Below are some worthy options you want to consider when choosing the emulator for your platform. The beautiful thing is that writing an emulator is an incremental process. You don’t have to implement everything all at once for games to work! You can start with a basic CPU that only supports branch instructions and some mortal kombat snes rom ALU , write some assembly test cases for it, and improve upon it and add more instructions slowly and carefully. My side-project got to a stage where it successfully emulates most games, and you can either run it on desktop, android and even from your browser.
I believe now would be a good time to b̶r̶a̶g̶ ramble on the experience, share some thoughts and insights, discuss internals, debugging techniques, etc. However, this isn’t a “complete guide on how to write an emulator from A-Z” kind of thing. For an iPhone GBA emulator, you might have luck using GBA4iOS.
It’s not available from the official App Store but if you can manage to get it installed on your device, you can use it for free to play GBA games on your iPhone without needing to jailbreak your phone. VBA-M has an option for Motion controls "Input, Set, Motion".
It currently works with all versions of the GBC title Kirby Tilt’n Tumble, which also was a special cartridge with a motion sensor built-in to control movement in-game. It has built-in cheats and various options to improve the rendering. Particularly useful is the Dropbox support, which allows you to sync your saves and configuration data to the cloud, then continue playing on another Android device. Game Boy and Game Boy Color were the handheld consoles that started it all.
There are the GBA emulators for Windows, Linux, and Mac platforms. There are also powerful solutions for Android and iOS devices. But you should know what criteria to look at when selecting your tools.