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Observes the data at a particular keypath. Unless specified otherwise, the callback will be fired immediately, with undefined as oldValue. Thereafter it will be called whenever the observed keypath changes.

Note that you can observe keypath patterns...

ractive.observe( 'items.*.status', function ( newValue, oldValue, keypath ) {
    var index = /items.(\d+).status/.exec( keypath )[1];
    alert( 'item ' + index + ' status changed from ' + oldValue + ' to ' + newValue );

...or multiple space-separated keypaths simultaneously:

ractive.observe( 'foo bar baz', function ( newValue, oldValue, keypath ) {
    alert( keypath + ' changed from ' + oldValue + ' to ' + newValue );

See Observers for more detail.

ractive.observe( keypath, callback[, options ])

Returns an object with a cancel method, for cancelling the observer

keypath String

The keypath to observe, or a group of space-separated keypaths. Any of the keys can be a * character, which is treated as a wildcard.

callback Function

The function that will be called, with newValue, oldValue and keypath as arguments (see Observers for more nuance regarding these arguments), whenever the observed keypath changes value. By default the function will be called with ractive as this. Any wildcards in the keypath will have their matches passed to the callback at the end of the arguments list as well.

options Object

init Boolean

Defaults to true. Whether or not to initialise the observer, i.e. call the function with the current value of keypath as the first argument and undefined as the second

defer Boolean

Defaults to false, in which case observers will fire before any DOM changes take place. If true, the observer will fire once the DOM has been updated.


Defaults to ractive. The context the observer is called in (i.e. the value of this)

ractive.observe( map[, options ])

Returns an object with a cancel method, for cancelling all observers

map Object

A map of keypath: observer pairs

options Object

As above.