Hi All,

This is regarding the below lightning trailhead. Have found a solution towards it. Hope it helps upcoming future trailhead learners as well.

CampingList.cmp:

<aura:attribute name="items" type="Camping_Item__c[]"/>

<aura:handler name="init" action="{!c.doInit}" value="{!this}" />  
<aura:handler name="addItem" event="c:addItemEvent" action="{!c.handleAddItem}"/>   


<!-- Page Header -->
<div class="slds-page-header" role="banner" >
    <div class="slds-grid">
        <div class="slds-col" >
            <h1 class="slds-text-heading--medium">Camping List</h1>
        </div>
    </div>
</div>  
<!--/ Page Header -->

<!-- New Camping Form -->
    <c:campingListForm />
<!--/ New Camping Form -->

<!-- Camping List Items -->      
<aura:iteration items="{!v.items}" var="itm">
      <c:campingListItem item="{!itm}"/><br/>
</aura:iteration>

CampingListController.js:

({

    //-- Load Camping list Items.
    doInit: function(component,event,helper)    
    {
        //-- Create the Action.
        var action = component.get("c.getItems");

        //-- Add callback behavior for when response is received.
        action.setCallback(this,function(response)
        {
            var state = response.getState();
            if(component.isValid() && state === "SUCCESS")
            {
                component.set("v.items",response.getReturnValue());
                console.log("doInit: "+response.getReturnValue());
            }
        });

        //-- Send action off to be execute.
        $A.enqueueAction(action);
    },

     //-- Handle Create Expense Actions..
    handleAddItem: function(component, event, helper)
    {
        console.log("\nEntry into CampingListController.js -> handleAddItem()");        

        var item = event.getParam("item");
        console.log("\nCampingListController.js -> handleAddItem() item: "+ JSON.stringify(item));

        var action = component.get("c.saveItem");
        action.setParams
         ({
            "item": item
         });

        action.setCallback(this, function(response)
        {
            var state = response.getState();
            if (component.isValid() && state === "SUCCESS")
            {       
               var items = component.get("v.items");
               console.log("Campaigns(items..) before  create: "+JSON.stringify(items));
               items.push(response.getReturnValue());
               console.log("Campaigns(items..) after  create: "+JSON.stringify(items));
               component.set("v.items",items);
            }
        });
        $A.enqueueAction(action);
    }

    /*handleAddItem: function(component, event, helper)
    {
        console.log("\nEntry into CampingListController.js -> handleAddItem()");        

        var newItem = event.getParam("item");
        console.log("\nCampingListController.js -> handleAddItem()\n the Item: "+ JSON.stringify(newItem));
        helper.createItem(component, newItem);        

    }*/


})

CampingListController.apxc (apex):

public with sharing class CampingListController
{
    @AuraEnabled
    public static List<Camping_Item__c> getItems()
    {
        String[] fieldsToCheck = new String[]{'Id','Name','Packed__c','Price__c','Quantity__c'};
        Map<String,Schema.SObjectField> fieldDescribeTokens = Schema.SObjectType.Camping_Item__c.fields.getMap();

        for(String field : fieldsToCheck)
        {
            if(!fieldDescribeTokens.get(field).getDescribe().isAccessible())
            {
                throw new System.NoAccessException();
                return null;
            }            
        }        
        return [SELECT Id,Name,Packed__c,Price__c,Quantity__c FROM Camping_Item__c];
    }


    @AuraEnabled  
    public static Camping_Item__c saveItem(Camping_Item__c item)
    {   
        System.debug('Campaign List Item from Apex: '+item);
        upsert item;
        System.debug('Campaign List Item from Apex Id: '+item.Id);
        return item;
    }

}

CampingListForm.cmp:

<aura:component controller="CampingListController" >

    <aura:attribute name="newItem"  type="Camping_Item__c" default="{ 'sobjectType': 'Camping_Item__c',
                                                                      'Name':'',
                                                                      'Quantity__c': 0,
                                                                      'Price__c': 0,
                                                                      'Packed__c': false}"  />  

    <aura:registerEvent name="addItem" type="c:addItemEvent"/>


    <!-- New Camping Form-->  
    <div class="slds-col slds-col--padded slds-p-top--large" >  
        <!-- Boxed Area-->
        <fieldset class="slds-box slds-theme--default slds-container--small">

            <legend id="newexpenseform" class="slds-text-heading--small" >
                Add Camping List
            </legend>

            <!-- Create New Expense Form -->
            <form class="slds-form--stacked">
                <!-- Name -->
                <div class="slds-form-element slds-is-required" >
                    <div class="slds-form-element__control" >
                      
      <lightning:input aura:id="itemform"
                       label="Name"
                       name="itemname"
                       value="{!v.newItem.Name}"
                       required="true"/>
                 
                        </div>
                             </div>
                            

                <!-- Quantity -->
                <div class="slds-form-element__label" >
                    <div class="slds-form-element__control" >
                        
                        <lightning:input type="number"
                       aura:id="itemform"
                       label="Quantity"
                       name="quantity"
                       value="{!v.newItem.Quantity__c}"
                       min="1"
                       required="true"/>

