Application Crash on Fast switching between UISegment - iphone

Question

I have a segment and a table view, I have to reload the data in the table view its working but when i switch the segment buttons fast then application crashes.
On segment i have three buttons and on their action I am triggering a request to a url and fetching the json from that url (3 different urls resp. and returns 10, 17 and 8 results in json).
I have a custom table cell which have the images in it and i am trying to lazily load theose images.
In JSON result each object of json have various keys in it like name, id, imageurl etc.
the id is unique throughout the application and I am trying to load the images and save the images in local memory based on that unique id, so that i can show the cashed images and no need to redownload the images on segment change because some enteries on all three segments are same.
Here is my complete code
- (IBAction)segmentedControlIndexChanged {
NSString *offerRequestUrl = nil;
switch (self.mySegment.selectedSegmentIndex) {
case 0:
offerRequestUrl = #"url_one";
self.feedRequestUrl = [NSURL URLWithString:offerRequestUrl];
break;
case 1:
offerRequestUrl = #"url_two";;
self.feedRequestUrl = [NSURL URLWithString:offerRequestUrl];
break;
case 2:
offerRequestUrl = #"url_three";;
self.feedRequestUrl = [NSURL URLWithString:offerRequestUrl];
break;
default:
break;
}
[self parseJSONWithURL:self.feedRequestUrl];
}
- (void) parseJSONWithURL:(NSURL *) jsonURL
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSData* data = [NSData dataWithContentsOfURL: jsonURL];
[self performSelectorOnMainThread:#selector(fetchedData:) withObject:data waitUntilDone:YES];
});
}
- (void)fetchedData:(NSData *)responseData {
//parse out the json data
NSError* error;
NSDictionary* json = [NSJSONSerialization JSONObjectWithData:responseData
options:kNilOptions
error:&error];
self.feedsArray = [json objectForKey:#"result"];
[self.myTableView reloadData];
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
int count = self.feedsArray.count;
// if there's no data yet, return enough rows to fill the screen
if (count == 0)
{
return 1;
}
return count;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
// customize the appearance of table view cells
//
static NSString *CellIdentifier = #"MyTableCell";
static NSString *PlaceholderCellIdentifier = #"PlaceholderCell";
// add a placeholder cell while waiting on table data
int nodeCount = [self.feedsArray 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 = UITextAlignmentCenter;
cell.selectionStyle = UITableViewCellSelectionStyleNone;
}
cell.detailTextLabel.text = #"Loading . . .";
[self performSelector:#selector(checkAndDisplayAlert) withObject:self afterDelay:10.0];
return cell;
}
CustomCell *cell = (CustomCell*)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (nodeCount > 0)
{
// Configure the cell...
cell.carImageView.image = [UIImage imageNamed:#"Placeholder"]; //acts as default image
if([self.imageDict valueForKey:[NSString stringWithFormat:#"%#", self.feedsArray[indexPath.row][#"id_str"]]]==nil)
{
[NSThread detachNewThreadSelector:#selector(displayingSmallImage:) toTarget:self withObject:indexPath];
} else {
cell.carImageView.image = [self.imageDict valueForKey:[NSString stringWithFormat:#"%#", self.feedsArray[indexPath.row][#"id_str"]]];
}
cell.imageView.clipsToBounds = YES;
cell.titleLabel.text = self.feedsArray[indexPath.row][#"title_key"];
}
return cell;
}
- (void) displayingSmallImage:(NSIndexPath *)indexPath
{
NSString *imageUrl = self.feedsArray[indexPath.row][#"icon_url"];
NSURL *url = [NSURL URLWithString:imageUrl];
UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:url]];
if(image == nil)
{
image = [UIImage imageWithContentsOfFile:[[[NSBundle mainBundle] resourcePath] stringByAppendingPathComponent:#"Placeholder"]];
}
[self.imageDict setObject:image forKey:[NSString stringWithFormat:#"%#", self.feedsArray[indexPath.row][#"id_str"]]];
[self performSelectorOnMainThread:#selector(imageReceived:) withObject:indexPath waitUntilDone:NO];
}
- (void) imageReceived:(NSIndexPath *)indexPath
{
UIImage *image = (UIImage *)[self.imageDict objectForKey: [NSString stringWithFormat:#"%#", self.feedsArray[indexPath.row][#"id_str"]]];
UIImageView *imgs = (UIImageView *)[[self.myTableView cellForRowAtIndexPath:indexPath] viewWithTag:IMG_TAG];
[imgs setImage:image];
[self.myTableView reloadRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationNone];
}
- (void) checkAndDisplayAlert
{
if (!self.feedsArray && self.isAlertPresent == NO) {
UIAlertView *nilAlert = [[UIAlertView alloc] initWithTitle:#"Sorry!"
message:#"No result returned from server."
delegate:self
cancelButtonTitle:#"OK"
otherButtonTitles:nil];
[nilAlert setTag:kNilAlertTag];
self.isAlertPresent = YES;
[nilAlert show];
}
}
The main things I have to do is
lazily download the images
cache the images in local memory based on unique ID
Check the cashed images dictionary for already downloaded image and display it
If possible save these downloaded images in the local memory (nsuserdefaults etc) so that if user restart the app then also images are there and can be displayed based on their unique ID and no need to download again.
Any help would be appreciated.

Answer 1

Please post the error log, if it is array out of bounds exception I suspect these two methods
- (void)fetchedData:(NSData *)responseData & your tableViewDatasource
To find which line of code causes this crash do this. Go to break points tab on the left side and click + in the bottom.
Then you would get something like this:
Just hit done, run your code again and tell us which line caused the exception.
I understood your problem, why do you want to take the URL from the array in this function
- (void) displayingSmallImage:(NSIndexPath *)indexPath
you could pass the Url to load to that function, from cellForRowAtIndex like this:
NSArray *argsToPass=[NSArray arrayWithObjects:indexPath,self.feedsArray[indexPath.row][#"icon_url"]];
[NSThread detachNewThreadSelector:#selector(displayingSmallImage:) toTarget:self withObject:argsToPass];
and your function is modified as:
- (void) displayingSmallImage:(NSArray *)args
{
if([args count] == 0)
{
NSString *imageUrl = [args objectAtIndex:1];
NSIndexPath *indexPath=[args objectAtIndex:0];
NSURL *url = [NSURL URLWithString:imageUrl];
UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:url]];
if(image == nil)
{
image = [UIImage imageWithContentsOfFile:[[[NSBundle mainBundle] resourcePath] stringByAppendingPathComponent:#"Placeholder"]];
}
[self.imageDict setObject:image forKey:[NSString stringWithFormat:#"%#", self.feedsArray[indexPath.row][#"id_str"]]];
[self performSelectorOnMainThread:#selector(imageReceived:) withObject:indexPath waitUntilDone:NO];
}
}

Answer 2

you can set self.view.userInteractionEnabled=NO when you are sending request to server.
and when u get the data just enable the user interaction .so that multiple requests are avoided.