So, I'm trying to get all the facebook pictures I'm tagged so I can see them on the iPad, but I wanted to make this function so I can call it everytime I would need to get the url's. The problem is, after I call this function, the array is nil, because the values I get are inside a block. How do I make an array to store the data I get for later use?
-(NSArray *)getFacebookTaggedPictures
{
__block NSArray *taggedPictures = [[NSArray alloc]init];
[FBRequestConnection startWithGraphPath:#"me/photos" completionHandler:^(FBRequestConnection *connection, id result, NSError *error)
{
if(!error)
{
taggedPictures = [(NSArray *)[result data]copy];
//NSLog(#"the tagged pictures are: %#",result);
}
}];
return taggedPictures;
}
As you have correctly noticed, the array that you return is empty because your method returns before the response from the API has been received and parsed into taggedPictures inside the completionHandler block.
You must save the array inside that block. I'd recommend changing your method as follows:
-(NSArray *)getFacebookTaggedPicturesWithCompletion:(void (^)(NSArray* photos))completion;
where the calling methods would pass the appropriate completion block to handle the obtained pictures. (i.e. save them to disk, display them, do some processing on them etc.):
void (^loggerBlock)(NSArray* photos) = ^(NSArray *array) {
NSLog(#"obtained photos: %#",[array description]);
//save here instead of logging
};
[self getFacebookTaggedPicturesWithCompletion:loggerBlock];
Hope this helps.
Related
i have a pointer key in PFUser and I'm trying to retrieve the object it's pointing to. I've seen many examples about querying for it but there shouldn't be any need for that, since parse has the fetch method and it's a pointer of PFUser class, so I'm using this:
PFObject *menu = [[[PFUser currentUser] objectForKey:#"menuID"] fetchIfNeeded];
I know my current user has an object being pointed to in that key but i get a null menu object all the time
Wain was correct in saying that you need to fetch the currentUser. However, you have to keep in mind that we're working with multiple threads if you want to use fetchInBackground. To keep in on a single thread, simply use [[PFUser currentUser] fetch], but keep in mind that this can cause hanging or blocking for your user when internet connection is bad. Below is a sample of how to use this more effectively. (Same issue with the fetch vs. fetchInBackground for the menu) We have to fetch the menu as well, since it is a pointer and so the currentUser will only fetch the pointer and not the whole object.
[[PFUser currentUser] fetchInBackgroundWithBlock:^(PFObject *object, NSError *error) {
if(!error){
PFObject *menu = object[#"menuID"];
[menu fetch];
// Execute any code that needs the menu here, or store it in your viewController (or other class variable) if you need to save it for later
// Alternately, you could use this:
dispatch_async(dispatch_get_main_queue(), ^{
[menu fetchInBackgroundWithBlock:^(PFObject *fetchedMenu, NSError *menuError) {
if(!menuError){
// Execute any code that needs the menu here, or store it
} else {
NSLog(#"%#", menuError);
}
}];
});
} else {
NSLog(#"%#", error);
}
}];
By default currentUser doesn't have any custom added columns of data populated. You need to fetch the user to download that data and then you can use it locally.
Alternatively your oils us cloud code and save a network request.
I'm using AFNetworking to fetch remote data, and Magical Record to import and save it locally.
Basically, I'm calling magical record save with a block, within the completion block from the AFNetworking method, which leads to my Unit Tests hanging forever.
Here's the code of the sync method. It is defined in a SyncEngine class, used as a singleton.
NSArray *operations = … ; // An array of AFHTTPRequestOperation defined before
__block BOOL syncSuccess = … ; // Set by operations
__block NSDictionary *syncErrors = … ; // Set by operations
NSArray *batchedOperations =
[AFURLConnectionOperation batchOfRequestOperations:operations
progressBlock:^(NSUInteger numberOfFinishedOperations, NSUInteger totalNumberOfOperations) {
// …
}
completionBlock:^(NSArray *operations) {
NSMutableDictionary *allErrors = [NSMutableDictionary dictionaryWithDictionary:syncErrors];
[_localContext saveToPersistentStoreWithCompletion:^(BOOL success, NSError *error){
if (!success && error)
allErrors[#"MR_saveToPersistentStore"] = error;
syncCompletionBlock(syncSuccess && success, allErrors);
}];
}
];
// _manager is a AFHTTPRequestOperationManager
[_manager.operationQueue addOperations:batchedOperations waitUntilFinished:YES];
So the issue comes from the fact that I'm calling "saveToPersistentStoreWithCompletion" within "completionBlock".
