IOS：将图像从parse.com加载到tableview中

Question

I am trying to load associated images from parse into a custom table cell. When I execute the code parse is successfully queried and the other information for the table cell such as book name and author are correctly displayed, but when i try and add in the images the app crashes.

This is the code I use to get the image data.

-(void)queryForNewBooks{
_bookNameArray = [[NSMutableArray alloc] init];

_authorNameArray  = [[NSMutableArray alloc] init];
_isbnNumberArray = [[NSMutableArray alloc] init];
_bookImageData = [[NSMutableArray alloc] init];
PFQuery *query = [PFQuery queryWithClassName:@"BooksForSale"];
[query orderByDescending:@"createdAt"];
[query findObjectsInBackgroundWithBlock:^( NSArray *objects, NSError *error) {

    if (objects.count >=1) {

        for (PFObject *object in objects) {
            PFFile *imageFile = [object objectForKey:@"image"];
            [imageFile getDataInBackgroundWithBlock:^(NSData *result, NSError *error) {
                if (!error) {
                    NSData *data = result;
                    NSLog(@"HEYYYYYY");
                    [ _bookImageData addObject:data];


                }
            }];

Here is the table cell code

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *simpleTableIdentifier = @"TableViewCell";

 TableViewCell *cell = (TableViewCell *)[tableView dequeueReusableCellWithIdentifier:simpleTableIdentifier];
if (cell == nil)
{
    NSArray *nib = [[NSBundle mainBundle] loadNibNamed:@"TableViewCell" owner:self options:nil];
    cell = [nib objectAtIndex:0];
}
NSLog(@"Here are the book names, %@",_bookNameArray);

cell.bookNameLabel.text = [_bookNameArray objectAtIndex:indexPath.row];
cell.authorNameLabel.text = [_authorNameArray objectAtIndex:indexPath.row];
UIImage *image = [UIImage imageWithData: [_bookImageData   objectAtIndex:indexPath.row]];
 cell.bookImageLabel.image = image;



return cell;
}

I believe it is these two lines that are causing the issue

  UIImage *image = [UIImage imageWithData: [_bookImageData objectAtIndex:indexPath.row]];
     cell.bookImageLabel.image = image;

when I comment these two line the app doesn't crash.

Can anyone offer suggestions as to what I am doing incorrectly?

Thanks

Error log:

*** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[__NSArrayM objectAtIndex:]: index 0 beyond bounds for empty array'
*** First throw call stack:
(
    0   CoreFoundation                      0x000000010f951f45 __exceptionPreprocess + 165
    1   libobjc.A.dylib                     0x000000010f3cbdeb objc_exception_throw + 48
    2   CoreFoundation                      0x000000010f8359e4 -[__NSArrayM objectAtIndex:] + 212
    3   ParseStarterProject                 0x000000010c45fd41 -[ParseStarterProjectViewController tableView:cellForRowAtIndexPath:] + 753
    4   UIKit                               0x000000010dba9e2a -[UITableView _createPreparedCellForGlobalRow:withIndexPath:willDisplay:] + 782
    5   UIKit                               0x000000010dba9f3f -[UITableView _createPreparedCellForGlobalRow:willDisplay:] + 74
    6   UIKit                               0x000000010db7f307 -[UITableView _updateVisibleCellsNow:isRecursive:] + 3187
    7   UIKit                               0x000000010dbb2d1c -[UITableView _performWithCachedTraitCollection:] + 92
    8   UIKit                               0x000000010db9a884 -[UITableView layoutSubviews] + 223
    9   UIKit                               0x000000010db08e40 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 710
    10  QuartzCore                          0x000000010cf4559a -[CALayer layoutSublayers] + 146
    11  QuartzCore                          0x000000010cf39e70 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 366
    12  QuartzCore                          0x000000010cf39cee _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 24
    13  QuartzCore                          0x000000010cf2e475 _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 277
    14  QuartzCore                          0x000000010cf5bc0a _ZN2CA11Transaction6commitEv + 486
    15  QuartzCore                          0x000000010cf5c37c _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv + 92
    16  CoreFoundation                      0x000000010f87d947 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23
    17  CoreFoundation                      0x000000010f87d8b7 __CFRunLoopDoObservers + 391
    18  CoreFoundation                      0x000000010f87350b __CFRunLoopRun + 1147
    19  CoreFoundation                      0x000000010f872e08 CFRunLoopRunSpecific + 488
    20  GraphicsServices                    0x0000000110f79ad2 GSEventRunModal + 161
    21  UIKit                               0x000000010da5430d UIApplicationMain + 171
    22  ParseStarterProject                 0x000000010c45dfbf main + 111
    23  libdyld.dylib                       0x00000001114fa92d start + 1
    24  ???                                 0x0000000000000001 0x0 + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException

It appears that the data is not being added to the nsmutablearray which should occur here

for (PFObject *object in objects) {
    PFFile *imageFile = [object objectForKey:@"image"];
    [imageFile getDataInBackgroundWithBlock:^(NSData *result, NSError *error) {
        if (!error) {
            NSData *data = result;
            NSLog(@"HEYYYYYY");
            [ _bookImageData addObject:data];
        }

Answer 1

getDataInBackgroundWithBlock is an async process. Its need time to receive the response. So before request finished, your _bookImageData is empty array.

Before _bookImageData have data, - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath is being called, and BOOM your app crashed!

 UIImage *image = [UIImage imageWithData: [_bookImageData objectAtIndex:indexPath.row]];
 cell.bookImageLabel.image = image;

The solution is

① Try to refresh your tableview after receive _bookImageData by: [yourtableview reloadData] or something like that.

② Make your app not crash by using try .. catch, or check _bookImageData before your call objectAtIndex.

Recommend: Use some placeHolder image to make your app cooler. :)

   @try {
     UIImage *image = [UIImage imageWithData: [_bookImageData objectAtIndex:indexPath.row]];
     cell.bookImageLabel.image = image;
   } catch ...