How to launch an app straight from another app on your phone with soft navigation keys
There are plenty of sidebar launchers and add-ons popping up like mushrooms lately in the Play Store. These solve several issues, besides being unorthodox and cool to use - they give you access to apps and functions on your phone while you are buried into doing something else. Now, there is an easier way to do that kind of app-within-an-app multitasking launch, especially if you own a Nexus 5, Z2, Moto G, or another phone with soft navigational keys.
If you want to easily find and launch an app from within another app you are perusing at the moment, you don't need to go back to the home key, or launch the task screen. App Swap is a new add-on that works with any launcher, and pops up a window full of app shortcuts each time you do the Google Voice Search launch gesture - i.e. swiping up from the navigational bar at the bottom of the display. Here's what you need to do:
Install App Swap, open it and purchase the full version. We kid, but the full functionality goes ad-free only this way;1.
2. The first time you swipe up from the bar, you'd be presented with a choice by Android Intent over whether to actually launch Google Voice Search, as usual, or assign App Swap to this gesture. Naturally, App Swap will be assigned as the default action, and will pop up any time you swipe up, no matter where you are in the interface, or which app you are deep-diving into at the moment;
3. App Swap can be customized by color, and you can search for apps within its windows, if the list is too long. By default, it lists your most used applications, though, unlike other side launchers, which makes it that more useful. In addition, the premium option supports hiding apps, icon packs, and a context menu with app info and uninstall options.
source: ebtokyo (XDA-Devs)