After closing the App NSTimer updating in iphone - objective-c

Question

When my app become active I get last image from ALAssetsLibrary which is stored in sqlite and I start a NSTimer to check a new image is added to ALAssetsLibrary. After screenshot is taken in my app it displays alert because image name in sqlite and ALAssetsLibrary last image name are mismatching. When app is closed I stop timer and user open it again I start the timer again to check screenshot image is deleted or not. If it is not deleted it will display an alert
-(void)getAllImagesName
{
ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
NSMutableArray *imgArray = [[NSMutableArray alloc]init];
[library enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
// Within the group enumeration block, filter to enumerate just photos.
[group setAssetsFilter:[ALAssetsFilter allPhotos]];
for (int i=0; i<[group numberOfAssets]; i++) {
// Chooses the photo at the last index
[group enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:i] options:0 usingBlock:^(ALAsset *alAsset, NSUInteger index, BOOL *innerStop) {
// The end of the enumeration is signaled by asset == nil.
if (alAsset) {
// ALAssetRepresentation *representation = [alAsset defaultRepresentation];
[imgArray addObject:alAsset.defaultRepresentation.filename];
}
}];
}
NSString *img;
img=[[DBModel database]getPreviousName];
NSLog(#"select img=%#",img);
NSArray *results;
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"SELF contains[cd] %#",img];
results = [contentArray filteredArrayUsingPredicate:predicate];
//NSLog(#"predicate results = %#",results);
if ([results count] != 0) {
[self displayAlertMsg];
}
else{
}
}
} failureBlock: ^(NSError *error) {
// Typically you should handle an error more gracefully than this.
//NSLog(#"No groups");
}];
}
timer1 = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:#selector(getAllImagesName) userInfo:nil repeats:YES];
runLoop = [NSRunLoop currentRunLoop];
[runLoop addTimer:timer1 forMode:NSRunLoopCommonModes];
[runLoop run];
My problem is after user taken screen shot in other app and try to use my app it is displaying an alert. Is timer running after closing app? please let me know anyone not understanding my problem.