The Observer Pattern facilitates event-based programming. You use this pattern whenever you wire up events in a language like C# or JavaScript (using the EventEmitter in Node or listening to DOM events in the browser).

Many frameworks have the mechanics for observation al- ready built in, but let’s look at how we can construct an ob- server by hand by adding methods to our GroovyQuery that get fired when certain events occur. These are commonly referred to as callbacks

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