Parent/Child Message Passing using Attributes

LightningPCAttributes.txt

<!-- Parent component "parent.cmp" -->
<aura:component>
    <aura:attribute name="myAttr" type="String" default="world" />
    <c:child myAttrChild="{!v.myAttr}">
</aura:component>

<!-- Child component "child.cmp" -->
<aura:component>
    <aura:attribute name="myAttrChild" type="String"/>
    <lightning:input name="input1" label="Enter some text" value="{!myAttrChild}" />
</aura:component>

/**
 * parentController.js
 */
({
    fooMethod : function(component, event, helper) {
        var myAttrChild = component.get("v.myAttr");
        console.log("myAttrChild: ", myAttrChild);
    }
})

If user enters some value in input1 in child component and then fooMethod is called,
myAttr in parent will have the same value as myAttrChild.