element and the options binding. When the user selects or de-selects an item in the multi-select list, this adds or removes the ." />

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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. Knockout options binding preselect 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.

What you would expect knockout options binding preselect that no matter at what index the Font object was that it would select the proper item in the select menu when knockout options binding preselect loaded the page.

This is not the case. Javascript does not 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 knockout options binding preselect to pre select one based on a pre determined key.

Faced with this problem, one knockout options binding preselect 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 separate 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 knockout options binding preselect is not knockout options binding preselect 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 it 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 knockout options binding preselect 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. This 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 this issue previously, and created my own workaround. So, since we knockout options binding preselect use workarounds, I thought I would share mine.

Notify me of follow-up comments by 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 options binding controls what options should appear in a drop-down list i. The value you assign should be an array or observable array. For a multi-select list, to set which of the options are selected, or to read which of the options are selected, use the selectedOptions binding.

For a single-select list, you can also read and write the selected option using the value binding. Note that the only difference between examples 3 and 4 is the optionsText value.

You should supply an array or observable array. Any previous options will be removed. However, if you want to let the user choose from an array of arbitrary JavaScript objects not merely strings , then see the optionsText and optionsValue parameters below. Sometimes, you might not want to select any particular option by default. But a single-select drop-down list usually starts with some item selected, so how can you avoid preselecting something? This easy to do: So, if myChosenValue holds the value undefined which observables do by default , then the dummy option will be selected.

See Example 3 above to see how you can bind options to an array of arbitrary JavaScript object - not just strings. Example 3 shows how you can specify that property name by passing an additional parameter called optionsText. See Example 4 above, which shows how you could generate the displayed text by concatenating together multiple property values.

You can also specify a JavaScript function to determine this value. Sometimes you may want to mark an array entry as deleted, but without actually losing record of its existence. This is known as a non-destructive delete. For details of how to do this, see the destroy function on observableArray.

By default, the options binding will skip over i. If you want to show destroyed entries, then specify this additional parameter like:. If you need to run some further custom logic on the generated option elements, you can use the optionsAfterRender callback. See Note 2 below. For a multi-select list, you can read and write the selection state using selectedOptions.

Technically this is a separate binding, so it has its own documentation. The callback function is invoked each time an option element is inserted into the list, with the following parameters:. Extending the binding syntax Components Overview: What components and custom elements offer Defining and registering components The component binding Using custom elements Advanced: The "options" binding Purpose The options binding controls what options should appear in a drop-down list i.