                    </div>                
                </div>

                <!-- Price -->
                <div class="slds-form-element slds-is-required" >
                    <div class="slds-form-element__control" >
                        
                       <lightning:input type="number"
                       aura:id="itemform"
                       label="Price"
                       name="price"
                       value="{!v.newItem.Price__c}"
                       formatter="currency"
                       step="0.01"/>

                    </div>
                </div>

                <!-- Packed -->
                <div class="slds-form-element" >
                    
                    <lightning:input type="checkbox"
                       aura:id="itemform"
                       label="Packed?"
                       name="packed"
                       checked="{!v.newItem.Packed__c}"/>
                    
                </div>

                <!-- Button Create Expense -->
                <div class="slds-form-element">
                    
                    <lightning:button label="Create Camping Item"
                        variant="brand"
                        onclick="{!c.clickCreateItem}"/>
                    
                </div>

            </form>            
            <!--/ Create New Expense Form -->
        </fieldset>
        <!--/ Boxed Area-->     
    </div>
    <!--/ New Camping Form-->  

</aura:component>

CampingListFormController.js:

({
    clickCreateItem : function(component, event, helper)
    {
        //-- Simplistic error checking.
        console.log("\nIn CampingListFormController.js -> submitForm()");
        var validCampign = true;

        //-- Name must not be blank.
        var nameField   = component.find("name");
        var campaignname = nameField.get("v.value");
        if($A.util.isEmpty(campaignname))
        {
            validCampign = false;
            nameField.set("v.errors",[{message: "Camping name can't be blank."}]);     
        }
        else
        {
            nameField.set("v.errors",null);
        }

        //-- Quantity must not be blank.
        var qtyField = component.find("quantity");
        var quantity = qtyField.get("v.value");
        if($A.util.isEmpty(quantity))
        {
            validCampign = false;
            qtyField.set("v.errors",[{message: "Quantity can't be blank."}]);
        }
        else
        {
            qtyField.set("v.errors",null);            
        }

        //-- Price must not be blank
        var priceField = component.find("price");
        var price      = priceField.get("v.value");
        if($A.util.isEmpty(price))
        {
            validCampign = false;
            priceField.set("v.errors",[{message: "Price can't be blank."}]);
        }
        else
        {
            priceField.set("v.errors",null);            
        }

        //-- If we pass error checking, do some real work.
        if(validCampign)
        {
            //-- Create the new expense.
            var newCampaign = component.get("v.newItem");
            console.log("In CampingListFormController.js -> submitForm()\n The Item: " + JSON.stringify(newCampaign));
            helper.createItem(component, newCampaign);
        }               



    }

})

CampingListFormHelper.js:

({
    createItem : function(component,item)
    {   
        console.log("\nEntry into CampingListFormHelper.js -> createItem()");
        console.log("In CampingListFormHelper.js -> createItem() the item is: "+JSON.stringify(item));
        var createEvent = component.getEvent("addItem");        
        createEvent.setParams({ "item": item });
        createEvent.fire();        
        component.set("v.newItem",{'sobjectType':'Camping_Item__c','Name': '','Quantity__c': 0,'Price__c': 0,'Packed__c': false});
    }
})

CampingApplication.app:

<aura:application extends="force:slds">
    <c:campingHeader />
    <c:campingList />
</aura:application>

CampingListHelper.js:

({
    createItem : function(component,campaign)
    {
        console.log("\nEntry into CampingListHelper.js -> createItem()");        
        console.log("\nCampingListHelper.js -> createItem()\n the Item: "+ JSON.stringify(campaign));

        this.saveItem(component, campaign, function(response)
        {
            var state = response.getState();
            if (component.isValid() && state === "SUCCESS")
            {
                var campaigns = component.get("v.items");
                console.log("Campaigns before create: "+JSON.stringify(campaigns));
                var retcamp = response.getReturnValue();
                campaigns.push(retcamp);
                console.log("Campaigns after  create: "+JSON.stringify(campaigns));
                component.set("v.items",campaigns);   
            }
        });        
    },

    saveItem : function(component,campaign,callback)
    {
        console.log("\nEntry into CampingListHelper.js -> saveItem()");        
        console.log("\nCampingListHelper.js -> saveItem() the campaign is: "+ JSON.stringify(campaign));

         var action = component.get("c.saveItem");
         console.log("Helper->saveItems"+campaign);
         action.setParams
         ({
            "campaign": campaign
         });

        if (callback)
        {
            action.setCallback(this, callback);
        }
        $A.enqueueAction(action);        

    }

})

Reference Links:

1) https://developer.salesforce.com/forums/ForumsMain?id=906F0000000kDmtIAE
2) https://salesforce.stackexchange.com/questions/149023/facing-problem-with-trailhead-lightning-module-challenge-connect-components-wit
3) https://salesforce.stackexchange.com/questions/192815/how-can-i-use-lightninginput-instead-of-uiinputsmthg-for-the-challenge-ente
4) https://salesforce.stackexchange.com/questions/191861/the-campinglist-component-appears-to-be-using-ui-components-instead-of-base-ligh

Hope you find this comment useful and clear this trailhead successfully. All the best.

Regards,
Muthukumaran K
Consultant (Salesforce)