Take a look at example on http://jsfiddle.net/2NJ7y/3/ (version of AngularJS 1.0.1). There is simple app, which is waiting for entering of lucky number. If the number is equal to 7, I reset lucky number to null. If I enter number 7 several times, sometime/randomly the lucky number stay in input field. Why? How this behavior solve? Thanks.
I've done some debugging.
Firstly for me lucky number stay in input field not randomly.
enter 3 (model==3, input==3) => enter 7 (alert, model==null, input="")
=> enter 3 (model==3, input==3) => remove 3 (model=="", input=="")
=> enter 7 (alert, model==null, input="")
=> enter 7 (alert, model==null, input="7")
7 stay in input field only if previous model value was null.
What happens: when you input 7 fired input event which is handled by listener function of input directive. Listener function calls $setViewValue. $setViewValue sets $viewValue, $modelValue, model value and calls $viewChangeListeners (ngChangeDirective simply adds handler to $viewChangeListeners). Alert is displayed, luckynumber is set to null. After all that if luckynumber differs from previous value on previous dirty checking $watch handler and $render are called.
In my examples $render called if previous model value was "3" or "". If previous model value was null $render isn't called.
Why $timeout with 0 delay works: when you call $timeout with 0 delay function with changing luckynumber to null is postponed at the end of events queue (all javascript in a browser executes on a single thread). $viewChangeListener is not change model value from 7 to null. $digest finishes. Then $timeout handler is called. Model value is set to null. $watch handler and $render are called. $render sets input value to "".
At long last, a solution. Use $watch instead of ng-change:
$scope.$watch('luckynumber', function() {
if ($scope.luckynumber == 7) {
alert('The lucky number mustn\'t be equal 7.');
$scope.luckynumber = null;
}
})
This other SO answer by @Valentyn made me think of trying that solution to this question.
If you simply put
$scope.luckynumber = undefined;
prior to the alert you don't eliminate the race condition but you do change it so that 7 does get cleared properly, but sometimes you get the alert twice.
If the alert code is replaced by something idempotent, like altering the DOM to display an error, then this issue won't be important.