我是IOS开发人员初学者，请向我介绍“如何仅从数据库中获取数据”
我为此MainViewController和BookViewController有2个班级
这是一个很好的脱机模式可以显示我只有数据的工作。
MainViewController.m
- (void)viewDidLoad
{
[super viewDidLoad];
db = [[DBManager alloc]init];
[db initWithDB];
bookArray = [db getBookData];
}
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *path = [paths objectAtIndex:0];
_databaseVersion = [[bookArray objectAtIndex:indexPath.item]bookVersion];
NSLog(@"%f",_databaseVersion);
BookCell *bookcell = [collectionView dequeueReusableCellWithReuseIdentifier:@"ReuseCell" forIndexPath:indexPath];
return bookcell;
}
-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
return [bookArray count];
}
- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
return 1;
}
此
MainViewController正在工作但是当我将这样的数据发送到
BookViewController-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender{
if ([segue.identifier isEqualToString:@"course1"]){
BookViewController *controller = (BookViewController *)segue.destinationViewController;
[controller setBookArray:bookArray];
controller.checkType = @"1";
controller.statusMode = statusMode;
controller.databaseVersion = _databaseVersion;
}
}
但是bookArray对
BookViewController不起作用BookViewController.m
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *path = [paths objectAtIndex:0];
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
BookCell *bookcell = [collectionView dequeueReusableCellWithReuseIdentifier:@"ReuseCell" forIndexPath:indexPath];
BookCell *cell = (BookCell *)[collectionView cellForItemAtIndexPath:indexPath];
NSInteger row= indexPath.row;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSURL *imageURL = [NSURL URLWithString:[[bookList objectAtIndex:indexPath.item]coverURL]];
NSData *imageData = [NSData dataWithContentsOfURL:imageURL];
UIImage *image = [UIImage imageWithData:imageData];
dispatch_async(dispatch_get_main_queue(), ^{
bookcell.bookCover.image = [UIImage imageWithData:imageData];
bookcell.bookCover.alpha = 0.5f;
bookcell.bookDetailLabel.hidden = NO;
NSLog(@"Row == %d ",row);
if ([[bookList objectAtIndex:indexPath.row]bookVersion] > [[bookArray objectAtIndex:indexPath.row]bookVersion] && [[bookArray objectAtIndex:indexPath.row]bookVersion] > 0) {
bookcell.bookCover.image = [UIImage imageWithData:imageData];
bookcell.bookCover.alpha = 1.0f;
bookcell.bookDetailLabel.hidden = NO;
bookcell.bookDetailLabel.text = @"Update";
}
});
});
}
return bookcell;
}
-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
return [bookList count];
}
- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
return 1;
}
//bookList is data from Webservice and bookArray is data from database.但是我不知道为什么我不能使用数据库中的数据
我在
[[bookArray objectAtIndex:indexPath.item]bookVersion];的cellForItemAtIndexPath:中尝试这样的BookViewController.m，但是bookArray总是崩溃，因为如果我这样做，bookArry只有一个数据是有效的[[bookArray objectAtIndex:0]bookVersion];将获得一个数据而不是崩溃，但是我想使用 if([[bookList objectAtIndex:indexPath.row]bookVersion] > [[bookArray objectAtIndex:indexPath.row]bookVersion])
{ .... }
但这总是崩溃，因为
[[bookList objectAtIndex:indexPath.row]bookVersion]在Web服务中有10个数据，但是[[bookArray objectAtIndex:indexPath.row]bookVersion]只有一个数据。请帮助我如何获取[[bookArray objectAtIndex:indexPath.row]bookVersion]数据，如果没有数据，则不会崩溃，将显示null请请。在此
[[bookArray objectAtIndex:indexPath.item]bookVersion]上崩溃 NSString *testData = [[bookArray objectAtIndex:indexPath.item]downloadFlag];
NSLog(@"test flag %@",testData);
2014-01-20 11:14:29.837 GreenAcademy PowerShelf[596:60b] test flag 1
2014-01-20 11:14:29.842 GreenAcademy PowerShelf[596:60b] -------------------------
2014-01-20 11:14:29.844 GreenAcademy PowerShelf[596:60b] test flag 1
2014-01-20 11:14:29.849 GreenAcademy PowerShelf[596:60b] -------------------------
2014-01-20 11:14:29.851 GreenAcademy PowerShelf[596:60b] test flag 1
2014-01-20 11:14:29.854 GreenAcademy PowerShelf[596:60b] -------------------------
2014-01-20 11:14:29.863 GreenAcademy PowerShelf[596:60b] *** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[__NSArrayM objectAtIndex:]: index 3 beyond bounds [0 .. 2]'
*** First throw call stack:
(0x2e349f4b 0x38a1b6af 0x2e280533 0xb6911 0x30b1de65 0x30b1c5d9 0x30b18921 0x30abcda3 0x30743c6b 0x3073f47b 0x3073f30d 0x3073ed1f 0x3073eb2f 0x3073885d 0x2e3151cd 0x2e312b71 0x2e312eb3 0x2e27dc27 0x2e27da0b 0x32f51283 0x30b21049 0x80b7d 0x38f23ab7)
libc++abi.dylib: terminating with uncaught exception of type NSException
最佳答案
如果您不希望代码在尝试访问超出项数组的项时生成异常，请首先检查count属性。因此，而不是：
NSInteger row = indexPath.row
if ([[bookList objectAtIndex:row] bookVersion] > [[bookArray objectAtIndex:row] bookVersion])
{ .... }
您可以首先添加
count属性的检查：if ([bookArray count] >= (row + 1) && [[bookList objectAtIndex:row] bookVersion] > [[bookArray objectAtIndex:row] bookVersion])
{ .... }
或者，也许更简洁一些，使用新的数组下标符号：
if ([bookArray count] >= (row + 1) && [bookList[row] bookVersion] > [bookArray[row] bookVersion])
{ .... }