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You can think of a mixin as a kind of mashup for a component; it combines the new behavior of the mixin with the existing behavior of the component, and bundles it all in one place. Mixins may be used to add specialized validation to user input fields, dynamically modify the HTML output of a component, or to add Ajax effects and behaviors to of all sorts of to components.
Tapestry comes with several mixins, such as the Autocomplete mixin which adds autocomplete behavior to an ordinary TextField Component. In addition, you can easily create your own.
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Component Mixins 