How to call child component controller function/helper from Parent Component Controller

Question

I have requirement to call a child controller function from parent controller whenever the event occurs in parent side.

for e.g

ABC is parent component and DCE is child component, this child component has Span of Text tag and X symbol. when I click on x it is clearing the value using child controller. ABC parent component has Form element and child component is one one of in form.

My requirement is to clear the this child component value when parent component button is clicked.

here is the below sample code of what am trying because i cannot post actual code here

ParentComponent.cmp

 <aura:registerEvent name="clrValues" type="c:ClearValues"/>
    <div>
         <c:ChildComponent textValue="test"/>
         <ui:button label="Save"/>
         <ui:button label="Clear" press="{!c.clear}"/>      
    </div>
 </aura:component>

ParentController.js

({
    clear : function(component, event, helper) {
        var evtFire = component.getEvent("clrValues");

        evtFire.fire();
    }
})

ClearValues.evt

<aura:event type="APPLICATION" description="Event template" />

ChildComponent.cmp

<aura:component >
    <aura:handler name="setValues" event="c:ClearValues" action="{!c.clear}"/>
    <aura:attribute name="textValue" type="String" default="test"/>
    <div >
    <ui:inputText label="Expense Name" value="{!v.textValue}" aura:id="input1"/>
    </div>

</aura:component>

ChildController.js

({
    clear : function(component, event, helper) {
        helper.clearValues(component,event);
    }
})

Answer 1

The handler name in your child component should match the registered name in the parent component. i.e., the handler in child component should be like this:

  <aura:handler name="clrValues" event="c:ClearValues" action="{!c.clear}"/>

--UPDATE--

I think a better approach for parent-to-child communication is to use <aura:method> as listed in lightning docs and here

To do this, you should define a method in child component like this:

  <aura:method name="sampleMethod" action="{!c.clear}" access="PUBLIC"> 

You should then have an id when you refer it inside your parent component like this:

  <c:ChildComponent textValue="test" aura:id="cComp"/>

Then in the clear method of the parent component, you should invoke the child method like this:

 var childCmp = component.find("cComp")
 childCmp.sampleMethod();

Answer 2

Application Event handler should not contain name attribute just the event and action attribute is enough as mentioned here.

This is how an application event should be fired in your clear method:

({
    clear : function(component, event, helper) {
        $A.get("e.c:clrValues").fire();
    }
})

For more info,look at the section on application event in the doc.