访问通过工厂初始化的变量，该工厂启动异步请求

Question

我有一个工厂发送了一个邮政请求以获取一些JSON键值对：

.factory('Rest', ['$resource',
function($resource) {
    // returns JSON key-value pairs, e.g. "{'foo', 'bar'}"
    return $resource('rest/get', {}, {
        get: {
            method: 'POST'
        }
    });
}])

我有另一个工厂打算接触控制器，以便访问其钥匙的键值：

.factory('Pairs', ['Rest',
function(Rest) {
    var pairs;
    Rest.get(function(response) {
        pairs = response;
    });
    return {
        get: function(key) {
            return pairs[key];
        }
    };
}])

问题是，当我调用Pairs.get('foo')时，Pairs工厂的pairs对象尚未初始化（因为Rest.get是异步），因此导致TypeError: Cannot read property 'foo' of undefined：

.controller('AnyController', ['Pairs',
function (Pairs) {
    console.log(Pairs.get('foo')); // error
}])

任何想法如何解决？

Answer 1

正如您在问题中所述的那样，Rest.get是异步的，因此您的Pairs.get也必须异步。您可以按以下方式实现它：

.factory('Pairs', ['Rest', '$q',
function(Rest, $q) {
var pairs;
var deferredList = [];
Rest.get(function(response) {
    pairs = response;
    angular.forEach(deferredList, function(o) {
        o.deferred.resolve(pairs[o.key]); // resolve saved defer object
    });
    deferredList = null; // we don't need the list anymore
});
return {
    get: function(key) {
        if (pairs) {
            return $q.when(pairs[key]); // make immediate value as a promise
        }
        var deferred = $q.defer(); // create a deferred object which will be resolved later
        deferredList.push({ // save both key and deferred object for later
            key: key,
            deferred: deferred
        });
        return deferred.promise;
    }
};
}])

然后这样使用：

Pairs.get('foo').then(function(value) {
    console.log(value);
});

Answer 2

您想将异步功能包裹在承诺中。这是我做类似的事情的方式。注意：如果需要时，安全触发$摘要周期，以便对任何数据更改都可以观看。

 var safeApply = function (scope, fn) {
                    if (scope.$$phase || scope.$root.$$phase) { 
                        fn(); 
                    } else {  
                        scope.$apply(fn); 
                    }
                };
ret.getAll = function(type) {
    var deferred = $q.defer();
    var where = "apm_type = '" + type + "'";
    query(type, where, function(err, response) {
        var objs = [];
        if (err) {
            safeApply($rootScope, function() { deferred.reject(err);});
        } else {
            safeApply($rootScope, function() { deferred.resolve(response);});
        }
    }); 
    return deferred.promise;
};