The app itself cannot expire, but functionality unlocked within the app can expire so long as the app remains useful--or the app is based on subscriptions, in which case you can make the expired app do nothing at all.
In the case of OmniGraffle 7 the free functionality that it retains after the trial expires is that the app can continue to be used as a free viewer for OmniGraffle documents (in much the same way as people use free PDF viewer apps).
When we first shipped OmniGraffle 7, some people thought that perhaps we'd just managed to slip its free trials past some Apple's reviewer's radar. But we were very careful and intentional about designing the app to follow their guidelines, and we've shipped several updates that have passed review since then--and at the end of the year Apple awarded the app a "Best of 2016" App Store award.
So now that we know for certain that Apple is OK with this approach, we're in the process of bringing this model to the rest of our apps on both Mac and iOS.
Ah, that's very interesting. I did not realize that reverting the expired trial version to "view-only" mode was considered within the guidelines. Thanks for the clarification, I may have to consider this model for my own apps.
I assume Apple did not have a problem with your use of the "Free Trial" wording in the app? After the trial period expires, does the app display a launch screen directing you to purchase the full version?
No, Apple didn't have a problem with that term: we just had to make sure the description was clear about the functionality which was retained after the trial ended.
After the trial period expires, all documents are read-only with a small badge in the title bar that you can click on to bring up the in-app purchase choices.
In the case of OmniGraffle 7 the free functionality that it retains after the trial expires is that the app can continue to be used as a free viewer for OmniGraffle documents (in much the same way as people use free PDF viewer apps).
When we first shipped OmniGraffle 7, some people thought that perhaps we'd just managed to slip its free trials past some Apple's reviewer's radar. But we were very careful and intentional about designing the app to follow their guidelines, and we've shipped several updates that have passed review since then--and at the end of the year Apple awarded the app a "Best of 2016" App Store award.
So now that we know for certain that Apple is OK with this approach, we're in the process of bringing this model to the rest of our apps on both Mac and iOS.