angularjs - Angular : watch not working inside loop

Question

如果我动态注册 watch 事件(在我的情况下是 for 循环)，则 watch 无法工作。请看一下 fiddle 。有任何想法吗？

var myApp = angular.module('myApp',[]);


function MyCtrl($scope, $parse) {

    // 1. binding watch inside loop (doesn't work):
    $scope.aaa = 1;
    $scope.bbb = 2;

    $scope.dependsOn = [
        function () { return $scope.aaa; },  
        function () { return $scope.bbb; }
    ];        

    for (var i = 0; i < $scope.dependsOn.length; i++) {        

        $scope.$watch(
            function () {
                return $scope.dependsOn[i];                    
            },
            function (newVal) {
                if (newVal !== undefined) {
                    console.log("doesn't work");
                }
            }
        );
    }    

    $scope.aaa = 5;
    $scope.bbb = 6;    

    // binding watch not inside loop (works):

    $scope.ccc = 1;
    $scope.ddd = 2;    

    $scope.dependsOn = [
        function () { return $scope.ccc; },
        function () { return $scope.ddd; }
    ];    

    $scope.$watch(
        function () {
            return $scope.dependsOn[0];                    
        },
        function (newVal) {
            if (newVal !== undefined) {
                console.log("works");
            }
        }
    );    

    $scope.$watch(
        function () {
            return $scope.dependsOn[1];                    
        },
        function (newVal) {
            if (newVal !== undefined) {
                console.log("works");
            }
        }
    );     

    $scope.ccc = 5;
    $scope.ddd = 6;      
}

fiddle

Answer 1

您遇到的问题是因为您正在捕获变量 i在一个封闭。然后i递增到值 2 并退出循环，您的每个代表将有 2 作为他们的 i实际执行委托(delegate)时的值。

证明:
http://jsfiddle.net/df6L0v8f/1/
(补充:)

$scope.$watch(
     function () {
         console.log(i);
         return $scope.dependsOn[i];                    
     },
    function (newVal) {
         if (newVal !== undefined) {
              console.log("doesn't work");
         }
    }
);

您可以通过使用自调用闭包在迭代时捕获值并为您的委托(delegate)维护该值来解决此问题和变量提升问题。

http://jsfiddle.net/df6L0v8f/4/

for (var i = 0; i < $scope.dependsOn.length; i++) {        
     var watchDelegate = (function(itemDelegate){
          return function () {
              return itemDelegate;                    
          };
      })($scope.dependsOn[i]);

      $scope.$watch(
          watchDelegate,
          function (newVal) {
              if (newVal !== undefined) {
                  console.log(newVal());
              }
          }
      );
  }