Knockout.js optgroup select with optioncaption

Question

I have implemented a grouped select box in knockout.js which was inspired by ( I figured it out by looking at ) KnockoutJS - Binding value of select with optgroup and javascript objects by RP Niemeyer but it's a little different. it looks like this.

<select name="Field" data-bind="foreach: FieldList.Groups, value:Field" >
    <optgroup data-bind="attr: {label: Label}, foreach: Children">
        <option data-bind="text: Text, value: Value"></option>
    </optgroup>
 </select>

my viewmodel would look like

var viewmodel = {
    Field: 2,
    Groups:{
      Label:"Field 1",
      Children:[
         { Text:"Field1", Value:1 },
         { Text:"Field2", Value:2 }
      ]
    }

Something like that, in any case it works great. However, I really need to put a "Please Select ..." as the first option.

Given that it's a foreach loop

a) optionsCaption binding doesn't work and

b) I can't just add the option in there cuz it would be repeated for each group.

just to make sure no one can help me i'll add this constraint. I am actually code gen'ing ( just some C# on server ) the html and while I can do a lot of custom stuff I can't add arbitrary pieces of text, ie comments into the output. which is to say I can't do the containerless foreach because I can only emmit htmltags not html comments.

ugg.

If anyone has any idea's please let me know I would greatly appreciate it. thanks, R

Answer 1

Alrighty, so I decided to just do my own binding. It was much easier then I expected. I will post it here. I just ripped this straight from the source code and modified it slightly. The changes are really quite simple, I strongly suggest you compare the two to see what I"m doing. Now, there are some caveats.

a) I have basically hardcoded some "conventions" for my own convenience and because I wasn't sure quite how to make it generic enough for any type model. Basically you should look at the viewmodel I"m using and if you can't/don't want to reproduce something similar you will probably have to change the code to reflect that. but it's dead simple (with firebug :) )

b) It is updating the model's selected value for a single select but I have not tried this using multiple select. I'm using something else for multiple select. But I left that code in from the original "options" binding.

---update---

figured out c. it works to update the viewmodel.

----end update ---

c) I'm not sure if it will update the select if you change the model. Frankly I was testing that when I realized I couldn't seem to make it update the select options on a regular select box so I'm a bit stymied on that. I'll update when/if I figure it out.

d) you must include the ensureDropdownSelectionIsConsistentWithModelValue function because it's a "private" function in ko and you can't reach it from out side.

function ensureDropdownSelectionIsConsistentWithModelValue(element, modelValue, preferModelValue) {
    if (preferModelValue) {
        if (modelValue !== ko.selectExtensions.readValue(element))
            ko.selectExtensions.writeValue(element, modelValue);
    }

    // No matter which direction we're syncing in, we want the end result to be equality between dropdown value and model value.
    // If they aren't equal, either we prefer the dropdown value, or the model value couldn't be represented, so either way,
    // change the model value to match the dropdown.
    if (modelValue !== ko.selectExtensions.readValue(element))
        ko.utils.triggerEvent(element, "change");
}

ko.bindingHandlers['groupedSelect'] = {
'update': function (element, valueAccessor, allBindingsAccessor) {
    if (ko.utils.tagNameLower(element) !== "select")
        throw new Error("options binding applies only to SELECT elements");

    var selectWasPreviouslyEmpty = element.length == 0;
    var previousSelectedValues = ko.utils.arrayMap(ko.utils.arrayFilter(element.childNodes, function (node) {
        return node.tagName && (ko.utils.tagNameLower(node) === "option") && node.selected;
    }), function (node) {
        return ko.selectExtensions.readValue(node) || node.innerText || node.textContent;
    });
    var previousScrollTop = element.scrollTop;

    var value = ko.utils.unwrapObservable(valueAccessor());
    value = value.groups();
    var selectedValue = element.value;

