Question

我试图理解如何编写JavaScript对象的枚举器。我遇到了Sharepoint code where items of a SP List were being obtained on the client side的某个部分，并且正在对Sharepoint对象执行类似于下面的代码。

var enumerator = list.getEnumerator();
while(enumerator.moveNext()) {
    var currentItem = enumerator.getCurrent();
    // actions on currentItem
}

如果我可以编写一个可以在使用上面的枚举器时迭代的自定义对象，它看起来怎么样？这是我的尝试，但它不起作用：

var list = {
    items: ["A", "B", "C", "D", "E"],
    counter: -1,
    enumerator: {
        moveNext: function() {
            counter++; // outer object's variable not accessible
            if(counter >= items.length)
                return false;
            else
                return true;
        },
        getCurrent: function() {
            return items[counter]; // outer object's variable not accessible
        }
    },
    getEnumerator: function() { 
        return this.enumerator(); 
    }
}

var enumerator = list.getEnumerator();
while(enumerator.moveNext()) {
    var currentItem = enumerator.getCurrent();
    alert(currentItem);
}

你能帮助我理解如何将getEnumerator()写成适用于任何可枚举对象的函数原型吗？

Answer 1

尝试以下：

var list = function() {
    // these local variables could be accessed by inner functions.
    var items = ["A", "B", "C", "D", "E"],
        counter =  -1;
    return {
        enumerator: {
            moveNext: function() {
                counter++; 
                return counter < items.length;
            },
            getCurrent: function() {
                return items[counter]; 
            }
        },
        getEnumerator: function() { 
            // here should not be return this.enumerator() but just this.enumerator
            return this.enumerator; 
        }
    };
};

var enumerator = list().getEnumerator();
while(enumerator.moveNext()) {
    var currentItem = enumerator.getCurrent();
    alert(currentItem);
}

<强> THE JSFIDDLE DEMO.

Answer 2

您可以使用list.

访问变量

var list = {
    items: ["A", "B", "C", "D", "E"],
    counter: -1,
    enumerator: {
        moveNext: function() {
            list.counter++; // outer object's variable not accessible
            if(list.counter >= list.items.length)
                return false;
            else
                return true;
        },
        getCurrent: function() {
            return list.items[list.counter]; // outer object's variable not accessible
        }
    },
    getEnumerator: function() { 
        return list.enumerator; 
    }
}

或者您可以创建一个函数来创建它的实例

function makeList(items) {
    var list = {
        items: items
        counter: -1,
        enumerator: {
            moveNext: function() {
                list.counter++; // outer object's variable not accessible
                if(list.counter >= list.items.length)
                    return false;
                else
                    return true;
            },
            getCurrent: function() {
                return list.items[list.counter];
            }
    },
    getEnumerator: function() { 
        return list.enumerator; 
    }
    return list;
}


var enumerator = makeList(["A", "B", "C", "D", "E"]).getEnumerator();

Answer 3

感谢来自@xdazz和@Juan Mendes的指针，我能够计算出枚举器函数原型：

Array.prototype扩展名：

Array.prototype.getEnumerator = Array.prototype.getEnumerator ||
function() {
    var items = this;
    var counter = -1;
    var enumerator = {
        moveNext: function() {
            counter++; 
            return (counter < items.length);
        },
        getCurrent: function() {
            return items[counter]; 
        }
    }
    return enumerator;
};

<强>用法：

var list = ["A", "B", "C", "D", "E"];

var enumerator = list.getEnumerator(); // makes any array enumerable
while(enumerator.moveNext()) {
    var currentItem = enumerator.getCurrent();
    alert(currentItem);
}

<强> JSFIDDLE DEMO

编写JavaScript对象的枚举器

3 个答案: