Return middleware chain promise from callback()

docs/api/index.md

@@ -169,6 +169,12 @@ this.cookies.set('name', 'tobi', { signed: true });
 app.context.db = db();
 ```
 
+## app.usedAsMiddleware
+
+  Disables the default 404 status code and empty response.
+  This is very useful if you're using Koa as part of another framework, where Koa tries
+  to handle a request first and then falls back to the legacy framework.
+
 ## Error Handling
 
   By default outputs all errors to stderr unless __NODE_ENV__ is "test" or `app.silent` is `true`.

lib/application.js

@@ -130,10 +130,13 @@ app.callback = function(){
   if (!this.listeners('error').length) this.on('error', this.onerror);
 
   return function handleRequest(req, res){
-    res.statusCode = 404;
+    // don't set the default status code if Koa is part of a larger framework
+    if (!self.usedAsMiddleware) {
+      res.statusCode = 404;
+    }
     var ctx = self.createContext(req, res);
     onFinished(res, ctx.onerror);
-    fn.call(ctx).then(function handleResponse() {
+    return fn.call(ctx).then(function handleResponse() {
       respond.call(ctx);
     }).catch(ctx.onerror);
   }
@@ -215,6 +218,8 @@ function respond() {
 
   // status body
   if (null == body) {
+    // don't send back the default empty response if Koa is part of a larger framework
+    if (this.app.usedAsMiddleware) return;
     this.type = 'text';
     body = this.message || String(code);
     this.length = Buffer.byteLength(body);

test/application.js

@@ -1110,3 +1110,41 @@ describe('app.response', function(){
     .expect(204, done);
   })
 })
+
+describe('app.usedAsMiddleware', function(){
+  var app = koa()
+  app.use(function *(next) {
+    if (this.url.indexOf('/koa') == 0) {
+      this.statusCode = 200
+      this.body = 'Hello from Koa!'
+    } else {
+      yield next
+    }
+  })
+  app.usedAsMiddleware = true
+  var koaHandle = app.callback()
+  function handle(req, res){
+    koaHandle(req, res).then(function(){
+      if (!res.finished) {
+        res.end('Hello from other framework!')
+      }
+    })
+  }
+  var server = http.createServer(handle).listen(0)
+
+  it('should not touch res if middleware doesn\'t', function(done){
+    request(server)
+    .get('/')
+    .expect(200)
+    .expect('Hello from other framework!')
+    .end(done)
+  })
+
+  it('should let middleware handle res', function(done){
+    request(server)
+    .get('/koa')
+    .expect(200)
+    .expect('Hello from Koa!')
+    .end(done)
+  })
+})