objective c - How to synchronize two asynchronous delegate methods callbacks in iOS? -

i'm writing simple weather app try solidify understanding of protocols , delegates. i'm in following situation:

• i have 2 data sources weather data (just nsobjects @ time) , view controller want update once have received data both sources.
• the 2 data sources have protocols adhere in view controller. delegate methods called once have received data own web service. mean data source 1 gets data before data source 2, vice-versa, or @ same time (don't know 100% if possible)
• i want update view once have received data both sources.

what best way this? thinking of nesting delegate methods, data source 1 notify data source 2 when has data (through protocols), have data source 2 notify view controller update view when has data. don't think correct/best way things.

any ideas?

thanks

this sounds fit gcd dispatch groups.

first, create dispatch group dispatch_group_create. use dispatch_group_enter before start request each service. call dispatch_group_leave when receive response each service. can specify when both requests complete dispatch_group_notify.

the code might this:

dispatch_group_t weathergroup = dispatch_group_create();  dispatch_group_enter(weathergroup); [weatherservice1 requestwithcompletion:^(response *response){     // response     dispatch_group_leave(weathergroup); }];  dispatch_group_enter(weathergroup); [weatherservice2 requestwithcompletion:^(response *response){     // response     dispatch_group_leave(weathergroup); }];  dispatch_group_notify(weathergroup, dispatch_get_main_queue(),^{     // executed after both requests finished }); 

see docs more details: https://developer.apple.com/library/ios/documentation/performance/reference/gcd_libdispatch_ref/#//apple_ref/doc/uid/tp40008079-ch2-sw19