Add test class for AMP_Comment_Walker.

And test all of its methods.
@todo: tests for remaining methods in PR #909.

tests/test-class-amp-comment-walker.php

@@ -0,0 +1,133 @@
+<?php
+/**
+ * Tests for AMP_Comment_Walker class.
+ *
+ * @package AMP
+ */
+
+/**
+ * Tests for AMP_Comment_Walker class.
+ *
+ * @since 0.7
+ */
+class Test_AMP_Comment_Walker extends \WP_UnitTestCase {
+
+	/**
+	 * Instance of AMP_Comment_Walker
+	 *
+	 * @var AMP_Comment_Walker
+	 */
+	public $walker;
+
+	/**
+	 * Setup.
+	 *
+	 * @inheritdoc
+	 */
+	public function setUp() {
+		parent::setUp();
+		$this->walker = new AMP_Comment_Walker();
+	}
+
+	/**
+	 * Test AMP_Comment_Walker::start_el.
+	 *
+	 * @covers AMP_Comment_Walker::start_el()
+	 */
+	public function test_start_el() {
+		$GLOBALS['post'] = $this->factory()->post->create(); // WPCS: global override OK.
+		$output          = '<div></div>';
+		$base_args       = array(
+			'format'      => 'baz',
+			'avatar_size' => 100,
+			'max_depth'   => 5,
+		);
+
+		$args = array_merge(
+			$base_args,
+			array(
+				'style' => 'baz',
+			)
+		);
+
+		$comment = $this->factory()->comment->create_and_get();
+		$this->walker->start_el( $output, $comment, 0, $args );
+
+		$this->assertContains( '<li data-sort-time=', $output );
+		$this->assertContains( $comment->comment_ID, $output );
+		$this->assertContains( strval( strtotime( $comment->comment_date ) ), $output );
+
+		$output = '<div></div>';
+		$args   = array_merge(
+			$base_args,
+			array(
+				'style' => 'div',
+			)
+		);
+
+		$comment = $this->factory()->comment->create_and_get();
+		$this->walker->start_el( $output, $comment, 0, $args );
+		$this->assertContains( '<div data-sort-time=', $output );
+	}
+
+	/**
+	 * Test AMP_Comment_Walker::paged_walk.
+	 *
+	 * @covers AMP_Comment_Walker::paged_walk()
+	 */
+	public function test_paged_walk() {
+		$GLOBALS['post'] = $this->factory()->post->create(); // WPCS: global override OK.
+		$comments        = $this->get_comments();
+		$args            = array(
+			'format'      => 'div',
+			'style'       => 'baz',
+			'avatar_size' => 100,
+			'max_depth'   => 5,
+		);
+		$output          = $this->walker->paged_walk( $comments, 5, 1, 5, $args );
+
+		foreach ( $comments as $comment ) {
+			$this->assertContains( $comment->comment_author, $output );
+			$this->assertContains( $comment->comment_content, $output );
+		}
+	}
+
+	/**
+	 * Test AMP_Comment_Walker::build_thread_latest_date.
+	 *
+	 * @covers AMP_Comment_Walker::build_thread_latest_date()
+	 */
+	public function test_build_thread_latest_date() {
+		$comments      = $this->get_comments();
+		$reflection    = new ReflectionObject( $this->walker );
+		$tested_method = $reflection->getMethod( 'build_thread_latest_date' );
+		$tested_method->setAccessible( true );
+		$latest_time        = $tested_method->invoke( $this->walker, $comments );
+		$comment_thread_age = $reflection->getProperty( 'comment_thread_age' );
+		$comment_thread_age->setAccessible( true );
+		$comment_thread_value = $comment_thread_age->getValue( $this->walker );
+
+		foreach ( $comments as $comment ) {
+			$this->assertEquals( strtotime( $comment->comment_date ), $comment_thread_value[ $comment->comment_ID ] );
+		}
+
+		$last_comment = end( $comments );
+		$this->assertEquals( strtotime( $last_comment->comment_date ), $latest_time );
+	}
+
+	/**
+	 * Gets comments for tests.
+	 *
+	 * @return array $comments An array of WP_Comment instances.
+	 */
+	public function get_comments() {
+		$comments = array();
+		for ( $i = 0; $i < 5; $i++ ) {
+			$comments[] = $this->factory()->comment->create_and_get( array(
+				'comment_date' => gmdate( 'Y-m-d H:i:s', ( time() + $i ) ), // Ensure each comment has a different date.
+			) );
+		}
+		return $comments;
+	}
+
+}