Migrate process plugin: extract

1. +++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/Extract.php
   @@ -0,0 +1,41 @@
   +    if (!is_array($value)) {
   +      throw new MigrateException('Invalid input.');
   +    }
   ...
   +    if (!$key_exists) {
   +      throw new MigrateException('Extraction failed.');
   +    }
   

   I wonder whether we can give a more helpful explanation of what is exactly the invalid input and how it should look like?

2. +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/ExtractTest.php
   @@ -0,0 +1,67 @@
   +  function setUp() {
   ...
   +  function testExtract() {
   ...
   +  function testExtractFromString() {
   ...
   +  function testExtractFail() {
   

   It would be cool to have proper visibility.

Thank you!

1. +++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/Extract.php
   @@ -0,0 +1,41 @@
   + * This plugin extract a value from an array.
   

   extracts

2. +++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/Extract.php
   @@ -0,0 +1,41 @@
   + * @see https://drupal.org/node/2152731
   

   I'd rather see the basic example in @code?

1. +++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/Extract.php
   @@ -0,0 +1,42 @@
   +    $new_value = NestedArray::getValue($value, $this->configuration['indexes'], $key_exists);
   

   I wonder whether we should support also 'index' as many times you think more of a singular value.

2. +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/ExtractTest.php
   @@ -0,0 +1,67 @@
   + */
   +class ExtractTest extends MigrateProcessTestCase {
   

   (optional) I kind of love @see to the actual class so you can jump to the class really fast in case you use an IDE. Nice test coverage!

Thank you

Nothing to complain about here, this is the same as other process plugins, and thanks for linking to the full docs!

Only thing is that handbook page makes me nervous. It shows we're replacing a line of extremely basic PHP 101 with 8 lines of Drupal-specific YAML that has to be formatted "just so." I'm concerned about the DX impact of that. :\ But I guess this ship has already sailed on using YAML for these inputs, so not much can be done about that. Maybe worth a once-over once things are closer to ready to see if we can improve some of the verboseness, not sure.

In the meantime, however...

Committed and pushed to 8.x. Thanks!