemImage;
@synthesize name;
- (id)initWithStyle:(UITableViewCellStyle)style reuseIdentifier:(NSString *)reuseIdentifier
{
self = [super initWithStyle:style reuseIdentifier:reuseIdentifier];
if (self) {
// Initialization code
}
return self;
}
- (void)setSelected:(BOOL)selected animated:(BOOL)animated
{
[super setSelected:selected animated:animated];
// Configure the view for the selected state
}
@end