javascript - Isolate scope value is replaced only if it exists - AngularJS Directive -

template:

<ui-print text="print" panel="#paper1"            wait-for-data="adc.waitfordata"            callfront-fn="adc.somefunction()"></ui-print>  <ui-print text="print" panel="#paper2"></ui-print> 

directive:

scope: {     text: '@',     panel: '@',     callfrontfn: '&',     waitfordata: '=' },  link: function($scope) {     var invokeprint = function(panel) {         $window.print();         $scope.waitfordata = true;     }      $scope.printpanel = function(panel) {         if (!$scope.waitfordata) {             invokeprint(panel);         } else {             $scope.callfrontfn();         }     };      $scope.$watch('waitfordata', function(n) {         if (n === false) {             invokeprint($scope.panel);         }     }); } 

hi guys, trying understand how isolate scope works here. if @ above code there 2 element directives. 1 has waitfordata , other doesn't.

in 2nd directive, when invokeprint() , there $scope.waitfordata=true, not triggering $watch. in 1st directive does.

is when isolate scope not passed value cannot replaced in directive? not understand why behaving that.

i wanted know why watch not triggered in 2nd directive when $scope.waitfordata = true, triggered in 1st directive.

sorry not post entire code due security reasons. code needs debugged.

thanks in advance!!!

waitfordata: '=' 

means require variable waitfordata. if don't supply 1 wont break straight away break try set value. open console , see error messages. if wish value optional append question mark so:

waitfordata: '=?' 

now if change waitfordata variable in second directive trigger watch. should append question mark of optional variables or teh directive break try , use them.