The best answer is probably the one that was provided by Apple in the Cocoa iOS Application Requirements Main Cycle section.
In the cycle of the main event, the application continuously sends incoming events to objects for processing and, as a result of this processing, updates its appearance and state. An event loop is just a launch loop: an event loop to schedule work and coordinate the reception of events from various input sources connected to the trigger loop. Each thread has access to a run loop. In everything except the main thread, the launch cycle must be configured and started manually according to your code. In Cocoa applications, the execution loop for the main thread — the main event loop — is started automatically by the application object. What distinguishes the main event loop is that its primary input source accepts events from the operating system that are generated by user actions - for example, clicking on a view or entering text from the keyboard.
By the way, if you are relatively new to iOS development, I would recommend reading this document, as it will answer many of the questions you have.
John parker
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