angularjs – $parser.unshift ??这是如何工作的？

link: function(scope,ctrl) {
    /**
     * This function is added to the list of the $parsers.
     * It will be executed the DOM (the view value) change.
     * Array.unshift() put it in the beginning of the list,so
     * it will be executed before all the other
     */
    ctrl.$parsers.unshift(function(viewValue) {

        scope.pwdValidLength = (viewValue && viewValue.length >= 8 ? 'valid' : undefined); // Check the length of the string
        scope.pwdHasLetter = (viewValue && /[A-z]/.test(viewValue)) ? 'valid' : undefined; // Check if the string contains letter. RegExp.test() simply returns a boolean if the string matches the regex.
        scope.pwdHasNumber = (viewValue && /d/.test(viewValue)) ? 'valid' : undefined; // Check if the string contains digit. Same remark.

        if(scope.pwdValidLength && scope.pwdHasLetter && scope.pwdHasNumber) { // If all is good,then…
            ctrl.$setValidity('pwd',true); // Tell the controlller that the value is valid
            return viewValue; // Return this value (it will be put into the model)
        } else { // … otherwise…
            ctrl.$setValidity('pwd',false); // Tell the controlller that the value is invalid
            return undefined; // When the value is invalid,we should return `undefined`,as asked by the documentation
        }

    });
}