ExtJS: 3 Value Checkbox

js:

{
    xtype: 'checkbox',
    fieldLabel: 'Active',
    name: 'Active',
    inputValue: 'true',
    cls: 'value-undefined',
    handler: function () {
        if (!this.uncheckedValue) {
            this.removeCls('value-undefined');
            this.uncheckedValue = 'false';
        }
    },
    reset: function () {
        this.setValue(undefined);
        this.addCls('value-undefined');
        if (this.uncheckedValue) this.uncheckedValue = undefined;
    }
}

css:

.x-field.value-undefined input
{
    opacity:0.4;
}