If I use "saveToPersistentStoreAndWait" instead, it works but then I don't get to know if the save was successful or not.
What's the proper way to nest those operations ?
I'm not familiar with AFNetworking 2.0, but won't that code potentially invoke save many times?
Wouldn't it be better to save once after completing all of the batchedOperations?
I think I might have an async problem going on here, which bites cause I thought I had solved it. Anyway, I am making a bunch of web service calls like so:
//get the client data
__block NSArray* arrClientPAs;
[dataManager getJSONData:strWebService withBlock:^(id results, NSError* error) {
if (error) {
UIAlertView* alert = [[UIAlertView alloc] initWithTitle:#"Getting Client Data Error!" message:error.description delegate:nil cancelButtonTitle:NSLocalizedString(#"Okay", nil) otherButtonTitles:nil, nil];
[alert show];
} else {
arrClientPAs = results;
}
}];
and getJSONData is like so:
- (void) getJSONData : (NSString*) strQuery withBlock:(void (^)(id, NSError *))completion {
NSDictionary* dictNetworkStatus = [networkManager checkNetworkConnectivity];
NetworkStatus networkStatus = [[dictNetworkStatus objectForKey:#"Status"] intValue];
if (networkStatus != NotReachable) {
//set up the url for webservice
NSURL* url = [NSURL URLWithString:strQuery];
NSMutableURLRequest* urlRequest = [NSMutableURLRequest requestWithURL:url];
//set up the url connection
__block id results;
[NSURLConnection sendAsynchronousRequest:urlRequest queue:[NSOperationQueue mainQueue] completionHandler:
^(NSURLResponse* response, NSData* jsonData, NSError* error) {
if (error) {
completion(nil, error);
return;
}
results = [NSJSONSerialization JSONObjectWithData:jsonData options:NSJSONReadingMutableContainers | NSJSONReadingMutableLeaves | NSJSONReadingAllowFragments error:&error];
completion(results, nil);
}];
} else {
//not connected to a network - data is going to have to come from coredata
}
}
In the first block, if I log arrClientData I can see the data that I am expecting but when I log arrClientData after it it is nil. I was following this SO thread - How to return a BOOL with asynchronous request in a method? (Objective-C) and a couple of others.
Obviously I am trying to get the data after the async call is made. Any help would be appreciated.
The problem lies, I think, in what "asynchronous" means. Here's a diagram:
Step One
__block result;
Step Two - do something asynchonous, including e.g. setting result
Step Three
What order do things happen in here? Step Three happens before Step Two gets finished. That is what asynchronous means: it means, "go right on with this code, don't wait for the asynchronous stuff to finish." So at the time Step Three happens, the result variable has not yet been set to anything.
So, you are just misleading the heck out of yourself with your __block result. __block or no __block, there is no way you are going to find out out what the result is afterwards, because there is no "afterwards". Your code has completed before your __block result is even set. That is why asynchronous code uses a callback (eg. your completion block) which does run afterwards, because it is sequentially part of (appended to) the asynchronous code. You can hand your result downwards through the callback, but you cannot usefully set it upwards from within the block and expect to retrieve it later.
So, your overall structure is like this:
__block NSArray* arrClientPAs; // it's nil
[call getJSONdata] = step one
[call sendAsynchronousRequest]
do the block _asynchronously_ = step two, tries to set arrClientPAs somehow
step three! This happens _before_ step two, ...
... and this entire method ends and is torn down ...
