KnockoutJS - Options Binding

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With the move into MVC4 I have taken an interest in knockout. Microsoft seemed fit to include it with MVC4 so it was worth taking a look at. To my surprise it will undoubtedly save me some time and effort for complex ajax UIs. I have already hit a major stumbling block though with it. That is the fact that when you bind an object to a select knockout does not work as you would expect.

Knockoutjs options binding not working you would expect is that no matter at what index the Font object was that it would select the proper item in the select menu when you loaded the page. This is not the case. Javascript does knockoutjs options binding not working consider two objects truly equal unless it is a reference to an exact object.

Personally I think knockout. That is, it is able to store objects as the values of a select, therefore it should have a mechanism to allow you to pre select one based on a pre determined key. Faced with this problem, one of the easiest ways around is to change the way you bind to the select. Doing below you will properly pre select the value you want.

However, the only issue here is that when you update the value in the select you essentially corrupt your data.

Obviously this solution is not ideal. In my search for a solution many people suggested holding the selected value in a knockoutjs options binding not working observable.

You pre populate that observable with a reference to an object in the FontList array. This would work for a lot of people, but not for me. When I fetch my data from the server it is already nicely formatted and held in a structured object. If I have to break that structure for every drop down then the usefulness of knockout starts to come into question. After toying with the idea of simply editing the knockout source code to make it work the way I expected I ended up finding out that making a custom binding handler would solve my issue.

Here it is below along with the usage. In my opinion this is not an idea solution because of all the data I had to pass back in the preSelect argument.

In my particular situation because of the way my objects are structured I had to know all three parameters. You situation may be different so adjust the code accordingly. I found that even though you set the selected index using knockouts method knockoutjs options binding not working does not update the referencing object, so the last line is there to do that.

I also had to deal with this situation and I chose to comply with the way knockout meant it to be. In my viewmodel, once the initialization callbacks are completed, I run a sync method. This sync method goes over all the objects that need to get synced and syncs them to their appropriate arrays, similar to what you did with this binding. I create a bindingHandler using init and update methods. On initI set the first value that come from observableand set change event to update the observables associated to value and text.

Knockoutjs options binding not working handler will execute twice. At first time, select has no options bidden, so it will have no effect. But, at second time, with options bindden and created, every works fine.

I also ran into knockoutjs options binding not working issue previously, and created my own workaround. So, since we must use workarounds, I thought I would share mine. Notify me of follow-up comments knockoutjs options binding not working email. Notify me of new posts by email. I'm Ben Morris an asp.

I run Ion Commerce for a living and enjoy software development. February 7, at 4: March 28, at April 14, at 2: About Me I'm Ben Morris an asp.

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The checked binding links a checkable form control — i. When the user checks the associated form control, this updates the value on your view model. Likewise, when you update the value in your view model, this checks or unchecks the form control on screen.

Any previous checked state will be overwritten. For checkboxes , KO will set the element to be checked when the parameter value is true , and unchecked when it is false. This means that nonzero numbers and non- null objects and non-empty strings will all be interpreted as true , whereas zero, null , undefined , and empty strings will be interpreted as false.

When the user checks or unchecks the checkbox, KO will set your model property to true or false accordingly. Special consideration is given if your parameter resolves to an array. In this case, KO will set the element to be checked if the value matches an item in the array, and unchecked if it is not contained in the array.

When the user checks or unchecks the checkbox, KO will add or remove the value from the array accordingly. When the user changes which radio button is selected, KO will set your model property to equal the value of the selected radio button.

Of course, this is most useful when you have multiple radio button elements bound to a single model property. To ensure that only one of those radio buttons can be checked at any one time, you should set all of their name attributes to an arbitrary common value e. This is useful if you want the value to be something other than a string such as an integer or object , or you want the value set dynamically. In the following example, the item objects themselves not their itemName strings will be included in the chosenItems array when their corresponding checkboxes are checked:.

If your checkedValue parameter is an observable value, whenever the value changes and the element is currently checked, the binding will update the checked model property. For checkboxes, it will remove the old value from the array and add the new value. For radio buttons, it will just update the model value. Extending the binding syntax Components Overview: What components and custom elements offer Defining and registering components The component binding Using custom elements Advanced: The "checked" binding Purpose The checked binding links a checkable form control — i.