ios - get initial data from server on app start - ios

Question

I'm new to iOS development and trying to solve following problem.
In my app (which speaks with REST API) I want to make initial request to server on app start to get user info. I decided to use separate service class with singleton method. It makes request to server once and then returns user instance.
#implementation LSSharedUser
+ (LSUser *)getUser {
// make request to api server on the first call
// on other calls return initialized user
static LSUser *_sharedUser = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
LSHTTPClient *api = [LSHTTPClient create];
[api getUser:^(AFHTTPRequestOperation *operation, id user) {
_sharedUser = [[LSUser alloc] initWithDictionary:user];
} failure:nil];
});
return _sharedUser;
}
#end
My question is it a proper way of initializing global data from server? As you see request is async (with AFNetworking lib) so it will return null until request is finished.
Another problem here is that once it failed (bad connection for example) user will be null forever.

Answer 1

update your code like this
static LSUser *_sharedUser ;
+ (LSUser *)getUser {
// make request to api server on the first call
// on other calls return initialized user
if(_sharedUser){
//this will execute only at first time
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
LSHTTPClient *api = [LSHTTPClient create];
[api getUser:^(AFHTTPRequestOperation *operation, id user) {
_sharedUser = [[LSUser alloc] initWithDictionary:user];
return _sharedUser;
} failure:nil];
});
}
//this will execute 2nd time
return _sharedUser;
}
for answer to line ->
Ques 2 )Another problem here is that once it failed (bad connection for example) user will be null forever.
->once _sharedUser is initialized user will get _sharedData. but until shared data is not initialized it will return null whenever called.
Ques 1 )My question is it a proper way of initializing global data from server? As you see request is async (with AFNetworking lib) so it will return null until request is finished.
a better way is to implement your own custom delegate methods. once request is fetched or when call that do your work in that delegate method.
for 1st time: execute calling delegate methods when request is fetched or failed.
for 2nd time: after if block call delegate methods.

Answer 2

The basic approach requires an asynchronous design.
Say you have an asynchronous method:
- (void) loadUserWithCompletion:(void (^)(NSDictionary* user, NSError* error))completion;
You execute whatever you need to execute in the "Continuation" (the completion block):
[self loadUserWithCompletion:^(NSDictionary* params, NSError*error) {
if (user) {
User* user = [[User alloc] initWithDictionary:params];
// better we ensure we execute the following on the main thread:
dispatch_async(dispatch_get_main_queue(), ^{
self.model.user = user;
// If we utilize KVO, observers may listen on self.model.user
// which now get a notification.
// We also may to notify the user through an alert:
...
});
}
else {
// handler error
}
}];
With the asynchronous programming style you need to be more carefully when letting the user execute arbitrary tasks (say, tabbing buttons). You may consider to disable tasks which require a user model. Alternatively, display an alert sheet when the user model is not yet available.
Usually, you use some "observer" technique to get notified when the user model is eventually available. You may use KVO or completion handlers, or use some dedicated third party library which is specialized for those problems (e.g. a Promise library).
You should also let the user cancel that task at any point. This however requires a more elaborated approach, where you have "cancelable" tasks and where you hold references to these tasks in order to able to send them a cancel message.