ViewDidAppear not called when the modal view is dismissed

First, I create a MainViewController. Then in MainViewController, I do

[self presentViewController:modalViewController animated:YES completion:nil];
modalViewController.modalPresentationStyle = UIModalPresentationFormSheet;

When I dismiss the modalViewController, On iPhones(except iPhone 6+), viewDidAppear of MainViewController is called. On iPads and iPhone 6+, viewDidAppear of MainViewController is not called.

The logic is to called a function when the modalViewController is dismissed. How can I know when the modalViewController is dismissed.