Add on_block_before_render event

[a3020]

This will trigger an event when the block is about to be rendered*. Hooking into this event makes it possible to prevent it from displaying.

Example code to prevent displaying

Events::addListener('on_block_before_render', function ($event) {
    $event->preventRendering();

    return $event;
});

Output

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*) for naming convention see also #6574

[aembler]

Can you provide a use case where this would be helpful? I'm worried about potential performance impacts.

[a3020]

I'm trying to develop an add-on that'd make it possible to only render / output a block if certain conditions are met. I try to come up with a solution that doesn't involve changing all the block controllers.

If you have suggestions for the current PR or the above problem, please let me know.

[a3020]

This were the execution costs for me to run the anonymous function per block:

I've used an anonymous function to prevent collisioning of variables with the extract function.