One of Android's key features is how it is relatively easily customised. Users, manufacturers and software engineers can amend a great many things about the operating system and how the device looks and operates. Some custom ROMs include a powerful theme engine, which makes it even easier for the look and feel of a device to be relatively quickly and easily changed. Another key feature of Android is how tightly many of the Google services are integrated into the device, as one might expect, as Android is originally a Google project. However, not all of Google's services and features are readily accessible beyond non-standard, or system, Google applications. One terrific example of this is the Google Now page, which is accessible from the Google Now Launcher by swiping from the left. However, this functionality is currently limited to the Google Now Launcher and may not be replicated by other third party launchers.
Kevin Barry, the founder of TeslaCoil (the company behind the powerful, flexible and popular Nova Launcher) has written a Google+ post to both explain why it is not currently possible to build a Google Now page into a third party launcher, and also to gather support for Google Now integration into third party launchers. His post explains that Google has included an API to allow device manufacturers to include Google Now as part of their launchers and this is something Sony are experimenting with, but a limitation of this is that the functionality is only available in system applications - in other words, the launcher that's built in to your device (or via downloading the Google Now Launcher from the Google Play Store). Kevin opens up the possibility that Sony has licensed the code that allows their launcher to trigger the Google Now page.
Kevin explains that it would be possible to rewrite the necessary API on a rooted device to enable left swipe support on devices, but by doing so, this could bother Google and ultimately might make the company less interested in working with third party launchers. Furthermore, Google could change the API and break this execution at any time. Google might tell license holders, if indeed Sony did license the code, but not anybody else until the functionality stopped working. Furthermore, changing this API could break other features of a given device such as software updates and rooting a device prevents Android Pay from working properly.
It is possible that Google will change the API at some point going forward, as it does not seem very Google-like to exclude third party launchers from one of their flagship Android products. Kevin adds a note of caution that if this were to happen, it could take several months if not years before Google updates the API. On the other hand, it could be that the current Sony arrangement is an experiment and if successful, might pave the way for a more immediate update to the Google Now code. Kevin finishes his post by advising readers that if they are interested in Google updating the API to support third party launchers, to +1 his post.