On iOS, Appium proxies command to a UIAutomation script running in Mac Instruments environment. Apple provides this application called ‘instruments’ which is used to do lot activities like profiling, controlling and building iOS apps but it also has an automation component where we can write some commands in javascript which uses UIAutomation APIs to interact with the App UI. Appiumutilizes these same libraries to automate iOS Apps.
In the above figure, we can see the architecture of the Appium in context to the iOS automation. If we talk about a command life-cycle, it goes like, Selenium webdriver picks a command form the code like (Element.click) and sends it in form of JSon via http request to the Appium server. Appium server knows the automation context like the iOS and Android and sends this command to the Instruments command server which will wait for the Instruments command client (written in node.js) to pick it up and execute it in bootstrap.js with in the iOS instruments environment. Once the command is executed the command client sends back the message to the Appium server which logs everything related to the command in its console. This cycle keeps going till the time all the commands gets executed.
