Unwatch an expression: Sometimes you want to watch an expression only a few times, and then forget it. The $watch function returns a callback just for that. You just have to execute it back to destruct the watcher.

unwatch.js

var watcher = $scope.$watch('data.counter', function(newValue, oldValue) {
    iElement.css('width', 50 * newValue + 'px');
    if (newValue >= 10) {
        // when data.counter reaches 10, destruct the watcher.
        watcher();
    }
});