Add a FormInterface replacement for confirm_form()

Neato.

+++ b/core/lib/Drupal/Core/Form/ConfirmFormBase.php
@@ -0,0 +1,96 @@
+  /**
+   * @todo.
+   */
+  abstract protected function getQuestion();

These should have an interface to define them, then the base class just to provide reasonable defaults.

+++ b/core/lib/Drupal/Core/Form/ConfirmFormBase.php
@@ -0,0 +1,96 @@
+  public function validateForm(array &$form, array &$form_state) {
+  }
+
+  public function submitForm(array &$form, array &$form_state) {

submitForm is required, so I wouldn't even define it here. The class is abstract so just go ahead and leave it out, so PHP will force implementers to add it. valdateForm I suppose makes sense to give a default-null implementation to, but submitForm should always need an implementation.

+++ b/core/lib/Drupal/Core/Form/ConfirmFormBase.php
@@ -0,0 +1,96 @@
+  /**
+   * @todo.
+   */
+  abstract protected function getQuestion();

This could use a ConfirmFormInterface to define the methods and documentation, then the base class for ease of implementation.

I love how we don't need to change anything in FAPI itself for this. OO++ :-)

Bah, I didn't even notice that they were protected! Yeah, then I guess no interface, just the base class. Oh well.

+++ b/core/lib/Drupal/Core/Form/ConfirmFormBase.php
@@ -0,0 +1,122 @@
+  /**
+   * Returns the page to go to if the user cancels the action.
+   *
+   * @return string|array
+   *   This can be either:
+   *   - A string containing a Drupal path.
+   *   - An associative array with a 'path' key. Additional array values are
+   *     passed as the $options parameter to l().
+   *   If the 'destination' query parameter is set in the URL when viewing a
+   *   confirmation form, that value will be used instead of this path.
+   */
+  abstract protected function getPath();

getPath() doesn't suggest that it's the cancellation path. I'd rename this to getCancellationPath() or similar for clarity.

Otherwise, this is looking awesome.

+1 and pass the sweet potatoes.

I had a longer review and then Chrome pinballed of death on me so I lost it. :(

Suffice to say, this patch looks awesome. I've always found confirm_form()'s "API" (big air quotes there) really confusing. It's so great to have consistency in our form definitions, despite the fact that the OO requirement of page controllers make me cranky. ;)

Committed and pushed to 8.x. Thanks!

I think? Probably can update the existing one.

Will there be a meta issue to convert all the confirm form?
Related #1842036: [META] Convert all confirm forms to use modal dialog.

Change notice looks good thanks.