Create public $DisplayPluginBase::getExtender($id)

I'm pretty sure that you can alternative set another property on the view executeable or make a public method, which returns all active display extenders, but just making the property public is not hte ideal solution

I'll work on this as part of Drupal Global Sprint Weekend

I was able to successfully apply the patch to my local Drupal 8.4.0-dev instance. The new getExtender public method was added to DisplayPluginBase.php class. I also included tested to see if the method would be available to use as a public function of the display handler DisplayPluginBase class.

Given that we add a new method, we should also have tests for it.

1. +++ b/core/modules/views/src/Plugin/views/display/DisplayPluginBase.php
   @@ -2641,6 +2641,18 @@ public function getExtenders() {
   +   * @return \Drupal\views\Plugin\views\display_extender\DisplayExtenderPluginBase[$id].
   

   The documentation is wrong. If you return a display extender you should not put [$id]. at the end.

2. +++ b/core/modules/views/src/Plugin/views/display/DisplayPluginBase.php
   @@ -2641,6 +2641,18 @@ public function getExtenders() {
   +  	return isset($this->extenders[$id]) ? $this->extenders[$id] : NULL;
   

   This issues spaces ... it should simply not do that

Please review! :)

1. +++ b/core/modules/views/src/Plugin/views/display/DisplayPluginBase.php
   @@ -2641,6 +2641,18 @@ public function getExtenders() {
   +   * Returns a specific display extender.
   

   Well, not a specific one… the requested one.

   Returns the requested display extender.

2. +++ b/core/modules/views/src/Plugin/views/display/DisplayPluginBase.php
   @@ -2641,6 +2641,18 @@ public function getExtenders() {
   +   *   The id of the requested extender.
   

   s/id/ID/

3. +++ b/core/modules/views/src/Plugin/views/display/DisplayPluginBase.php
   @@ -2641,6 +2641,18 @@ public function getExtenders() {
   +   * @return \Drupal\views\Plugin\views\display_extender\DisplayExtenderPluginBase.
   

   Must not have a trailing period.

Also, this still does not have tests.

Please also upload a failing test-only patch. That will prove that the test is testing what it should be testing :)

It's not clear why this method should solve hasExtender() logic but I disagree on implementation
This method should throw exception if requested extender no instantiated or using "broken/fallback" plugin

+++ b/core/modules/views/src/Plugin/views/display/DisplayPluginBase.php
@@ -2641,6 +2641,18 @@ public function getExtenders() {
+   * @return \Drupal\views\Plugin\views\display_extender\DisplayExtenderPluginBase
...
+    return isset($this->extenders[$id]) ? $this->extenders[$id] : NULL;

|null
not clear why that method is useful to check for some specific extenser

I'm sure that primary role of DisplayPlugin is manage list of extenders and not care about returning null
because it's role of caller's code to ensure that extender exists in display and it will anyway require a condition to split logic depending on result of null or object

Postponing on #2824920-25: Make StylePluginBase::renderFields public to give the discussion of the proposed API additions context. Thanks!