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Adding custom libraries to the flutter apk build

If you are developing an android app in flutter and your apk compilation step works, but then runtime halts on the following error: "ArgumentError (Invalid argument(s): Failed to load dynamic library 'libhub.so': dlopen failed: library "libc++_shared.so" not found"

Exception has occurred.
ArgumentError (Invalid argument(s): Failed to load dynamic library 'libhub.so': dlopen failed: library "libc++_shared.so" not found: needed by /data/app/[...]/lib/x86_64/libhub.so in namespace classloader-namespace)
Then you have something in your code that uses this library to run properly. Ideally flutter together with Gradle is supposed to do everything of this kind of library packaging for you, so step one is to check that your basic configuration is correct. It's generally advisable to not have to do custom solutions like this one, and first see what you can do within standard flutter dependency management using flutter pub get <package> commands and or by dart solutions like the dart:ffi package.

But if that leads to a dead end, I'll offer you a more hands-on approach here. By manually adding the libc++_shared.so to your flutter project so that it gets included into the apk on build.

How to add libc++_shared.so to your flutter apk

Flutter lets you do some platform specific customizations, by editing the platform folders. We want to go to this folder android/app/src/main/ in our flutter project structure, and mkdir jniLib. Then cd into that folder and create the following file structure: 

/jniLib/.
        ├── arm64-v8a
        ├── armeabi-v7a
        ├── x86
        └── x86_64
Whatever is placed in these folders, Gradle will make sure to add within the /libs/ folder of your apk file, making its content available for your android app as libraries. Create the folders manually or cd to jniLib and run the following bash script: 
mkdir -p ./{arm64-v8a,armeabi-v7a,x86,x86_64} && echo "Folder structure created successfully."
Then it's time to populate your folders with the files you need. You can either add whetever you needed or, do what I'm going to instruct on next: add the libc++_shared.so.

Adding the libc++_shared.so file to your jniLib/ folder

The standard share library libc++_shared.so is part of the android development kit, that gets installed by android studio (I assume this is already done and that you know about how it works). You can obtain the library by copying directly from your android SDK. Go to wherever you installed the android_sdk and look for this folder android_sdk/ndk/<version-number>/toolchains/llvm/prebuilt/<OS>/sysroot/usr/lib and in that lib/ folder and search for libc++_shared.so. You can also search for the file using your operating system search function higher up in the ndk file structure if you want, the main objective is to find it.

Once found you can simply copy & paste it into the folders you created in the jniLib/ folder within the flutter project and then retry building your app. It's likely that you either have a working app, or at least a new error message that isn't related to missing this library anymore.

Personally I came across this problem while working with rinf for flutter and as a reference I figured things out while troubleshooting this github issue.