Magento 2: How to get system.xml config values?

Question

I am trying to create a custom system configuration but I am having trouble getting the values.

I created a simple module like in this tutorial. So:
app/code/Mageplaza/HelloWorld/etc/module.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Mageplaza_HelloWorld" setup_version="1.0.0" />
</config>

app/code/Mageplaza/HelloWorld/registration.php

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'Mageplaza_HelloWorld',
    __DIR__
);
?>

app/code/Mageplaza/HelloWorld/etc/frontend/routes.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:App/etc/routes.xsd">
    <router id="standard">
        <route id="mageplaza" frontName="helloworld">
            <module name="Mageplaza_HelloWorld" />
        </route>
    </router>
</config>

Then is followed this tutorial to create the system config:

app/code/Mageplaza/HelloWorld/etc/adminhtml/system.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <tab id="mageplaza" translate="label" sortOrder="10">
            <label>Mageplaza</label>
        </tab>
        <section id="helloworld" translate="label" sortOrder="130" showInDefault="1" showInWebsite="1" showInStore="1">
            <class>separator-top</class>
            <label>Hello World</label>
            <tab>mageplaza</tab>
            <resource>Mageplaza_HelloWorld::hello_configuration</resource>
            <group id="general" translate="label" type="text" sortOrder="10" showInDefault="1" showInWebsite="0" showInStore="0">
                <label>General Configuration</label>
                <field id="enable" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                    <label>Module Enable</label>
                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                </field>
                <field id="display_text" translate="label" type="text" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                    <label>Display Text</label>
                    <comment>This text will display on the frontend.</comment>
                </field>
            </group>
        </section>
    </system>
</config>

app/code/Mageplaza/HelloWorld/etc/config.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
    <default>
        <helloworld>
            <general>
                <enable>1</enable>
                <display_text>Hello World</display_text>
            </general>
        </helloworld>
    </default>
</config>

Mageplaza/HelloWorld/Helper/Data.php

<?php

namespace Mageplaza\HelloWorld\Helper;

use Magento\Framework\App\Helper\AbstractHelper;
use Magento\Store\Model\StoreManagerInterface;
use Magento\Framework\ObjectManagerInterface;
use Magento\Framework\App\Helper\Context;
use Magento\Store\Model\ScopeInterface;

class Data extends AbstractHelper
{
    protected $storeManager;
    protected $objectManager;

    const XML_PATH_HELLOWORLD = 'helloworld/';



    public function __construct(Context $context,
        ObjectManagerInterface $objectManager,
        StoreManagerInterface $storeManager
    ) {
        $this->objectManager = $objectManager;
        $this->storeManager  = $storeManager;
        parent::__construct($context);
    }

    public function getConfigValue($field, $storeId = null)
    {
        return $this->scopeConfig->getValue(
            $field, ScopeInterface::SCOPE_STORE, $storeId
        );
    }


    public function getGeneralConfig($code, $storeId = null)
    {
        return $this->getConfigValue(self::XML_PATH_HELLOWORLD . $code, $storeId);
    }


}

I can see the config but when I try to request the values like this:

$helper = $this->objectManager->create('Mageplaza\HelloWorld\Helper\Data');
echo $helper->getGeneralConfig('enable');
echo $helper->getGeneralConfig('display_text');

I get an error:

1 exception(s):
Exception #0 (Exception): Notice: Undefined property: Mageplaza\HelloWorld\Controller\Index\Display\Interceptor::$objectManager

Answer 1

Your need to inject \Magento\Framework\ObjectManagerInterface into Mageplaza\HelloWorld\Controller\Index\Display before use it. Like this:

public function __construct(
    \Magento\Framework\ObjectManagerInterface $objectManager,
) {
    $this->objectManager = $objectManager;
    parent::__construct($context);
}

P/S: If your class is a controller extends \Magento\Framework\App\Action\Action class, you don't need to inject ObjectManagerInterface, use directly

$helper = $this->_objectManager->create('Mageplaza\HelloWorld\Helper\Data');

Answer 2

/**
 * scope config 
 *
 * @var \Magento\Framework\App\Config\ScopeConfigInterface
 */
protected $_scopeConfig;

public function __construct(
    \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig 
) { 
    $this->_scopeConfig = $scopeConfig;
}  

$config_val =  $this->_scopeConfig->getValue('section/group/field', \Magento\Store\Model\ScopeInterface::SCOPE_STORE);