After 1.4+, FxOS has started to support DSDS (Dual Sim feature) that you can insert 2 simcards into your FxOS device. By default, because FxOS is a unified OS that should be used in different devices, we will automatically hide this entry point by default if there is no more than one physical simcard slot.
If you check source codes in settings/js/panels/simcard_manager, you would notice that we have sim_ui_model.js and simcard_manager.js. Let me briefly explain the responsibilities of these two different scripts below.
For SimUIModel, this is a abstract interface between hardware information and our UI. Based on UX’s spec, there are so many different scenarios that we have to show / hide some parts of UI like simcard is not inserted, simcard is locked, simcard is blocked …etc. So, that’s why we need SimUIModel to control every sub-states.
While for SimcardManager, it will bridge the internal state of SimUIModel and real UI to make sure what should be hidden and what should be updated. In addition to this, SimcardManager also needs to watch some hardware events (like AirplaneMode, cardstatechange … etc) to update its UI to make sure they do reflect the real status.
In SimcardManager, there are three important select/option called Outgoing Calls, Outgoing Messages and Data. For some apps (like dialer), they will watch these mozSettingsKey to show / hide some hints to users, for example, if you choose Always Ask for outgoing calls, every time when you are trying to dial out a number, it will keep popping up a simcard picker UI so that you can choose which number you are going to use to dial.
But, there is one black magic that you need to keep in mind when you are dealing with these three select/option. In system app, there is one file called sim_settings_helper.js which does lots of works with these three options. Based on UX’s spec, there is a special request that we have to automatically switch options if from 2 simcards to 1 simcard. Interestingly, because from Gaia (including Gecko) aspect, we can’t directly make sure the second card is indeed inserted (We did encountered one edge case that one of colleague’s old simcard is hard to be recognized by our hardware and it takes more than 3 ~ 5 seconds to get its information from Gecko), due to this, sim_settings_helper.js will wait for upcoming event - simslotready from simslot_manager.js to make sure we did wait longer enough for the second card, and we can based on current situation to decided whether to switch option to another card or not.