Emit post-processing 'info' event for Stream-based output

docs/api.md

@@ -27,6 +27,7 @@ The object returned by the constructor implements the
 [stream.Duplex](http://nodejs.org/api/stream.html#stream_class_stream_duplex) class.
 
 JPEG, PNG or WebP format image data can be streamed out from this object.
+When using Stream based output, derived attributes are available from the `info` event.
 
 ```javascript
 sharp('input.jpg')
@@ -37,6 +38,19 @@ sharp('input.jpg')
   });
 ```
 
+```javascript
+// Read image data from readableStream,
+// resize to 300 pixels wide,
+// emit an 'info' event with calculated dimensions
+// and finally write image data to writableStream
+var transformer = sharp()
+  .resize(300)
+  .on('info', function(info) {
+    console.log('Image height is ' + info.height);
+  });
+readableStream.pipe(transformer).pipe(writableStream);
+```
+
 #### metadata([callback])
 
 Fast access to image metadata without decoding any compressed image data.

docs/changelog.md

@@ -10,6 +10,10 @@
   [#338](https://github.com/lovell/sharp/issues/338)
   [@lookfirst](https://github.com/lookfirst)
 
+* Emit post-processing 'info' event for Stream output.
+  [#367](https://github.com/lovell/sharp/issues/367)
+  [@salzhrani](https://github.com/salzhrani)
+
 ### v0.13 - "*mind*"
 
 #### v0.13.1 - 27<sup>th</sup> February 2016

index.js

@@ -803,10 +803,11 @@ Sharp.prototype._pipeline = function(callback) {
     if (this.options.streamIn) {
       // output=stream, input=stream
       this.on('finish', function() {
-        sharp.pipeline(that.options, function(err, data) {
+        sharp.pipeline(that.options, function(err, data, info) {
           if (err) {
             that.emit('error', err);
           } else {
+            that.emit('info', info);
             that.push(data);
           }
           that.push(null);
@@ -814,10 +815,11 @@ Sharp.prototype._pipeline = function(callback) {
       });
     } else {
       // output=stream, input=file/buffer
-      sharp.pipeline(this.options, function(err, data) {
+      sharp.pipeline(this.options, function(err, data, info) {
         if (err) {
           that.emit('error', err);
         } else {
+          that.emit('info', info);
           that.push(data);
         }
         that.push(null);

test/unit/io.js

@@ -109,6 +109,26 @@ describe('Input/output', function() {
     readable.pipe(pipeline).pipe(writable);
   });
 
+  it('Stream should emit info event', function(done) {
+    var readable = fs.createReadStream(fixtures.inputJpg);
+    var writable = fs.createWriteStream(fixtures.outputJpg);
+    var pipeline = sharp().resize(320, 240);
+    var infoEventEmitted = false;
+    pipeline.on('info', function(info) {
+      assert.strictEqual('jpeg', info.format);
+      assert.strictEqual(320, info.width);
+      assert.strictEqual(240, info.height);
+      assert.strictEqual(3, info.channels);
+      infoEventEmitted = true;
+    });
+    writable.on('finish', function() {
+      assert.strictEqual(true, infoEventEmitted);
+      fs.unlinkSync(fixtures.outputJpg);
+      done();
+    });
+    readable.pipe(pipeline).pipe(writable);
+  });
+
   it('Handle Stream to Stream error ', function(done) {
     var pipeline = sharp().resize(320, 240);
     var anErrorWasEmitted = false;