simple expression evaluation for extended lightning component is sometimes wrong

Question

In this simple and straightforward scenario the evaluation of an expression is not getting updated correctly.

I have two components, an (abstract) base component and an extended one:

cmBaseComponent:

<aura:component extensible="true" abstract="true">    

<aura:attribute name="selected" type="Boolean" />

<div>
    <lightning:input type="checkbox" name="selectedCB" label=" " checked="{!v.selected}" />
</div>

<div>
    Base component: {!v.selected} // {!v.selected ? 'true' : 'false'}
</div>

{!v.body}

cmExtendedComponent:

<aura:component extends="c:cmBaseComponent">    

<aura:set attribute="selected" value="true" />

Extended: {!v.selected} // {!v.selected ? 'true' : 'false'}

NOTE: I have no other logic - no controller or other bindings.

The result on the screen is this:

So far so good, means initial evaluation is correct. BUT: when I toggle the checkbox the first time the result is:

Toggling again and again will always result in the right behavior - it just fails on the first data change.

Fiddling around finding out that:

• this only occurs when you set the attribute to true - <aura:set attribute="selected" value="false" /> always renders correctly
• this only occurs in the extended class
• this only occurs when you use the expression in a tenary operator or other expression functions like {! if(v.selected, 'true', 'false) } - not when you use the expression {!v.selected} directly as in sample
• no difference toggling the value by using ui:inputCheckbox or lightning:input

Is this a bug in the framework or could anyone explain why this behavior is the way it is?

Answer 1

Agreed, bug in the framework.

I'll file a bug.

Answer 2

I think that when u use aura:set , it should be for attribute values that can not be updated later ( maybe that during rerender of Base component, the aura:set is taken in account again )

I suggest you set your initial value in you extended component init handler so that there won't be problems once it's user updated.