Bind textfield value length

Code:

Ext.define('Ap.view.TestWindow', {
    extend: 'Ext.window.Window',
    alias: 'widget.testwindow',

    title: 'Test window',

    viewModel: {
        type: 'default',
        formulas: {
            valueLength: function(get) {
                return get('text.value').length;
            }
        }
    },

    layout: 'fit',
    modal: true,
    width: 400,
    height: 300,

    buttons: [{
        text: 'Save'
    }, {
        text: 'Cancel',
        handler: function() {
            this.up('window').close();
        }
    }],


    items: {
        xtype: 'form',
        layout: 'anchor',
        bodyPadding: 15,
        autoScroll: true,
        border: 0,
        items: [{
            xtype: 'textarea',
            anchor: '100%',
            reference: 'text',
            publishes: ['value'],
            height: 140
        }, {
            xtype: 'displayfield',
            fieldLabel: 'Number of chars:',
            bind: '{valueLength} chars'
        }]
    }


});

Ext.widget('testwindow').show();