在两个视图控制器iOS之间不断传递数据

I'm new in objective-c. I try to send data between 2 view controllers (for example controllerA and controllerB), but I need this data constantly, so I use _displaylink in controllerB to call method from controllerA. I use player in controllerA and I want to display time etc. in controllerB. How to get existing instance of controllerA? I'm new in objective-c. I try to send data betw