I've an array of user objects. I need to iterate over this array, and then, for each user, I need to make a request to fetch data, based on its user id. This request is an async request (and for reasons I can't make it sync). Here is the edited code using dispatch_group calls:
EDITED
for (User *user in self.userList) {
dispatch_group_enter(self.usersGroup);
[self.wrapper getUserDataWithId:user.userid completion:^(Owner *owner, NSError *error) {
user.owner = owner;
dispatch_group_leave(self.usersGroup);
}];
}
dispatch_group_notify(self.usersGroup, dispatch_get_main_queue(), ^{
// All requests have finished
// Stuff with updated userlist
});
The main issue is obvious: when first requests have finished, the user reference it has is not the one that has launched the request, so I don't have the correct association.
Is there any easy and elegant way to solve this issue? Thanks in advance!
As #Paulw11 suggested the loop will not wait for you to execute the block so i'm suggesting you to use recursive function like this instead of for loop.
- (void)getOwnerForObectAtIndex:(NSInteger)index {
if (index < self.userList) {
User *user = [self.userList objectAtIndex:index];
[self.wrapper getUserDataWithId:user.userid completion:^(Owner *owner, NSError *error) {
user.owner = owner;
[self getOwnerForObectAtIndex:index+1];
}];
}
else {
//Now all User have its corresponding owner, Handle array Now.
}
}
Now call this function like this [self getOwnerForObectAtIndex:0];
Related
I've the following code below (example code) that sends an API GET request multiple times.
- (void)listOfPeople:(NSArray *)array {
for (int i = 0; i < array.count; i++) {
Person *person = [array objectAtIndex:i];
[personClient getPersonData:person.fullName onSuccess:^(id result) {
// change data here
} onFailure:^(NSError *error) {
}];
}
}
The code doesn't work very well because the API requests finishes in a different order every time. I need to complete each api request in order. I believe I need to wait until either the completion block or the failure block is finished before continuing the for loop. Can someone point me in the right direction unless there is a better way to accomplish this task. I've tried dispatch group, but it didn't complete each request in order.
Get rid of the for loop, and instead make a recursive function that calls itself from the completion handler to get the next Person. That way when each call completes, it will make the call to get the next one.
Something like this:
- (void)getPersonFromArray:(NSArray *)array atIdx:(NSInteger)idx {
if (idx < array.count)
{
Person *person = [array objectAtIndex:idx];
[personClient getPersonData:person.fullName onSuccess:^(id result)
{
// Do something useful with Person here...
// ...
[self getPersonFromArray:array atIdx(idx + 1)];
} onFailure:^(NSError *error) {
// Handle errors here
// ...
}];
}
}
I have a method in an iOS app that is supposed to return a bool value depending upon whether or not a web call succeeds.
The web call is structured in a way such that it takes a block as a callback parameter and that callback is called when the web call has a result. Based on that result my method needs to return a True/False value.
So, I need to stop execution from progressing any further without first having a result to return.
I am trying to achieve this via semaphores, after looking at some examples that others have shared, but the callback is never called, if I remove the semaphore then the callback is always called.
What am I missing here?
+ (BOOL)getUserInformation {
__block BOOL flag = false;
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
[[WebServicesManager sharedManager] getUserInformationWithCallback:^(NSInteger statusCode, NSString *response, NSDictionary *responseHeaders, id obj, NSError *error) {
if (error) {
//Handle error case and perform appropriate cleanup actions.
}
else
{
//Save user information
flag = true;
}
dispatch_semaphore_signal(semaphore);
}];
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
return flag;
}
I put a break point on if(error) to check if the callback gets called, it doesnt, unless I remove the semaphore.
I could give this method its own callback block or I could give the containing class a delegate and achieve what I need but I would really like to make this approach work.
The WebServicesManager is probably dispatching it's block on the same thread the semaphore is waiting on.
As #Rob is correctly mentioning in the comments, this is most likely not a good idea to do on the main thread; rather make use of the asynchronous model and not block the main thread for possibly minutes until the connection may time out under certain circumstances, freezing your UI.
You are undoubtedly deadlocking because you're using semaphore on same thread to which the web services manager (or the API that that is using) dispatches its completion handler.
If you want a rendition that avoids the deadlock scenario, but also avoids the pitfalls of blocking the main thread, you can do something like:
+ (void)getUserInformation:(nonnull void (^)(BOOL))completionHandler {
[[WebServicesManager sharedManager] getUserInformationWithCallback:^(NSInteger statusCode, NSString *response, NSDictionary *responseHeaders, id obj, NSError *error) {
if (error) {
completionHandler(false);
} else {
//Save user information
completionHandler(true);
}
}];
}
Then, rather than doing something like:
BOOL success = [YourClass getUserInformation];
if (success) {
...
}
You can instead do:
[YourClass getUserInformation:^(BOOL success) {
if (success) {
...
