IOS:将图像从 parse.com 加载到 TableView 中

Question

我正在尝试将相关图像从解析加载到自定义表格单元格中。当我执行代码时，解析被成功查询并且表格单元格的其他信息(如书名和作者)正确显示，但是当我尝试添加图像时，应用程序崩溃了。

这是我用来获取图像数据的代码。

-(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];


                }
            }];

这是表格单元格代码

- (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;
}

我相信是这两行导致了这个问题

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

当我评论这两行时，应用程序不会崩溃。

任何人都可以就我做错了什么提出建议吗？

谢谢

错误日志:

 *** 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 是一个异步进程。它需要时间来接收响应。所以在请求完成之前，您的 _bookImageData 是空数组。

在 _bookImageData 有数据之前，- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath 被调用，结果你的应用崩溃了!

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

解决办法是

① 在收到 _bookImageData 后尝试通过以下方式刷新您的 tableview:[yourtableview reloadData] 或类似的东西。

② 在调用 objectAtIndex 之前使用 try..catch 或检查 _bookImageData 使您的应用程序不会崩溃。

建议:使用一些占位符图像让您的应用更酷。 :)

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