Support async transformFn when it returns Promise

README.md

@@ -16,7 +16,7 @@ contents and returns the desired contents.
 pipelines in both buffer mode and streaming mode.
 * **Economical**. Reduce the need for gulp-specific plugins by pairing
 gulp-transform with ordinary node packages and functions.
-
+* **Async**-ready. Transform is supported by returning `Promise` from transform function.
 ## Install
 
 Install via [npm][NPM link]:
@@ -89,9 +89,9 @@ gulp.task('cheerio', function() {
 
 ##### transformFn `function`
 
-The callback responsible for the transformation. The return value must be a
-string or a Buffer, which will replace the file's contents. The callback
-is invoked once per file with the following arguments:
+The callback responsible for the transformation. The return value must be value or
+Promise resolvable to string or a Buffer, which will replace the file's contents.
+The callbackis invoked once per file with the following arguments:
 
 * **contents** `Buffer` | `string` <br>
 The initial contents of the file. Contents are passed as a Buffer unless the

src/file-stream.js

@@ -18,10 +18,16 @@ export class FileStream extends Transform {
   }
 
   _flush(done) {
+    let self = this;
     let contents = Buffer.concat(this.data);
-    this.push(transform(this.fn, contents, this.file, this.opts));
-
-    done();
+    transform(this.fn, contents, this.file, this.opts)
+      .then(function(contents) {
+        self.push(contents);
+        done();
+      })
+      .catch(function(error) {
+        done(error)
+      })
   }
 
 }

src/plugin-stream.js

@@ -15,14 +15,17 @@ export class PluginStream extends Transform {
     let {fn, opts} = this;
 
     if (file.isBuffer()) {
-      file.contents = transform(fn, file.contents, file, opts);
-    }
-
-    if (file.isStream()) {
+      transform(fn, file.contents, file, opts)
+        .then(function(contents) {
+          file.contents = contents;
+          next(null, file)
+        })
+        .catch(function(error) {
+          next(error);
+        })
+    } else if (file.isStream()) {
       file.contents = file.contents.pipe(new FileStream(fn, file, opts));
+      next(null, file);
     }
-
-    next(null, file);
   }
-
 }

src/transform.js

@@ -3,9 +3,10 @@ import {err} from './err';
 
 export function transform(fn, contents, file, opts) {
   let encoded = opts.encoding ? contents.toString(opts.encoding) : contents;
-  let transformed = fn.call(opts.thisArg, encoded, file);
-
-  return isBuffer(transformed) ? transformed :
-    isString(transformed) ? new Buffer(transformed) :
-    err('transformFn must return a string or a Buffer');
+  return Promise.resolve(fn.call(opts.thisArg, encoded, file))
+    .then(function(transformed) {
+      return isBuffer(transformed) ? transformed :
+        isString(transformed) ? new Buffer(transformed) :
+        err('transformFn must return a string or a Buffer');
+    })
 }

test/index.coffee

@@ -32,8 +32,13 @@ describe 'plugin: gulp-transform', ->
 
     context 'returns neither a string nor a Buffer', ->
 
-      it 'throws PluginError', ->
-        err -> transform((content) -> 42).write buffered()
+      it 'throws PluginError', (done) ->
+          t = transform((content) -> 42)
+          t.on 'error', (err) ->
+            done()
+          t.on 'data', () ->
+            done new Error('expected PluginError')
+          t.write buffered()
 
     context 'returns a Buffer or string', ->
       [fn, file] = [null, null]