Snap Interfaces

Publishing a Snap

# Login to your Snapcraft Account
snapcraft login

# Build Snap Package
cd root/of/your/snap
snapcraft

# Publish to Stable Chnnel
snapcraft register snap_name
snapcraft push snap_name.snap --release stable

# Publishing Privately
snapcraft register --private snap_name
snapcraft push snap_name.snap --release stable

Using Shared Library

snap-a offers the stringsnap library. snap-b utilizes the stringsnap library and builds the stringrunner binary. We will use the content interface for connecting slots and plugs.

Exposing stringsnap library using slot

slots:
  lib0-1804:
    interface: content
    content: lib0-1804
    source:
      read:
        - $SNAP/lib

lib0-1804 exposes a slot on the content interface which has the capability to read the $SNAP/lib directory.

Utilizing the slot using a plug

plugs:
  lib0-1804:
    interface: content
    content: lib0-1804
    default-provider: stringsnap:lib0-1804
    target: $SNAP/extra-libs

Apart from this, we have to set the LD_LIBRARY_PATH for the stringrunner snap so that it can find the required library at the extra-libs folder created.

apps:
 my-app:
   command: bin/stringrunner
   plugs:
     - lib0-1804
   environment:
       LD_LIBRARY_PATH: $SNAP/extra-libs/lib

Establishing an Interface connection

By default, for a content interface, if the slots and plugs are coming from snaps from the same publisher, they are automatically connected. For the snaps to have a publisher associated to them, they have to be installed from the Snap Store. For manually connecting two snaps:

sudo snap connect snap1-name:plug-name snap2-name:slot-name

Notes

1. snap_name shouldn't follow the regex snap*
2. Library name should follow the regex lib*.la
3. For content interfaces which are responsible for exposing libraries, it's preferred to keep the interface name as libx-yyzz where x is a version number, yyzz represents Ubuntu yy.zz LTS used for building the snap package.
4. While building the dependent snap, we have to mention the absolute path of our required library in the Makefile.
5. Since snaps have a confined environment in which they can work, they can't use the hardcoded path of the dependent snap at runtime. To solve this issue, we establish an interface to expose dependent snap's libraries to our required snap.

References

1. https://forum.snapcraft.io/t/publishing-private-snaps/1905
2. https://forum.snapcraft.io/t/installing-private-snap-package/5816
3. https://snapcraft.io/docs/supported-interfaces