Add transformPath option (#60)

fixture/different.transformPath/src/a

@@ -0,0 +1 @@
+a

fixture/different.transformPath/src/b

@@ -0,0 +1 @@
+b

fixture/different.transformPath/trg/c/a

@@ -0,0 +1 @@
+a

fixture/different.transformPath/trg/c/b

@@ -0,0 +1 @@
+b!

index.js

@@ -66,6 +66,10 @@ module.exports = (dest, opts) => {
 		throw new gutil.PluginError('gulp-changed', '`dest` required');
 	}
 
+	if (opts.transformPath !== undefined && typeof opts.transformPath !== 'function') {
+		throw new gutil.PluginError('gulp-changed', '`opts.transformPath` needs to be a function');
+	}
+
 	return through.obj(function (file, enc, cb) {
 		const dest2 = typeof dest === 'function' ? dest(file) : dest;
 		let newPath = path.resolve(opts.cwd, dest2, file.relative);
@@ -74,6 +78,14 @@ module.exports = (dest, opts) => {
 			newPath = gutil.replaceExtension(newPath, opts.extension);
 		}
 
+		if (opts.transformPath) {
+			newPath = opts.transformPath(newPath);
+
+			if (typeof newPath !== 'string') {
+				throw new gutil.PluginError('gulp-changed', '`opts.transformPath` needs to return a string');
+			}
+		}
+
 		opts.hasChanged(this, cb, file, newPath);
 	});
 };

readme.md

@@ -112,6 +112,23 @@ You can also supply a custom comparator function which will receive the followin
 - `sourceFile` *([Vinyl file object](https://github.com/wearefractal/vinyl#file))*
 - `destPath` *(string)* - Destination for `sourceFile` as an absolute path
 
+##### transformPath
+
+Type: `Function`
+
+Function to transform the path to the destination file. Should return the absolute path to the (renamed) destination file.
+
+Useful if you rename your file later on, like in the below example:
+
+```js
+gulp.task('marked', () =>
+	gulp.src('src/content/about.md')
+		.pipe(changed('dist', {transformPath: newPath => path.join(path.dirname(newPath), path.basename(newPath, '.md'), 'index.html')}))
+		.pipe(marked())
+		.pipe(rename(newPath => path.join(path.dirname(newPath), path.basename(newPath, '.md'), 'index.html'))))
+		.pipe(gulp.dest('dist'))
+);
+```
 
 ## In-place change monitoring
 

test.js

@@ -107,6 +107,22 @@ describe('compareSha1Digest', () => {
 		});
 	});
 
+	it('should only pass through changed files using transformPath', () => {
+		const stream = gulp.src('fixture/different.transformPath/src/*')
+			.pipe(changed('fixture/different.transformPath/trg', {
+				hasChanged: changed.compareSha1Digest,
+				transformPath: newPath => {
+					const pathParsed = path.parse(newPath);
+					return path.join(pathParsed.dir, 'c', pathParsed.base);
+				}
+			}));
+
+		return getStream.array(stream).then(files => {
+			assert.equal(files.length, 1);
+			assert.equal(path.basename(files[0].path), 'b');
+		});
+	});
+
 	it('should only pass through changed files using extension .coffee', () => {
 		const stream = gulp.src('fixture/different.ext/src/*')
 			.pipe(changed('fixture/different.ext/trg', {