add convert.back()

index.js

@@ -9,14 +9,17 @@ function convert (mw) {
   if (typeof mw !== 'function') {
     throw new TypeError('middleware must be a function')
   }
-  if (mw.constructor.name === 'GeneratorFunction') {
-    return function (ctx, next) {
-      return co.call(ctx, mw.call(ctx, createGenerator(next)))
-    }
-  } else {
+  if (mw.constructor.name !== 'GeneratorFunction') {
     // assume it's Promise-based middleware
     return mw
   }
+  return function (ctx, next) {
+    return co.call(ctx, mw.call(ctx, createGenerator(next)))
+  }
+}
+
+function * createGenerator (next) {
+  return yield next()
 }
 
 // convert.compose(mw, mw, mw)
@@ -28,6 +31,26 @@ convert.compose = function (arr) {
   return compose(arr.map(convert))
 }
 
-function * createGenerator (next) {
-  return yield next()
+convert.back = function (mw) {
+  if (typeof mw !== 'function') {
+    throw new TypeError('middleware must be a function')
+  }
+  if (mw.constructor.name === 'GeneratorFunction') {
+    // assume it's generator middleware
+    return mw
+  }
+  return function * (next) {
+    let ctx = this
+    let called = false
+    // no need try...catch here, it's ok even `mw()` throw exception
+    yield Promise.resolve(mw(ctx, function () {
+      if (called) {
+        // guard against multiple next() calls
+        // https://github.com/koajs/compose/blob/4e3e96baf58b817d71bd44a8c0d78bb42623aa95/index.js#L36
+        return Promise.reject(new Error('next() called multiple times'))
+      }
+      called = true
+      return co.call(ctx, next)
+    }))
+  }
 }

package.json

@@ -27,6 +27,7 @@
   },
   "devDependencies": {
     "koa": "^2.0.0-alpha.2",
+    "koa-v1": "^1.0.0",
     "mocha": "^2.3.3",
     "standard": "^5.3.1",
     "supertest": "^1.1.0"

test.js

@@ -4,6 +4,7 @@
 
 const co = require('co')
 const Koa = require('koa')
+const KoaV1 = require('koa-v1')
 const assert = require('assert')
 const convert = require('./index')
 const request = require('supertest')
@@ -143,6 +144,60 @@ describe('convert.compose()', () => {
   })
 })
 
+describe('convert.back()', () => {
+  it('should work with koa 1', done => {
+    let app = new KoaV1()
+
+    app.use(function * (next) {
+      this.body = [1]
+      yield next
+      this.body.push(6)
+    })
+
+    app.use(convert.back((ctx, next) => {
+      ctx.body.push(2)
+      return next().then(() => {
+        ctx.body.push(5)
+      })
+    }))
+
+    app.use(convert.back(co.wrap(function * (ctx, next) {
+      ctx.body.push(3)
+      yield next()
+      ctx.body.push(4)
+    })))
+
+    request(app.callback())
+      .get('/')
+      .expect(200, [1, 2, 3, 4, 5, 6])
+      .end(done)
+  })
+
+  it('should guard multiple calls', done => {
+    let app = new KoaV1()
+
+    app.use(function * (next) {
+      try {
+        this.body = [1]
+        yield next
+      } catch (e) {
+        this.body.push(e.message)
+      }
+    })
+
+    app.use(convert.back(co.wrap(function * (ctx, next) {
+      ctx.body.push(2)
+      yield next()
+      yield next() // this should throw new Error('next() called multiple times')
+    })))
+
+    request(app.callback())
+      .get('/')
+      .expect(200, [1, 2, 'next() called multiple times'])
+      .end(done)
+  })
+})
+
 describe('migration snippet', () => {
   let app = new Koa()