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Blocks in Drupal 8 are instances of the block plugin .
The Drupal block manager scans your modules for any classes that contain a @Block Annotation .
The example snippet below makes use of the @Block annotation along with the properties "id" and "admin_label" to define a custom block.
Create the file src/Plugin/Block/HelloBlock.php within the module skeleton created earlier and add the code below. You will, of course, need to rebuild the cache.
To add 'Hello block' you can go to Structure -> Block Layout ( admin/structure/block ) and click on 'Place Block' button associated with each available region.
Clicking on 'Place Block' button for any given region a "Place Block" dialogue pop-up will appear, with a listing of all available blocks. To quickly find your block, simply use 'Filter by block name' option or use mouse-scroll to locate 'Hello block'. This way you can add any number of instances of the custom block anywhere on your site.
The class name and the file name must be the same ( class HelloBlock and /src/Plugin/Block/HelloBlock.php). If the class name is different, the block will appear in the list of available blocks, however you will not be able to add it.
Be sure to double check all paths and filenames. Your .php must be in the correctly labelled directory (/src/Plugin/Block/), otherwise it won't be discovered by Drupal.
Note: Using Twig templates with custom blocks
- Add a _theme hook in your .module file. Don't name the theming function like 'block__. ' - this will not pass any variables down to the twig templates. Instead, you might use the module name as prefix.
- Use '#theme' in the render array in the build method and pass the variables on the same level as the '#theme' - '#varname'.