}
}];
// but do not try to use `success` here ... put everything
// contingent upon success inside the above completion handler
I have the following requirement:
Given a hierarchical tree-like structure, I am performing a breadth-first-search to walk through the WHOLE dataset. The data is being provided by an API with a method : (makes a request to a server using AFNetworking, saves the result to Core Data and calls back the completion block on success with the stored entries)
-(void) getChildrenForNodeId:(NSNumber*)nodeId
completion:(void (^)(NSArray *nodes))completionBlock;
The method which a controller executes to fetch data:
-(void)getAllNodesWithCompletion:(void (^)(NSArray *nodes))completionBlock{
NSNumber *rootId = ...
[MyNetworkManager getChildrenForNodeId:rootId completion:^(NSArray *nodes){
for(Node *node in nodes){
[self iterateOverNode:node.nodeId];
}
//execute completionBlock with nodes fetched from database that contain all their children until the very last leaf
}];
}
Here is the problem:
-(void)iterateOverNode:(NSNumber*)nodeId {
NSMutableArray *elements = [NSMutableArray array];
[elements addObject:nodeId];
while ([elements count]) {
NSNumber *current = [elements objectAtIndex:0];
[MyNetworkManager getChildrenForNodeWithId:current completion:^(NSArray *nodes) {
/**
In order to continue with the loop the elements array must be updated. This can only happen once we have retrieved the children of the current node.
However since this is in a loop, all of these requests will be sent off at the same time, thus unable to properly keep looping.
*/
for(Node *node in nodes){
[elements addObject:node.nodeId];
}
[elements removeObjectAtIndex:0];
}];
}
}
Basically I need the result of the callback to control the flow of the while loop but I am not sure how to achieve it. My understanding is that the request to getChildrenForNodeWithId:completion: from within the while-loop should happen in a new thread in a SERIAL order so that another should commence after the first one has completed. I am not sure how to achieve this neither with NSOperation nor with GCD. Any help will be greatly appreciated.
What you need here is some recursion. This problem is tricky as we also need a way to track and detect the point at which we have explored every branch to a leaf node.
I'm not a expert with tree search algorithms, so some folks could probably improve on my answer here. Kick this off by calling it with the root node id. self.trackingArray is an NSMutableArray property with __block qualifier. Each time we start a request for a Node, we add it's nodeId into this array, and when it returns, we remove it's nodeId, and add the nodeIds of it's children. We can then know that when count of the tracking array reaches 0, every request made has returned, and has not added child ids to the array. Once you detect we are finished, you could call a saved block or a delegate method.
This solution does not include any error handling. If any request fails, it won't be retried, and all child nodes will be ignored.
- (void)getNodesRecursively:(NSNumber *)nodeId {
// Only do this once
if (self.trackingArray == nil) {
self.trackingArray = [NSMutableArray new];
[self.trackingArray addObject:nodeId];
}
__weak typeof(self) weakSelf = self;
[MyNetworkManager getChildrenForNodeWithId:nodeId completion:^(NSArray *nodes) {
[self.trackingArray removeObject:nodeId];
for (Node *node in nodes) {
[weakSelf.trackingArray addObject:node.nodeId];
[weakSelf getNodesRecursively:node.nodeId];
}
if (weakSelf.trackingArray.count == 0) {
// We are done.
// Reset the state of the trackingArray
self.trackingArray = nil;
// call a method here to notify who ever needs to know.
[weakSelf allNodesComplete];
}
}];
}
Your other methods would look something like this
-(void)getAllNodesWithCompletion:(void (^)(NSArray *nodes))completionBlock{
// Save the block to a property
self.completion = completionBlock;
// Call -getNodesRecursively with root id
[self getNodesRecursively:rootNodeId];
}
Then you could have a second method
- (void)allNodesComplete {
// Call saved block
// Completion should load nodes from core data as needed
self.completion();
}
I haven't tested the code, but does that approach seem sensible? I'm assuming we don't need to capture the here nodes, as they can be loaded from core data as required.
I need to retrieve a couple of NSDicionaries that are compared against an id.
First, I'm calling a NSArray with these id's in them. I'm looping over them to see get the details of that id, and with that i'm calling another pfcloud function. Up until this point, all goes well. However, when I'm logging the payment details of the payment id's, the order sequence is is in a different order than the array I putted it in.
for(__block NSString *paymentId in success){
[self getPaymentDetails:paymentId];
}
So for instance: success = #[#"1",#"2",#"3"]
the method getPaymentDetails will log me#[#"details about 1", #"details about 3", #"details about 2"]
However, I need them to be in the exact same order.
This is my getPaymentDetails code:
-(void)getPaymentDetails:(NSString *)paymentId{
PFUser *currentUser = [PFUser currentUser];
[PFCloud callFunctionInBackground:#"getpaymentdetails"
withParameters:#{#"objectid": paymentId, #"userid": currentUser.objectId}
block:^(NSDictionary *success, NSError *error) {
if(success){
NSDictionary *payment = success;
NSString *amount = [payment objectForKey:#"amount"];
if (![amount isKindOfClass:[NSNull class]]) {
[self.amountArray addObject:amount];
}
else {
[self.amountArray addObject:#""];
}
NSString *from = [payment objectForKey:#"from"];
if (![from isKindOfClass:[NSNull class]]) {
[self.fromArray addObject:from];
}
else {
[self.fromArray addObject:#""];
}
} else{
NSLog(#"Error logged getpaymentdetails: %#", error);
}
}];
}
The values stored in the amountArray for instance, do not match the index of the paymentId
How come and how do I solve this?
