Behind the Curtain: Inter-App Communication

I’ve been working on another Android app the past couple weeks. We’ve added a couple of features, included enhanced logging and a couple of night-mode themes.

One thing that has been often requested is a way to launch Scanner Radio from our app.
I got in contact with the developer of Scanner Radio and he gave me a snippet of code that allows Active911 to launch the Scanner Radio selector.
The main problem I’ve been working on overcoming today is how to store the Intent that is given back in the ActivityResult of the selector.

There’s not really a good way to serialize an Intent in Android, it seems that they are intended to be passed and immediately used.
The design pattern for most Inter-Process Communication is defining an Intent schema that can be used by external applications.
Another pattern used is exposing an external Service, which I haven’t looked into much.

Regardless, right now I have an Intent that, when used immediately, works as intended. However, I can only use that intent as long as the app is open, so I need to find a way to persist the Intent while maintaining all of its required information.

At first I tried just using the Intent.toUri() method. That gets me halfway there, as the URI still doesn’t contain any of the Extras in the Intent.
The next thing I tried was manually serializing the data into tab-delimited name value pairs. However, this doesn’t work as I don’t necessarily know if the extras are all Strings, as well as how many layers of nesting there are.

My final strategy is to take the Bundle and serialize it into a byte array, and then store that byte array as a UTF-8 string, then reversing the process to get it back out.
This stackoverflow answer was the most helpful in figuring out how to do so

The downside to this is that, since the class implementing Parcelable may change between platform versions, this is not guaranteed to work across system updates.
Also, after digging through the Parcel and Bundle source code, it seems that the Intent may be too large to serialize and deserialize in this way.

It looks as though the extras only has a single item, so instead I will be just serializing and deserializing this one item.
It contains a set of 3 name value pairs, so I will serialize that into JSON, though serializing it into byte data did work initially.

In the end, I ditched the Parceling and byte storing of the data, since that is highly discouraged, and just stored the name value pairs.
As long as the bundle doesn’t get deeper, have a change in the name of the extra, or use anything other than Strings in the sub-bundle it will work.

The Android update with a Scanner Radio setting is hitting our new Beta group today, and will likely be updated in the Google Play store next week.

2 thoughts on “Behind the Curtain: Inter-App Communication

  1. Do you have any plans for Active911 to be accessible to other programs (like Tasker) via Intents (or some other inter-app communication method)? It would nice to be able to really customize our device based upon receiving an alarm through your program.

  2. I’m running Android 4.4.2 w/ Active911 v1.2.8.002, without GPS enabled, but
    scanner radio on alert shows as not installed
    Q: why is scanner radio on alert showing as not installed and how can i get it to work?
    BTW, i read the post above (https://blog.active911.com/?p=86) nice job!
    I’m willing to volunteer for testing if needed.

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