According to recent rumors, Apple has also published the patent for the foldable phone.

A foldable iPhone needs to be a great foldable phone, period . As per Apple’s designed you can use that foldable phone as mobile and tablet. So when can we expect Apple to launch a foldable iPhone? Apple's strategy is to be the best, not the first.
The patent describes how Apple’s foldable phone will works, its safety, flexibility, features and much more. However, the patent is not necessarily perfect; they vary and so the design of device changes. There has been a huge anticipation about seeing the release of a new Apple foldable phone either in 2019 or 2020.

Working with Foldable News, Gilsing has demonstrated how the application of Apple’s design aesthetic on this form factor has the potential to create a truly transformative device. Apple Foldable Phone Release Date: September 2020 or 2021. However, it looks like the Cupertino company is still busy working on the development of the device. The earliest predictions are that Apple will launch a foldable phone in 2020. Whether it's an iPhone, Android or some other future OS, a good foldable phone will need to hit plenty of basic design considerations. So, the release will still take longer.