Add Gradle bindings for Gherkin formatter

As we've created a Gherkin formatter as part of diffplug#907, we should make it
possible to use it natively in Gradle, which requires we add it as a new
supported type in `SpotlessExtension`.

Author: jamietanna

plugin-gradle/README.md

@@ -69,6 +69,7 @@ Spotless supports all of Gradle's built-in performance features (incremental bui
   - [SQL](#sql) ([dbeaver](#dbeaver), [prettier](#prettier))
   - [Typescript](#typescript) ([tsfmt](#tsfmt), [prettier](#prettier))
   - [JSON](#json)
+  - [Gherkin](#gherkin)
   - Multiple languages
     - [Prettier](#prettier) ([plugins](#prettier-plugins), [npm detection](#npm-detection), [`.npmrc` detection](#npmrc-detection))
       - javascript, jsx, angular, vue, flow, typescript, css, less, scss, html, json, graphql, markdown, ymaml
@@ -558,6 +559,34 @@ spotless {
 }
 ```
 
+## Gherkin
+
+- `com.diffplug.gradle.spotless.GherkinExtension` [javadoc](https://javadoc.io/doc/com.diffplug.spotless/spotless-plugin-gradle/5.15.0/com/diffplug/gradle/spotless/GherkinExtension.html), [code](https://github.com/diffplug/spotless/blob/main/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GherkinExtension.java)
+
+```gradle
+spotless {
+  gherkin {
+    target 'src/**/*.feature' // you have to set the target manually
+    simple() // has its own section below
+  }
+}
+```
+
+### simple
+
+Uses a Gherkin pretty-printer that optionally allows configuring the number of spaces that are used to pretty print objects:
+
+```gradle
+spotless {
+  gherkin {
+    target 'src/**/*.feature'
+    simple()
+    // optional: specify the number of spaces to use
+    simple().indentWithSpaces(6)
+  }
+}
+```
+
 <a name="applying-prettier-to-javascript--flow--typescript--css--scss--less--jsx--graphql--yaml--etc"></a>
 
 ## Prettier

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GherkinExtension.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2016-2021 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.gradle.spotless;
+
+import javax.inject.Inject;
+
+import com.diffplug.spotless.FormatterStep;
+import com.diffplug.spotless.gherkin.GherkinSimpleStep;
+
+public class GherkinExtension extends FormatExtension {
+	private static final int DEFAULT_INDENTATION = 4;
+	static final String NAME = "gherkin";
+
+	@Inject
+	public GherkinExtension(SpotlessExtension spotless) {
+		super(spotless);
+	}
+
+	@Override
+	protected void setupTask(SpotlessTask task) {
+		if (target == null) {
+			throw noDefaultTargetException();
+		}
+		super.setupTask(task);
+	}
+
+	public SimpleConfig simple() {
+		return new SimpleConfig(DEFAULT_INDENTATION);
+	}
+
+	public class SimpleConfig {
+		private int indent;
+
+		public SimpleConfig(int indent) {
+			this.indent = indent;
+			addStep(createStep());
+		}
+
+		public void indentWithSpaces(int indent) {
+			this.indent = indent;
+			replaceStep(createStep());
+		}
+
+		private FormatterStep createStep() {
+			return GherkinSimpleStep.create(indent, provisioner());
+		}
+	}
+
+}

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessExtension.java

@@ -175,6 +175,12 @@ public void json(Action<JsonExtension> closure) {
 		format(JsonExtension.NAME, JsonExtension.class, closure);
 	}
 
+	/** Configures the special Gherkin-specific extension. */
+	public void gherkin(Action<GherkinExtension> closure) {
+		requireNonNull(closure);
+		format(GherkinExtension.NAME, GherkinExtension.class, closure);
+	}
+
 	/** Configures a custom extension. */
 	public void format(String name, Action<FormatExtension> closure) {
 		requireNonNull(name, "name");

plugin-gradle/src/test/java/com/diffplug/gradle/spotless/GherkinExtensionTest.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2021 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.gradle.spotless;
+
+import java.io.IOException;
+
+import org.junit.Test;
+
+public class GherkinExtensionTest extends GradleIntegrationHarness {
+	@Test
+	public void defaultFormatting() throws IOException {
+		setFile("build.gradle").toLines(
+				"buildscript { repositories { mavenCentral() } }",
+				"plugins {",
+				"    id 'java'",
+				"    id 'com.diffplug.spotless'",
+				"}",
+				"spotless {",
+				"    gherkin {",
+				"    target 'examples/**/*.feature'",
+				"    simple()",
+				"}",
+				"}");
+		setFile("src/main/resources/example.feature").toResource("gherkin/minimalBefore.feature");
+		setFile("examples/main/resources/example.feature").toResource("gherkin/minimalBefore.feature");
+		gradleRunner().withArguments("spotlessApply").build();
+		assertFile("src/main/resources/example.feature").sameAsResource("gherkin/minimalBefore.feature");
+		assertFile("examples/main/resources/example.feature").sameAsResource("gherkin/minimalAfter.feature");
+	}
+
+	@Test
+	public void formattingWithCustomNumberOfSpaces() throws IOException {
+		setFile("build.gradle").toLines(
+				"buildscript { repositories { mavenCentral() } }",
+				"plugins {",
+				"    id 'java'",
+				"    id 'com.diffplug.spotless'",
+				"}",
+				"spotless {",
+				"    gherkin {",
+				"    target 'src/**/*.feature'",
+				"    simple().indentWithSpaces(6)",
+				"}",
+				"}");
+		setFile("src/main/resources/example.feature").toResource("gherkin/minimalBefore.feature");
+		gradleRunner().withArguments("spotlessApply").build();
+		assertFile("src/main/resources/example.feature").sameAsResource("gherkin/minimalAfter6Spaces.feature");
+	}
+}