    // Remove all existing <option>s.
    // Need to use .remove() rather than .removeChild() for <option>s otherwise IE behaves oddly (https://github.com/SteveSanderson/knockout/issues/134)
    while (element.length > 0) {
        ko.cleanNode(element.options[0]);
        element.remove(0);
    }

    if (value) {
        var allBindings = allBindingsAccessor();
        if (typeof value.length != "number")
            value = [value];
        if (allBindings['optionsCaption']) {
            var option = document.createElement("option");
            ko.utils.setHtml(option, allBindings['optionsCaption']);
            ko.selectExtensions.writeValue(option, undefined);
            element.appendChild(option);
        }
        for (var a= 0, b = value.length; a < b; a++) {
            var optGroup = document.createElement("optgroup");
            ko.bindingHandlers['attr'].update(optGroup, ko.observable({label: value[a].label()}));
            var children = ko.utils.unwrapObservable(value[a].children());
            for (c=0, d=children.length; c<d; c++){
                var option = document.createElement("option");

                // Apply a value to the option element
                var optionValue = typeof allBindings['optionsValue'] == "string" ? value[a].children()[c][allBindings['optionsValue']] : value[a].children()[c];
                optionValue = ko.utils.unwrapObservable(optionValue);
                ko.selectExtensions.writeValue(option, optionValue);

                // Apply some text to the option element
                var optionsTextValue = allBindings['optionsText'];
                var optionText;
                if (typeof optionsTextValue == "function")
                    optionText = optionsTextValue(value[a].children()[c]); // Given a function; run it against the data value
                else if (typeof optionsTextValue == "string")
                    optionText = value[a].children()[c][optionsTextValue]; // Given a string; treat it as a property name on the data value
                else
                    optionText = optionValue;                // Given no optionsText arg; use the data value itself
                if ((optionText === null) || (optionText === undefined))
                    optionText = "";

                ko.utils.setTextContent(option, optionText);
                optGroup.appendChild(option);
            }
            element.appendChild(optGroup);
        }

        // IE6 doesn't like us to assign selection to OPTION nodes before they're added to the document.
        // That's why we first added them without selection. Now it's time to set the selection.
        var newOptions = element.getElementsByTagName("option");
        var countSelectionsRetained = 0;
        for (var i = 0, j = newOptions.length; i < j; i++) {
            if (ko.utils.arrayIndexOf(previousSelectedValues, ko.selectExtensions.readValue(newOptions[i])) >= 0) {
                ko.utils.setOptionNodeSelectionState(newOptions[i], true);
                countSelectionsRetained++;
            }
        }

        element.scrollTop = previousScrollTop;

        if (selectWasPreviouslyEmpty && ('value' in allBindings)) {
            // Ensure consistency between model value and selected option.
            // If the dropdown is being populated for the first time here (or was otherwise previously empty),
            // the dropdown selection state is meaningless, so we preserve the model value.
            ensureDropdownSelectionIsConsistentWithModelValue(element, ko.utils.unwrapObservable(allBindings['value']), /* preferModelValue */ true);
        }

        // Workaround for IE9 bug
        ko.utils.ensureSelectElementIsRenderedCorrectly(element);
    }
}

Usage

<select data-bind="groupedSelect:FieldList,optionsText:'Text',optionsValue:'Value',optionsCaption:'-- Please Select --',value:FieldEntityId">

Viewmodel

"FieldList":{
    "groups":[
        {"label":"SomeGroup1","children":[
            {"Text":"field1","Value":"1"},
            {"Text":"field2","Value":"2"}
        ]},
        {"label":"SomeGroup 2","children":[
            {"Text":"field3","Value":"3"},
            {"Text":"field4","Value":"4"}
        ]}
    ]
}

I guess if you have any questions or comments let me know.
Also let me explain why there model looks, well, kind of strange. I am really just serializing from C# model that looks like this

public class GroupSelectViewModel
{
   public GroupSelectViewModel()
   {
       groups = new List<SelectGroup>();
   }

   public List<SelectGroup> groups { get; set; }
}   
public class SelectGroup
{
    public string label { get; set; }
    public IEnumerable<SelectListItem> children { get; set; }
}

SelecListItem is a c# class which uses pascal notation.