It may be simpler to just move the whole for loop into the background and then call the Parse function synchronously
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group,queue, ^{
for(__block NSString *paymentId in success){
[self getPaymentDetails:paymentId];
}
});
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
// Release the group when it is no longer needed.
dispatch_release(group);
Then in your getPaymentDetails you would call callFunction:withParameters:error: instead of callFunctionInBackground:withParameters:
This isn't an ideal solution however, as you are eliminating concurrency and so it will take longer to execute.
A better solution is to deal with the fact that the array is unordered at the conclusion of the loop and sort it once all of the data has been retrieved
The request callFunctionInBackground will do is executed asynchronously and there is no guarantee that the first call you make in your loop will finish first. This is not really related to Parse itself, that is just the nature of how this is done. You may end up with the same order by coincidence or a completely random one each time you execute this code.
If you want the order to stay the same, either pass in all IDs to your Cloud Function and update your Cloud Function to handle it or always wait for one call to finish, add the result to your array and then get the details with the next ID (basically a queue).
I have multiple methods, each with nested loops and facebook requests. There is an array of X id's and each method loops through each id, makes a request for that id then does stuff with the result data.
I need to be notified when each method has completed... ie, when the method has finished looping through all the id's in the array, making the facebook request for each, received the results and finished its tasks with the resulting data. I can't seem to figure out how to make this happen. here are examples of the methods:
- (void)runLoopForFacebookFriendsContent1 {
for (NSString *fbIdStr in self.fbIdsArr){
FBRequest *fbRequest = [FBRequest requestWithGraphPath:graphPathString parameters:nil HTTPMethod:#"GET"];
[fbRequest startWithCompletionHandler:^(FBRequestConnection *connection, id result, NSError *error) {
if (error) {
//show alert
} else {
//Do stuff with the resulting data
}
}];
}
}
- (void)runLoopForFacebookFriendsContent2 {
for (NSString *fbIdStr in self.fbIdsArr){
FBRequest *fbRequest2 = [FBRequest requestWithGraphPath:graphPathStringNumber2 parameters:nil HTTPMethod:#"GET"];
[fbRequest2 startWithCompletionHandler:^(FBRequestConnection *connection, id result, NSError *error) {
if (error) {
//show alert
} else {
for (PF_FBGraphObject *obj in [result objectForKey:#"data"]){
NSLog(#"facebook result: %#",result);
NSMutableDictionary *dict = [[NSMutableDictionary alloc]init];
[dict setValue:#"type2" forKey:#"ContentType"];
[dict setValue:obj forKey:#"data"];
[self.theFacebookDataArray addObject:dict];
}
}
}];
}
}
I call these methods in viewWillAppear. is there a way to setup some sort of completion handler to put the call for these methods inside? and then post an NSNotification when they are all done?
One option is to take a look at the ReactiveCocoa framework. I find it helps tremendously with this kind of pattern, after you get past a bit of a learning curve.
The following is an example directly from the linked file:
// Perform 2 network operations and log a message to the console when they are
// both completed.
//
// +merge: takes an array of signals and returns a new RACSignal that passes
// through the values of all of the signals and completes when all of the
// signals complete.
//
// -subscribeCompleted: will execute the block when the signal completes.
[[RACSignal
merge:#[ [client fetchUserRepos], [client fetchOrgRepos] ]]
subscribeCompleted:^{
NSLog(#"They're both done!");
}];
You could adapt this to the Facebook SDK fairly easily.
This behavior is expected since you are calling dispatch_group_enter and then instantly calling dispatch_group_leave which means the group has nothing to wait for.
You should call dispatch_group_enter before every block and call dispatch_group_leave at the end of every block.
Check the accepted answer here:
Wait until multiple networking requests have all executed - including their completion blocks
Update:
For the given example, you can call dispatch_group_enter before every call to startWithCompletionHandler:, and call dispatch_group_leave at the end of the completion block:
for (...) {
FBRequest *fbRequest = ...;
dispatch_group_enter(group);
[fbRequest startWithCompletionHandler:^(...) {
...
dispatch_group_leave(group);
}
}
Didn't want to have to answer my own question... don't really like doing that, and was hoping for a more elegant solution... but it doesnt seem to be coming and someone even downvoted the question. So i'm just going to post my solution and close the question out. If you end up with a similar problem and are reading this, I dealt with it by:
creating 2 int's for each method. One int is immediately set with the count of the array being iterated through. at the end of each iteration, I am posting an NSNotification. In the NSNotification handler method, i am ++ incrementing the second int & each time running an if condition, checking to see if they match.
It's a pain to keep track of all these when there are many methods like this happening... So if anyone ever finds a better solution, I'd love to hear it. Thanks to everyone who answered and tried to be helpful, I appreciate it!