LightningOutExample.cmp

<aura:component controller="LightningOutExampleController">
    <aura:attribute name="contacts" type="Contact[]" default="[]"></aura:attribute>

    <aura:handler name="init" value="{!this}" action="{!c.initialize}" />

    <aura:handler event="c:lightningAppEvent" action="{!c.lightningAppEventHandler}"/>

    <aura:registerEvent name="lightningAppExternalEvent" type="c:lightningAppExternalEvent" />

    <aura:iteration items="{!v.contacts}" var="contact">
        <h1>{!contact.Name}</h1><br/>
    </aura:iteration> 
    <ui:button label="Call Visualforce code from Lightning" press="{!c.callExternalFunction}"/>
   
</aura:component>