How to check if an ALAsset still exists using a URL - iphone

Question

I've got an array containing ALAsset urls (not full ALAsset objects)
So each time I start my application I want to check my array to see if it's still up to date...
So I tried
NSData *assetData = [[NSData alloc] initWithContentsOfFile:#"assets-library://asset/asset.PNG?id=1000000001&ext=PNG"];
but assetData is allways nil
thx for the help

Answer 1

Use assetForURL:resultBlock:failureBlock: method of ALAssetsLibrary instead to get the asset from its URL.
// Create assets library
ALAssetsLibrary *library = [[[ALAssetsLibrary alloc] init] autorelease];
// Try to load asset at mediaURL
[library assetForURL:mediaURL resultBlock:^(ALAsset *asset) {
// If asset exists
if (asset) {
// Type your code here for successful
} else {
// Type your code here for not existing asset
}
} failureBlock:^(NSError *error) {
// Type your code here for failure (when user doesn't allow location in your app)
}];

Answer 2

Having assets path you can use this function to check if image exist:
-(BOOL) imageExistAtPath:(NSString *)assetsPath
{
__block BOOL imageExist = NO;
ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
[library assetForURL:[NSURL URLWithString:assetsPath] resultBlock:^(ALAsset *asset) {
if (asset) {
imageExist = YES;
}
} failureBlock:^(NSError *error) {
NSLog(#"Error %#", error);
}];
return imageExist;
}
Remember that checking if image exist is checking asynchronyus.
If you want to wait until new thread finish his life call function "imageExistAtPath" in main thread:
dispatch_async(dispatch_get_main_queue(), ^{
[self imageExistAtPath:assetPath];
});
Or you can use semaphores, but this is not very nice solution:
-(BOOL) imageExistAtPath:(NSString *)assetsPath
{
__block BOOL imageExist = YES;
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);
dispatch_async(queue, ^{
ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
[library assetForURL:[NSURL URLWithString:assetsPath] resultBlock:^(ALAsset *asset) {
if (asset) {
dispatch_semaphore_signal(semaphore);
} else {
imageExist = NO;
dispatch_semaphore_signal(semaphore);
}
} failureBlock:^(NSError *error) {
NSLog(#"Error %#", error);
}];
});
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
return imageExist;
}

Answer 3

For iOS 8 or later, there is a synchronous method to check if an ALAsset exists.
#import Photos;
if ([PHAsset fetchAssetsWithALAssetURLs:#[assetURL] options:nil].count) {
// exist
}
Swift:
import Photos
if PHAsset.fetchAssetsWithALAssetURLs([assetURL], options: nil).count > 0 {
// exist
}
Swift 3:
import Photos
if PHAsset.fetchAssets(withALAssetURLs: [assetURL], options: nil).count > 0 {
// exist
}

Answer 4

Use this method to check if file exists
NSURL *yourFile = [[self applicationDocumentsDirectory]URLByAppendingPathComponent:#"YourFileHere.txt"];
if ([[NSFileManager defaultManager]fileExistsAtPath:storeFile.path
isDirectory:NO]) {
NSLog(#"The file DOES exist");
} else {
NSLog(#"The file does NOT exist");
}