In android sdk document, it's mentioned that all components(Activity, Service, etc) run on the main thread. The main thread is the thread where our code works on.
Conventionally, when talk about a Service, we think it this way:
// do something
In other words, a service is usually implemented as a infinite loop. If this is the case for android service, how can it run with other components on the same thread simultaneously?
And think about activity. It has to listen for incoming events like clicking a button, which also implies the pattern of an infinite loop. So, it's also not possible for activity to coexist with other components in the same thread as well.
In fact, components don't have their own loop, instead, they share the same main loop which is implemented in Looper. This also explains why google strongly recommends not do any long-running tasks in a component's life cycle events. Say, if we run a procedure for a long time in onBind function of a service, the main thread won't have any chance to deal with new message in the loop until the procedure finishes. The best idea is to spawn a new thread to do the task and let the main thread back to the main loop as soon as possible.
Looper is the starting point of an application. It acts as the role of message loop. It loops forever waiting for incoming messages and dispatch messages to corresponding components to handle. Upon receiving a message, the corresponding handler object's dispatchMethod will be invoked.
So, in android, a component is a object with a state machine and is able to handle all kinds of messages.