append block to specific container programatically

Question

I need to display the block in product detail page for bundle product only so I just add preference to overwrite Magento/Catalog/Controller/Product/View

Then alter the product detail page controller execute method like below.

// Render page
$page = $this->resultPageFactory->create();

if($this->_type->getById($productId)->getTypeId()=='bundle')
{
    $block=$page->getLayout()->createBlock('Vendor\Module\Block\Index\Index');
    $page->getLayout()->getBlock('content')->append($block);
}      

$this->viewHelper->prepareAndRender($page, $productId, $this, $params);
return $page;

but it always throws error like

PHP Fatal error: Call to a member function append() on boolean

I need to add my block to <referenceContainer name="content">

Answer 1

Why not just add your layout update to catalog_product_view_type_bundle.xml?

But in answer to your problem: You're creating a new $page from a factory. This page has not been added any child elements yet; other methods do that further on in the chain. That's why $page->getLayout()->getBlock('content') returns false: the 'content'-container does not yet exist.

Answer 2

Extending @GielBerkers answer, you can use an observer to modify layout programatically once it's ready.

Nspace/Module/etc/frontend/events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
<event name="core_layout_render_element">
    <observer name="Nspace_Module::modify_block" instance="Nspace\Module\Observer\ModifyBlock" />
</event>

Nspace/Module/Observer/ModifyBlock.php

<?php

namespace Nspace\Module\Observer;

use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;

class ModifyBlock implements ObserverInterface
{
  protected $_scopeConfig;
  private static $_layoutSet = false;

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

 public function execute(Observer $observer)
 {
    // Run this only once
    if(ModifyBlock::$_layoutSet) { return; }
    ModifyBlock::$_layoutSet = true;

    $layout = $observer->getLayout();

    // Optional step if you need to remove somethig
    $children = $layout->getChildBlocks('content');
    foreach($children as $key => $childB) 
    {
        if($childB->getType() === [some_type]) 
        {
            $layout->unsetElement($key);
            break;
        }
    }

    // Add a new block
    $layout->createBlock('Vendor\Module\Block\Index\Index', 'custom-index-index');
    $layout->setChild('content', 'custom-index-index', 'custom-index-index');
    return;
 }
}