gh9 gh9 - 1 year ago 131
Javascript Question

knockout sortable with computed observable not working

jsfiddle example. Like the title says I am trying to use a computed observable along with rniemeyer knockout sortable example. I keep getting

the write method needs to be implemented

This error is viewable in the developer console.

I have a write method implement on my ko.computed but it still errors out. What I am I doing wrong?

html and javascript below

<div id="main">
<div class="container" data-bind="sortable: tasks">
<div class="item">
<span data-bind="visible: !$root.isTaskSelected($data)">
<a href="#" data-bind="text: TestName"></a>
<span data-bind="visibleAndSelect: $root.isTaskSelected($data)">
<input data-bind="value: name, event: { blur: $root.clearTask }" />

var Task = function(first,last) {
var self = this;
self.firstName = ko.observable(first);
self.lastName = ko.observable(last);
self.TestName = ko.computed({
read: function (){
return self.firstName() + " " + self.lastName();
write: function (item) {

return self;

var ViewModel = function() {
var self = this;
self.testTasks = ko.observableArray([
new Task("test","one"),
new Task("test","two"),
new Task("test","three")

self.tasks = ko.computed({
read: function() { return self.testTasks();},
write: function(item) {console.log(item);}

self.selectedTask = ko.observable();
self.clearTask = function(data, event) {
if (data === self.selectedTask()) {

if ( === "") {
self.addTask = function() {
var task = new Task("new");

self.isTaskSelected = function(task) {
return task === self.selectedTask();

//control visibility, give element focus, and select the contents (in order)
ko.bindingHandlers.visibleAndSelect = {
update: function(element, valueAccessor) {
ko.bindingHandlers.visible.update(element, valueAccessor);
if (valueAccessor()) {
setTimeout(function() {
}, 0); //new tasks are not in DOM yet

ko.applyBindings(new ViewModel());

Answer Source

As the very author of this plugin says here, you can't use a computed observable; the sortable plugin depends on an actual observable array.

Which makes sense when you think about it: the plugin is actually manipulating the various indexes of the array as you re-sort the elements.

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