angular-watch-when

'Watch when' feature for AngularJS expressions that builds upon the ng 1.3 bind once expression syntax. Bind once is great, but sometimes you need your bindings to be a little smarter and get fired occasionally. This purposefully builds on angular-watch-require to recycle and reuse existing watches, but could be used in isolation if desired.

For example, you might not expect an id property on some selected entity to change; but what if the selected entity changes? You might like your bindings, links, etc. to reflect the changed reference, re-fire, and go away again. If used properly, this can significantly reduce the number of active watchers (like regular bind-once), while keeping the app reactive to the data you expect to change.

JSFiddle

Usage

bower install angular-watch-when

Add 'ngWatchWhen' to your application or module dependencies.

Expression syntax

The expression syntax is baked into $parse and can be used in $scope.$watch[*], templates, and directives. It's similar to the 'one-time' expressions described in the AngularJS 1.3 guide on expressions, One-time Binding.

::myModel::myModel.id will return a one-time expression on ::myModel.id that re-registers whenever the value of the expression myModel changes.

var unwatch = $scope.$watch('::myModel::myModel.id', function(){
    // called until 'myModel.id' is defined, for every new value of 'myModel'
});

unwatch(); 
// Will unregister everything, this watch listener will never be called again

Performance Gains

The simplistic, above example wouldn't result in a performance gains via a reduction in watches. In fact, it could only result in more or the same, considering the myModel watch lives forever. This module is designed to be used with angular-watch-require with an expression like the following ::?^myModel::myModel.id. This will attempt to re-use a myModel watcher, which will likely be used by many 'one-time' bindings. Make sure to import the module 'ngWatchRequire' before 'ngWatchWhen'

See also

angular-watch-require - Reuse watches by extending expression syntax to require and recycle existing watches.