Ext Designer, Combobox, set blank value in the JSON store

i have 开发者_如何学编程a combobox in the designer that loads from a JSON store, i want it to display a blank value in the list along with the data from the JSON store, so the user can go back to null after he selects something from the combo.

how will i go about doing this?

thanks.

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This solution don't do exactly what you asked, but your JSON can provide no value

{{"display":"", "value":-1},{"display":"first option", "value":1},{"display":"second option", "value":2}}

Then your code should consider value -1 as blank.

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Let's say you have a combobox with displayField: desc and selectField: name. Set a JSON record {root: {desc="", selectField=""}}.