Add support for Promises in "onSaveSpritesheet"

Closes #66.

README.md

@@ -119,7 +119,11 @@ Built-in filters:
 
 ###### hooks.onSaveSpritesheet
 
-> Hook that allows to rewrite the filepath of produced spritesheet.
+> Hook that allows to rewrite the data of produced spritesheet.
+
+> If returned value is string, it is used as filepath value, and if returned value is object, it is used as value which will be merged with current spritesheet data.
+
+> Returned value can also be Promise which should return either string or object.
 
 - Default: `null`
 - Required: `false`

src/core.js

@@ -324,17 +324,27 @@ export function runSpritesmith(opts, images) {
  */
 export function saveSpritesheets(opts, images, spritesheets) {
 	return Promise.each(spritesheets, (spritesheet) => {
-		spritesheet.path = _.isFunction(opts.hooks.onSaveSpritesheet)
-			? opts.hooks.onSaveSpritesheet(opts, spritesheet)
-			: makeSpritesheetPath(opts, spritesheet);
+		return (
+				_.isFunction(opts.hooks.onSaveSpritesheet) ?
+				Promise.resolve(opts.hooks.onSaveSpritesheet(opts, spritesheet)) :
+				Promise.resolve(makeSpritesheetPath(opts, spritesheet))
+			)
+			.then(( res ) => {
+
+				if ( _.isString(res) ) {
+					spritesheet.path = res;
+				} else {
+					_.assign(spritesheet, res);
+				}
 
-		if (!spritesheet.path) {
-			throw new Error('postcss-sprites: Spritesheet requires a relative path.');
-		}
+				if (!spritesheet.path) {
+					throw new Error('postcss-sprites: Spritesheet requires a relative path.');
+				}
 
-		spritesheet.path = spritesheet.path.replace(/\\/g, '/');
+				spritesheet.path = spritesheet.path.replace(/\\/g, '/');
 
-		return fs.outputFileAsync(spritesheet.path, spritesheet.image);
+				return fs.outputFileAsync(spritesheet.path, spritesheet.image);
+			});
 	}).then(spritesheets => {
 		return [opts, images, spritesheets];
 	});

test/08-save-spritesheets.js

@@ -105,3 +105,21 @@ test('should throw error if path is empty', async (t) => {
 
 	t.throws(saveSpritesheets(images, t.context.opts, spritesheets));
 });
+
+test('should use Promise result provided by book', async (t) => {
+	const cssContents = await readFileAsync('./fixtures/basic/style.css');
+	const ast = postcss.parse(cssContents, { from: './fixtures/basic/style.css' });
+	let images, spritesheets, opts;
+
+	t.context.opts.spritePath = './build/on-save-hook/';
+	t.context.opts.hooks.onSaveSpritesheet = (pluginOpts, spritesheetGroups) => {
+		return new Promise(( resolve ) => setTimeout(() => resolve(Promise.resolve(path.join(pluginOpts.spritePath, 'custom-name.png'))), 0));
+	}
+
+	[ opts, images ] = await extractImages(ast, t.context.opts);
+	[ opts, images, spritesheets ] = await runSpritesmith(t.context.opts, images);
+	[ opts, images, spritesheets ] = await saveSpritesheets(t.context.opts, images, spritesheets);
+
+	t.deepEqual(spritesheets[0].path, 'build/on-save-hook/custom-name.png');
+	t.truthy(fs.statAsync('./build/on-save-hook/custom-name.png'));
+});