js/listener.md

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# Listener
A generator for proxy objects that run callbacks when properties are changed.
## Interface
```js
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listener()
// Creates a new listener object
listener(some_object)
// Creates a new listener object that acts as a proxy for some_object
listener.listen(prop, callback)
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// Adds a callback on a specific property change
listener.listen(null, callback)
// Adds a callback on any property change
listener.listen(prop, callback, {once: true})
// Adds a one-time callback
```
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```js
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listener.forget(prop, callback)
// Removes a specific callback on a specific property
listener.forget(prop, null)
// Removes all callbacks from a property
// Note that undefined won't work, to avoid accidents
// it really has to be null
listener.forget(null, callback)
// This is not a special case, it simply removes a
// callback that was registered with lisetner.listen(null, callback)
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```
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Note: Forgetting one-time callbacks is not (yet) possible.