Add unit tests, demonstrating how to stub out deps.

Physikerwelt asked for some guidance on how to write good unit tests for
classes that depend on external resources. I wrote a few to serve as examples
for additional tests. Because they have an ulterior didactic purpose, the
comments are a bit more verbose than I would otherwise like, but despite that
the tests are good enough to merit being merged.

Change-Id: Ifa97eec1a68fb68b4744d1e5b192b410afe5ef68

tests/MathTexvcTest.php

@@ -0,0 +1,117 @@
+<?php
+/**
+ * PHPUnit tests for MathTexvc.
+ *
+ * @group Extensions
+ * @group Math
+ */
+
+/**
+ * @covers MathTexvc
+ */
+class MathTexvcTest extends MediaWikiTestCase {
+
+	/**
+	 * Tests behavior of render() upon a cache hit.
+	 * If the rendered object exists in the database cache, MathTexvc
+	 * just generates HTML from it, and skips shelling out to texvc
+	 * entirely.
+	 * @covers MathTexvc::render
+	 */
+	function testRenderCacheHit() {
+		global $wgMathCheckFiles;
+
+		// Disable file checks. (This is permissable, because PHPUnit
+		// will backup / restore global state on test setup / teardown.)
+		$wgMathCheckFiles = false;
+
+		// Create a MathTexvc mock, replacing methods 'readFromDB',
+		// 'callTexvc', and 'doHTMLRender' with test doubles.
+		$texvc = $this->getMockBuilder( 'MathTexvc' )
+			->setMethods( array( 'readFromDB', 'callTexvc', 'doHTMLRender' ) )
+			->disableOriginalConstructor()
+			->getMock();
+
+		// When we call render() below, MathTexvc will ...
+
+		// ... first check if the item exists in the database cache:
+		$texvc->expects( $this->once() )
+			->method( 'readFromDB' )
+			->with()
+			->will( $this->returnValue( true ) );
+
+		// ... if cache lookup succeeded, it won't shell out to texvc:
+		$texvc->expects( $this->never() )
+			->method( 'callTexvc' );
+
+		// ... instead, MathTexvc will skip to HTML generation:
+		$texvc->expects( $this->once() )
+			->method( 'doHTMLRender' );
+
+		$texvc->render();
+	}
+
+	/**
+	 * Test behavior of render() upon cache miss.
+	 * If the rendered object is not in the cache, MathTexvc will shell
+	 * out to texvc to generate it. If texvc succeeds, it'll use the
+	 * result to generate HTML.
+	 * @covers MathTexvc::render
+	 */
+	function testRenderCacheMiss() {
+		$texvc = $this->getMockBuilder( 'MathTexvc' )
+			->setMethods( array( 'readCache', 'callTexvc', 'doHTMLRender' ) )
+			->disableOriginalConstructor()
+			->getMock();
+
+		// When we call render() below, MathTexvc will ...
+
+		// ... first look up the item in cache:
+		$texvc->expects( $this->once() )
+			->method( 'readCache' )
+			->will( $this->returnValue( false ) );
+
+		// ... on cache miss, MathTexvc will shell out to texvc:
+		$texvc->expects( $this->once() )
+			->method( 'callTexvc' )
+			->will( $this->returnValue( MW_TEXVC_SUCCESS ) );
+
+		// ... if texvc succeeds, MathTexvc will generate HTML:
+		$texvc->expects( $this->once() )
+			->method( 'doHTMLRender' );
+
+		$texvc->render();
+	}
+
+	/**
+	 * Test behavior of render() when texvc fails.
+	 * If texvc returns a value other than MW_TEXVC_SUCCESS, render()
+	 * returns the error object and does not attempt to generate HTML.
+	 * @covers MathTexvc::render
+	 */
+	function testRenderTexvcFailure() {
+		$texvc = $this->getMockBuilder( 'MathTexvc' )
+			->setMethods( array( 'readCache', 'callTexvc', 'doHTMLRender' ) )
+			->disableOriginalConstructor()
+			->getMock();
+
+		// When we call render() below, MathTexvc will ...
+
+		// ... first look up the item in cache:
+		$texvc->expects( $this->any() )
+			->method( 'readCache' )
+			->will( $this->returnValue( false ) );
+
+		// ... on cache miss, shell out to texvc:
+		$texvc->expects( $this->once() )
+			->method( 'callTexvc' )
+			->will( $this->returnValue( 'error' ) );
+
+		// ... if texvc fails, render() will not generate HTML:
+		$texvc->expects( $this->never() )
+			->method( 'doHTMLRender' );
+
+		// ... it will return the error result instead:
+		$this->assertEquals( $texvc->render(), 'error' );
+	}
+}