How to detect the true end of a for loop? [closed] - objective-c

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Closed 10 years ago.
I have the following for loop contained within a method:
—(void)_va1idateUsers:(NSArray *)users withCurrentAccount:(ACAccount *)account comp1etionB1ock:(void (“)(TSCSpamUser *user, NSError *error))comp1etionBlock; {
for(TSCSpamUser *userID in users) {
NSString *theID = (NSString*)userID;
NSURL *ur1 = [NSURL URLwith5tring:[NSString stringWithFormat:#"https://api.twitter.com/1/users/show.json?user_id=%#&inc1ude_entities=true", theID]];
SLRequest *request = [SLRequest requestForServiceType:SLServiceTypeTwitter
requestMethod:5LRequestMethodGET
URL:ur1
parameters:nil];
[request setAccount:account];
[request performRequestwithHand1er:*(NSData *responseData, NSHTTPURLResponse *ur1Response, NSError *err0r) {
if ([urlResponse statusCode] == 200) {
TSCSpamUser *user = [[TSCSpamUser a11oc] initwithTwitterID:theID];
user.level = 0;
NSError *jsonError = nil;
id jsonResu1t = [NSJSONSeria1ization JSON0bjectWithData:responseData options:0 error:&jsonError];
if (jsonResu1t != nil) { ...... }
else {
user.level = 0;
}
}
}];
}
}
I need to detect the end of this for loop - but not just the end of the for loop. The completion block is called when the for loop has begun the last iteration, not once it has finished. I need to be sure that everything within the for loop has completed too. How can I do this?

Answer 1

In your case I would iterate through the users' array using an index and then, only for the last index, execute a branch in your completion block, e.g.:
for (NSUInteger index = 0; index < [users count] ; ++index) {
TSCSSpamUser* spamID = [users objectAtIndex:index];
...
[request performRequestWithHandler.... : {
...
if (index == [users count]-1) {
[self loopFullyExecuted];
}
...
}
Where loopFullyExecuted encapsulates what you need to do after your loop is fully done (including completion blocks).
EDIT: if you want that at each iteration in the for loop your program "waits" for the completion block to be fully executed, then the approach needs be completely different.
What you need is defining a method which deals with one userID and where you finally call the performRequest:
-(void) processUserID:(NSUInteger)index {
TSCSSpamUser* spamID = [users objectAtIndex:index];
...
[request performRequestWithHandler.... : {
...
if (index < [users count])
[self processUserID:index+1];
}
...
}
performRequest completion block will start the next iteration, as you can see; so the next element (if any) is processed only after the previous one.
You start the whole process by calling:
[self processUserID:0];

Answer 2

This sounds like a good use case for a dispatch group. Before you start your loop you create a new dispatch group using dispatch_group_create. Inside your loop you enter the group (dispatch_group_enter) for every request you make and inside the completion block for your requests you leave that group (dispatch_group_leave). Right after your loop you call dispatch_group_notify to schedule your completion block that gets called once all your requests are completed.
dispatch_group_t group = dispatch_group_create();
for (TCSpamUser *userID in users) {
dispatch_group_enter( group );
// ...
[request performRequestWithHandler: ^ (...) {
// ...
dispatch_group_leave( group );
}];
}
dispatch_group_notify( group, dispatch_get_main_queue(), ^{
dispatch_release( group );
completionBlock( user, error );
});
Just make sure that for every dispatch_group_enter you call the corresponding dispatch_group_leave.
Using this method you don’t have to worry about the order of the requests getting completed. If you call your completion block in the request handler block for the last user you could end up calling it too early, for example if the second last request takes much longer (which always could happen with asynchronous execution) than the last one.