Answer to Question 1
Events are occurrences that interrupt the existing condition of one or more objects. There three types of events:
First are call events, which invoke operations. When an object sends a message to another object, the recipient of the message views the arrival of the message as an event to which it must respond. As such, call events are the other side of the coin to messages. Moreover, such events are usually both targeted and synchronous (the event and the response to it work within the same phase or timeframe).
Second are signal events. These are anonymous, meaning that they are not directed to a specific object, but will be received by any object that catches them. (Call events may be compared to receiving a personal telephone call, while signal events are like buying newspapers or tuning to a radio station to learn about events.) Furthermore, signal events are usually asynchronous, meaning that the action that results in the event may not occur in the same phase or timeframe that the event occurs.
Third are time events, which are like signal events: disruptive, asynchronous, and anonymous. The important difference is that they are triggered by the passage of time, not by any prior action.
From the viewpoint of object-orientation, all the types above could be significant (or change) events if they change the state of an object (or a system).
Answer to Question 2
B
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