How to extend app with downloadable APKs (plugin sort of)

Question

I'm just wondering how the following scenario can be solved:

I want to write a very simple app for my daughter. The app displays 4 colored fields on the activity and through speech output says something like "tap green". She should then tap the green field to get positive feedback. So far, so easy.

I then thought it might be nice if the app would also "grow" as she grows older. Maybe later I want to have a game mode where I display animals, numbers, vehicles, etc., so it would be nice if I could have something like "Shape sets" - basically a set of images along with a description of what the app should say for each image. Also easy enough - all you need is a set of images and an XML file describing the images.

BUT
I'd like to be able to install these "Shape sets" as additional APKs later on, so that I don't have to modify the app every time. I'd like to install the APKs, so that the contents are added to a specific sub folder on the SD card, into which my app looks to enumerate available "Shape sets".

If I ever published the app to the Play Store, other people should also be able to download the "Shape sets" I create (no need for user contributions, though).

Is that possible? If so, what would I have to do to have the Android OS "copy" the contents of an APK to a specific folder (lets say "/TapGame/Shape Sets/Animals") on the SD card? Or is there even another way of achieving what I want that I didn't think of?

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The term "plugin architecture" just came to my mind as I wrote the question. Searching using that term I found this question: Extend my android app in different APK

It seems to provide a solution to my problem - I'll investigate this further, but please feel free to suggest other possible solutions!

Answer 1

Is that possible?

Um, sure.

If so, what would I have to do to have the Android OS "copy" the contents of an APK to a specific folder (lets say "/TapGame/Shape Sets/Animals") on the SD card?

Android won't do any of that. You have to do that. You would have to detect that a "shape set" APK was installed (either watching for package-installed broadcasts, scanning all installed apps for ones that seem to be a "shape set", etc.). Then you would have to arrange to copy whatever you wanted to wherever you wanted it, either by:

• Asking the "shape set" app to do it (e.g., send a command to some IntentService), or

• Using createPackageContext() and trying to do the copying from your main app

Or is there even another way of achieving what I want that I didn't think of?

Um, just use ZIP files that your app downloads itself from a well-known location. That corresponds to Dave Smith's final paragraph of his answer on the question you just linked to in your edit.

Or, just update the main app. I'm not quite certain what effort you think that you are saving otherwise.

Or, just keep the content online, using a Web service to indicate the available "shape sets" and downloading them as needed (with optional caching).

Answer 2

Fascinating question. If you really want to go for plug-ins then OSGi would probably be the way to go, but it's a lot of work to get to know and to use and seems like overkill in this case.

I don't know how your shapes are defined, but they are probably each defined in a separate file-set, providing the shape (maybe a png or jpg?) as well as the audio-file that will be used as a command for this shape. If the folder in which these file-sets are stored is fixed (TapGame/Shape Sets/...) the app could scan the folder each startup and the views could be generated accordingly (in this case, the activity cannot be build entirely in the XML-File, but must be partially done programmatically).

The Plugin-Aps would be rather easy. They are an apk which includes the file sets (jpg and mp3 or whatever). Started once they deposit all these files into the specified folder (they probably check before if these files exist) and then the apk can shut down again and be uninstalled.

on the next startup the Tap Game App would find the new symbols and include them into the game.

This seems rather straight forward to me. Another way would be to actually store the shapes and audio files on the internet and with each start of the App check if the number of shapes and audio-files has changed and create local copies of new ones. This would mean no downloads of apks... probably a more usual approach to the issue.

I am very interested to hear what you make of it, seems like a different approach then the norm, which is always cool to see.