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Where to use object manager in Magento 2

You can depend on and use the ObjectManager class in the following scenarios:
  • You can use the object manager in static magic methods like __wakeup(), __sleep(), etc.
  • An example can be found in the __wakeup() method in the Magento/Eav/Model/Entity/Attribute/AbstractAttribute class.
  • You can use the ObjectManager to maintain backward compatibility for a constructor.
  • In a global scope, like in fixtures of integration tests, you can use the object manager.
  • The object manager can be a dependency in classes used for the creation of objects, e.g. factories or proxies.
In nutshell al dependencies(except above cases) must be injected in the constructor. Dependency Injection will do the work. Then you will do specialised methods that return what you need.
More specifically in the scenario of providing data to templates, Magento recommends using ViewModels which are nothing but service classes that provide needed information in templates. These view models should be set in the layout files. see docblock of Magento\View\Framework\Element\Template

 * Standard Magento block.
 * Should be used when you declare a block in frontend area layout handle.
 *
 * Avoid extending this class.
 *
 * If you need custom presentation logic in your blocks, use this class as block, and declare
 * custom view models in block arguments in layout handle file.
 *
 * Example:
 * <block name="my.block" class="Magento\Backend\Block\Template" template="My_Module::template.phtml" >
 *      <arguments>
 *          <argument name="viewModel" xsi:type="object">My\Module\ViewModel\Custom</argument>
 *      </arguments>
 * </block>
Such a ViewModel class is Magento\Catalog\ViewModel\Breadcrums.
Remember all classes that are passed through layout files must implement the Magento\Framework\View\Element\Block\ArgumentInterface
More on Dependency Injection in Magento can be found here
A good official reference of Magento coding standards can be found here

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