knockout-2.0 - 淘汰赛2.2.0，在更改之前订阅获取值和新值

Question

jsfiddle链接：http://jsfiddle.net/T8ee7/

当我调用Knockout的subscription方法时，是否可以同时获取以前的值和新的值？现在，我只能单独调用get这些值。
如果新旧值不同，我想触发一些代码。

我想我可以做以下事情，但是会变得凌乱...
（http://jsfiddle.net/MV3fN/）

var sv = sv || {};
sv.PagedRequest = function (pageNumber, pageSize) {
    this.pageNumber = ko.observable(pageNumber || 1);
    this.numberOfPages = ko.observable(1);
    this.pageSize = ko.observable(pageSize || sv.DefaultPageSize);
};


var _pagedRequest = new sv.PagedRequest();
var oldValue;
_pagedRequest.pageNumber.subscribe(function (previousValue) {
    console.log("old: " + previousValue);
    oldValue = previousValue;
}, _pagedRequest, "beforeChange");


_pagedRequest.pageNumber.subscribe(function (newValue) {

    console.log("new: " + newValue);
    if (oldValue != newValue) {
        console.log("value changed!");
    }

});

_pagedRequest.pageNumber(10);
_pagedRequest.pageNumber(20);
​

Answer 1

我更喜欢使用observable extender。

http://jsfiddle.net/neonms92/xybGG/

扩展器：

ko.extenders.withPrevious = function (target) {
    // Define new properties for previous value and whether it's changed
    target.previous = ko.observable();
    target.changed = ko.computed(function () { return target() !== target.previous(); });

    // Subscribe to observable to update previous, before change.
    target.subscribe(function (v) {
        target.previous(v);
    }, null, 'beforeChange');

    // Return modified observable
    return target;
}

用法示例：

// Define observable using 'withPrevious' extension
self.hours = ko.observable().extend({ withPrevious: 1 });

// Subscribe to observable like normal
self.hours.subscribe(function () {
    if (!self.hours.changed()) return; // Cancel if value hasn't changed
    print('Hours changed from ' + self.hours.previous() + ' to ' + self.hours());
});