... and arrClientPAs is still nil! 🌻
I repeat: you cannot pass any information UP out of an asynchronous block. You can only go DOWN. You need your asynchronous block to call some method of some independently persistent object to hand it your result and tell it to use that result (and do it carefully, on the main thread, or you will cause havoc). You cannot use any automatic variable for this purpose, such as your declared NSArray variable arrClientPAs; there is no automatic scope any more, the method is over, the automatic variable is gone, there is no more code to run.
Check the value of the 'error 'variable after call:
results = [NSJSONSerialization JSONObjectWithData:jsonData options:NSJSONReadingMutableContainers | NSJSONReadingMutableLeaves | NSJSONReadingAllowFragments error:&error];
If 'error' isn't nil there is a problem with data which you get in your completion block.
You are mixing styles and confusing the purpose of __block.
Note: When you call a method that will be executed asynchronously you are creating a new execution path which will be executed at some point in the future (which includes immediately) on some thread.
In your getJSONData method you use a __block qualified variable, results, when you should not. The variable is only required within the block and should be declared there:
//set up the url connection
[NSURLConnection sendAsynchronousRequest:urlRequest queue:[NSOperationQueue mainQueue] completionHandler:
^(NSURLResponse* response, NSData* jsonData, NSError* error)
{
if (error) {
completion(nil, error);
return;
}
id results = [NSJSONSerialization JSONObjectWithData:jsonData options:NSJSONReadingMutableContainers | NSJSONReadingMutableLeaves | NSJSONReadingAllowFragments error:&error];
completion(results, nil);
}];
Declaring the variable outside of the block and adding __block just adds pointless complexity. After the call to sendAsynchronousRequest, returns before the request has been performed, the value of results would not be the value assigned in the block. The call to the completion block is performed on a different execution path and probably will not even be executed until after the call to getJSONData has returned.
However what is correct about your getJSONData method is its model - it takes a completion block which sendAsynchronousRequest's own completion handler will call. This is what is incorrect about your call to getJSONData - the completion block you pass does not pass on the results to another block or pass them to some object, but instead assigns them a local variable, arrClientPAs, declared before the call. This is the same situation as described above for getJSONData and will fail for the same reasons - it is not the arrClientPAs fails to "retain the data" but that you are reading it on in the current execution path before another execution path has written any data to it.
You can address this problem the same way getJSONData does - the enclosing method (not included in your question) can take a completion block (code entered directly into answer, expect typos!):
- (void) getTheClientData: ... completionHandler:(void (^)(id))handler
{
...
//get the client data
[dataManager getJSONData:strWebService withBlock:^(id results, NSError* error) {
if (error) {
UIAlertView* alert = [[UIAlertView alloc] initWithTitle:#"Getting Client Data Error!" message:error.description delegate:nil cancelButtonTitle:NSLocalizedString(#"Okay", nil) otherButtonTitles:nil, nil];
[alert show];
} else {
handler(results); // "return" the result to the handler
}
}];
There is another approach. If and only if getClientData is not executing on the main thread and you wish its behaviour to be synchronous and to return the result of the request then you can issue a sendSynchronousRequest:returningResponse:error: instead of an asynchronous one. This will block the thread getClientData is executing on until the request completes.
In general if you have an asynchronous method which you cannot replace by a synchronous one but require synchronous behaviour you can use semaphores to block your current thread until the asynchronous call completes. For an example of how to do this see this answer.
HTH
My app creates an object (PFUSER) for each user, and an (PF) object for each event they participate in. This works fine. then i have two files associated with that event. i save the first file to a PFFile, then associate it to the event pfobject. when i use blocks and do this in the background, how can then make sure control continues to do the same for the second file?
I am new to blocks so maybe it would be clearer to me why its not working with callbacks, but it seems the block runs the save in another thread and the current one is abandoned before the next steps are taken.
Of course i'd like to do both of these as "save eventually" to allow offline use.
any guidance / examples you can point me to greatly appreciated.
thanks!
saveEventually doesn't support PFFiles yet; it needs a bit more smarts to handle resuming uploads between restarts. One trick that is already available, however, is that PFObject knows how to save its children, including PFFiles. You can just say:
PFUser *user = PFUser.currentUser;
user[#"icon"] = [PFFile fileWithData:iconData];
user[#"iconThumb"] = [PFFile fileWithData:iconThumbData];
[user saveInBackgroundWithBlock:^(BOOL succeeded, NSError *error) {
// user will automatically save its files & only call this once the
// entire operation succeeds.
}];
I'm not 100% what you mean because you didn't post any codes, but I'd imagine if you want to associate multiple PFFile to PFObject this is all you have to do:
PFObject *object = [PFQuery getObjectOfClass:#"MyFile" objectId:id];
[object addObject:profilePicture forKey:#"Photo"];
[object addObject:coverPicture forKey:#"PhotoCover"];
[object saveEventually];
From Parse's documentation it seems like saveEventually does what you want:
Saves this object to the server at some unspecified time in the
future, even if Parse is currently inaccessible. Use this when you may
not have a solid network connection, and don’t need to know when the
save completes. If there is some problem with the object such that it
can’t be saved, it will be silently discarded. If the save completes
successfully while the object is still in memory, then callback will
be called.
As currently neither saveEvetually nor saving to the local data store are supported, below is a category of PFObject I am using to at least save offline what can be saved or returning error:
- (void) dr_saveWithCompletionHandler: (void(^)(NSError* error)) completionBlock {
__block BOOL canSaveEventually = YES;
[[self allKeys] enumerateObjectsUsingBlock:^(NSString* key, NSUInteger idx, BOOL *stop) {
id object = self[key];
if ([object isKindOfClass:[PFFile class]]) {
PFFile* file = (PFFile*) object;
if (!file.url || file.isDirty) {
canSaveEventually = NO;
}
}
}];
void (^localCompletionHandler) (BOOL, NSError*) = ^(BOOL succeeded, NSError *error) {
if (succeeded) {
if (completionBlock) completionBlock(nil);
} else {
if (completionBlock) completionBlock(error);
}
};
if (canSaveEventually) {
[self saveEventually:localCompletionHandler];
} else {
[self saveInBackgroundWithBlock:localCompletionHandler];
}
}
I am trying to create an array of all images from the saved photo album that match a certain criteria. Here is a simplified code for it. I add the photos to myImages array and confirmed via the "Added Image" log that the right images get logged. However the array returned by the function is always empty. Fairly new to Objective-C so any suggestions would be helpful.
NSMutableArray * myImages = [NSMutableArray array];
ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
// Enumerate just the photos by using ALAssetsGroupSavedPhotos.
[library enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
// Within the group enumeration block, filter to enumerate just photos.
[group setAssetsFilter:[ALAssetsFilter allPhotos]];
[group enumerateAssetsUsingBlock:^(ALAsset *alAsset, NSUInteger index, BOOL *innerStop) {
// The end of the enumeration is signaled by asset == nil.
if (alAsset) {
ALAssetRepresentation *representation = [alAsset defaultRepresentation];
UIImage *latestPhoto = [UIImage imageWithCGImage:[representation fullResolutionImage]];
NSLog(#"Added Image");
[myImages addObject:latestPhoto];
}
}];
}
failureBlock: ^(NSError *error) {
// Typically you should handle an error more gracefully than this.
NSLog(#"No groups");
}];
return myImages;
What is imagesTakenOnDate? Is that supposed to be myImages? If so, you cannot return it in this manner as the block code will execute after the method returns. The method is asynchronous. Rather than "return" you have 2 options to be able to access the modified array outside the function:
option 1: make your method take a completion block as a parameter, and then call the completion block inside the enumerateGroupsWithTypes block, and pass the completion block the array. For example:
typedef void (^CompletionBlock)(id, NSError*);
-(void)myMethodWithCompletionBlock:(CompletionBlock)completionBlock;
then when you're done with success call:
completionBlock(myImages, nil);
and in the failureBlock call:
completionBlock(nil, error);
option 2: make the array an ivar that is retained on your parent object, rather than a local variable, and then declare it as a __block variable so it can be modified within the block.
First thing. Do you really return imagesTakenOnDate? can`t see any reference to this ivar in your code. I would say that your put some breakpoints in your code. In the gdb debugger console you can type:
po myImages
than the debugger will print out the content of your array. Hope that helps