How to dynamically inject block into layout using observers

Question

I have a block class My\Extension\Block\MyClass which I want to insert into the layout. But I would not be knowing which page to insert in (will be specified by the admin), so I'm observing the layout_render_before event and in my observer class' execute method I'm doing the following

 $allowedBlock = $this->_configModel->getAllowedBlock(); // get the block name where to insert
 $allowedAction = $this->_configModel->getAllowedAction(); // get the full action name of the controller where to insert
 $layout = $this->_layout;
 $action = $this->_request->getFullActionName();
 if($allowedAction !== $action) return false; // if current page is not where to insert block, stop further processing
 $parentBlock = $layout->getBlock($allowedBlock); // get the block from the layout where to insert
 if ($parentBlock) { // if parent block exists
 $blockName = 'My\Extension\Block\MyClass';
 $blockToInsert = $layout->createBlock($blockName,'my_extensions_unique_block_name'); // create block to insert
 $parentBlock->append($blockToInsert); // insert block to parent
 }

And in my block I am setting the template like so,

public function _prepareLayout(){
 $this->setTemplate('My_Extension::subdirectory/my_template.phtml');
 parent::_prepareLayout();
}

And in my template file, I have a very simple string displayed

<h1>Hello World From Template</h1>

This exact code was working in Magento 1.9.x. The only difference was that I used the controller_action_layout_render_before event instead of layout_render_before. But the controller_action_layout_render_before was not working in Magento 2.1.x, so I used the layout_render_before event instead.

But now, my block is not visible on the frontend, even though if I write some code in my block's constructor (like echo "Hello World From Block's constructor";) it comes up on the page, meaning the block is being called, but the template is not visible on the frontend.

Considering the fact that the exact same code which was working on Magento 1.9.x, is not working in Magento 2.1.x, what am I doing wrong?

Answer 1

Use plugins. Scroll down for the working solution.

Two issues with your approach:

1. layout_render_before is called after layout is rendered. Check vendor/magento/framework/View/Result/Layout.php:

   160 \Magento\Framework\Profiler::start('LAYOUT');
   161 \Magento\Framework\Profiler::start('layout_render');
   162 
   163 $this->applyHttpHeaders($response);
   164 $this->render($response);
   165 
   166 $this->eventManager->dispatch('layout_render_before');
   167 $this->eventManager->dispatch('layout_render_before_' . $this->request->getFullActionName());
   168 \Magento\Framework\Profiler::stop('layout_render');
   169 \Magento\Framework\Profiler::stop('LAYOUT');
   

   So you append your block after the block has been rendered.

2. Magento does not render all block's children. At least not for vendor/magento/framework/View/Element/Template.php which is used for almost (correct me if I am wrong here) all blocks. You would have to put echo $this->getChildHtml('name_in_layout') in all templates.

Working solution. Tested in Magento 2.1.4

Use this plugin in your etc/di.xml:

<type name="Magento\Framework\View\Element\AbstractBlock">
 <plugin name="Goivvy_Inject" type="Goivvy\Custom\Plugin\InjectPlugin" /> 
</type>

app/code/Goivvy/Custom/Plugin/InjectPlugin.php:

 <?php
 namespace Goivvy\Custom\Plugin;
 class InjectPlugin {
 protected $_request;
 public function __construct(\Magento\Framework\App\RequestInterface $request){
 $this->_request = $request;
 }
 public function aftertoHtml(\Magento\Framework\View\Element\AbstractBlock $block, $result){
 if($block->getNameInLayout() == 'copyright' && $this->_request->getFullActionName() == 'catalog_product_view'){
 $bl = $block->getLayout()->createBlock('Goivvy\Custom\Block\Inject','goivvy_block_inject');
 return $result.$bl->toHtml();
 } 
 return $result;
 }
}

Instead of copyright and catalog_product_view use your dynamic block name and full action name.

• Jay Ghosh: I don't know why I was stupid enough to not think of using plugins. Anyway, it works now. Thank you for your help
• Jay Ghosh: Also, don't you think it's a little misleading to call it layout_render_before if it's dispatched after layout is rendered?
• goivvy.com: yeah it is probably a bug
• adjco: It is worth emphasizing the plugin class name must be suffixed with word "Plugin" as per Magento 2 plugin name convention.
• Arvind07: If you want to load dynamic content in the block then it wont work. It will always show the cached content if full_page cache is enabled.

Answer 2

The problem not came from your observer, it might came from your block class.

public function _prepareLayout(){
 $this->setTemplate('My_Extension::subdirectory/my_template.phtml');
 parent::_prepareLayout();
}

Don't set template inside _prepareLayout, try to use the core field instead. Something like

$this->_template = 'My_Extension::subdirectory/my_template.phtml';

Here is how it's working in M2

layout_template_construct

• Jay Ghosh: I tried that as well. Still it's not showing up. One more interesting phenomenon which is happening is, if I call the toHtml() method on the block's prepareLayout or constructor, the block is being rendered correctly, which means the template is set correctly