How to create APK Expansion Files for assets within flutter - flutter

Question

I have created a flutter app with video assets which are about 450mb in size. I have published the app successfully in the Apple appstore. But Google Play does not accept my APK as it is over the limit of 200mb. I tried to go with the approach of creating APK expansion files as the recommended workflow. I have read all available Android documention about expansion files but I still can not figure out how to implement them with flutter.
How do you implement and access assets within APK expansion files in a flutter app?

Answer 1

You're going to have a little bit of fun with this. Basically, until someone implements a plugin to access APK expansion files, you're going to have to write the java code to connect up to Flutter.
It's not prohibitively difficult, it just means that you're going to have to learn about Platform Channels and write a bit of native android code. The documentation probably does a better job of explaining platform channels than I do, but basically the easiest way is to use a MethodChannel to pass data from dart to native and vice-versa.
What you'd do to start is set up a method channel to initiate this process and call it with something like getObbFolder.
On the android side the first thing you need to do is make sure your app actually has the files downloaded. According to the android documentation, you can't guarantee that they will have been so you need to write the logic to download them. I'd recommend using the Download Library they provide as there's all sorts of things to worry about like the device running out of storage, network connectivity, showing progress, etc. I think the documentation for that is moderately straight forward (and if you have issues I'd recommend asking a new question specific to it.
Once you've done that, you need to get the path to the file, and request permission to read it if needed. Some android versions and some devices in other versions (it sounds like it's a bit of a crapshoot to be honest), you need permissions to read the file, while in others you don't. So it's best to just try and ask for permission if trying fails.
To get the directory it's saved in use context.getObbDir()
Then this one way to do it from the android docs:
File obb = new File(obb_filename);
boolean open_failed = false;
try {
BufferedReader br = new BufferedReader(new FileReader(obb));
open_failed = false;
ReadObbFile(br);
} catch (IOException e) {
open_failed = true;
}
if (open_failed) {
// request READ_EXTERNAL_STORAGE permission before reading OBB file
ReadObbFileWithPermission();
}
And for the versions that don't do runtime permission checking add this to your application manifest:
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
Android doesn't recommend always asking for the permission since sometimes the Obb folder is exempted from needing the READ_EXTERNAL_STORAGE permission.
Now you have two options. One is that you could pass that path back to flutter, and then use flutter's file reading to do something with the data, if the file is something you can directly read. If it's not, you could unpack it either using flutter or using java/kotlin, and then pass back the path to the unpacked files.
If you do choose to unpack the file from android you should do it to one of the directories flutter knows about (with the path_provider plugin for example), or write to wherever you want (and have permission to) and then simply pass the path back to flutter.
Hope that helps!