Disable button if combobox empty

Question

I have combobox and button on a form. And I want if the combobox is empty then the button is disabled but if combobox have records button is enabled.

{
    xtype: 'combobox',
    name : 'book',
    fieldLabel: 'BookName',
    value: name
}

buttons : [
{
    text: 'Add',
    action: 'Add_book'
}
]

Answer 1

You can add a listener to your combobox, you can change the function of the listener like you want

{
xtype: 'combobox',
name : 'book',
fieldLabel: 'BookName',
value: name,
listeners: {
afterrender: function() {
  if (this.getValue() === null) {
    Ext.getCmp('yourButton').setDisabled(true);
      }
   else {
       Ext.getCmp('yourButton').setDisabled(false);
        }
    }
}
}