Cross-platform contract

Hello guys! I've encountered a problem. I have a contract with a company to implement a solution for Android and iOS natively. The app must not be cross-platform. However, it has a lot of logic and similar implementation between the two platforms. That means I have to write the same logic twice (one for the Android and one for the iOS). <br><br>But what if I can write the logic only once and allow it to be used by both platforms? Now, I've heard of something called framework but I don't exactly know how it works.<br><br>I was wondering if I can create something like a framework that after some minor tweaks would run not only on Android but on iOS as well? Or would it violate the contract? <br><br>Please let me know if there's any solution. <br><br>Thank you in advance.<br><br>&#x200B;<br><br>Edit: <br><br>I haven't mentioned this. They don't care about the language but they want native solutions. That means I have to write it in Swift, Obj-C for iOS and Java, Kotlin for Android.