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This involves sources, listeners and observers and is an interesting multi-tier model of linkage/communication
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One can (in some cases) merge observer and listener but not the listener and source
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Registration involves sending an object name -- termed the callback object -- from the listener to the source.
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When event occurs, the source invokes a particular (event dependent) method of the callback object
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Events can be distinguished by using different callback methods (demultiplexing) or by inspecting the argument of a more generic method.
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Sources and Listeners use a "controlled syntax" but there are no special interfaces or methods for linkage of observers and listeners as usually(often) these are combined.
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