Implement app.middlewareFromConfig

Implement a function registering a middleware using a factory function
and a JSON config.

Example:

    app.middlewareFromConfig(compression, {
      enabled: true,
      phase: 'initial',
      config: {
        threshold: 128
      }
    });

lib/server-app.js

@@ -1,3 +1,4 @@
+var assert = require('assert');
 var express = require('express');
 var merge = require('util')._extend;
 var PhaseList = require('loopback-phase').PhaseList;
@@ -12,6 +13,47 @@ module.exports = function loopbackExpress() {
   return app;
 };
 
+/**
+ * Register a middleware using a factory function and a JSON config.
+ *
+ * **Example**
+ *
+ * ```js
+ * app.middlewareFromConfig(compression, {
+ *   enabled: true,
+ *   phase: 'initial',
+ *   config: {
+ *     threshold: 128
+ *   }
+ * });
+ * ```
+ *
+ * @param {function} factory The factory function creating a middleware handler.
+ *   Typically a result of `require()` call, e.g. `require('compression')`.
+ * @param {Array|*} config The configuration. Either an array of arguments
+ *   to pass to the factory function, or the value of the first argument
+ *   when the factory expects a single argument only.
+ */
+proto.middlewareFromConfig = function(factory, config) {
+  assert(typeof factory === 'function', '"factory" must be a function');
+  assert(typeof config === 'object', '"config" must be an object');
+  assert(typeof config.phase === 'string' && config.phase,
+    '"config.phase" must be a non-empty string');
+
+  if (config.enabled === false)
+    return;
+
+  var args = config.config;
+  if (args === undefined) {
+    args = [];
+  } else if (!Array.isArray(args)) {
+    args = [args];
+  }
+
+  var handler = factory.apply(null, args);
+  this.middleware(config.phase, handler);
+};
+
 /**
  * Register a middleware handler to be executed in a given phase.
  * @param {string} name The phase name, e.g. "init" or "routes".
@@ -23,6 +65,9 @@ module.exports = function loopbackExpress() {
 proto.middleware = function(name, handler) {
   this.lazyrouter();
 
+  assert(typeof name === 'string' && name, '"name" must be a non-empty string');
+  assert(typeof handler === 'function', '"handler" must be a function');
+
   var fullName = name;
   var handlerName = handler.name || '(anonymous)';
 

test/app.test.js

@@ -1,4 +1,5 @@
 var path = require('path');
+
 var http = require('http');
 var loopback = require('../');
 var PersistedModel = loopback.PersistedModel;
@@ -27,7 +28,7 @@ describe('app', function() {
       });
       app.use(namedHandler('main'));
 
-      executeHandlers(function(err) {
+      executeMiddlewareHandlers(app, function(err) {
         if (err) return done(err);
         expect(steps).to.eql([
           'initial', 'session', 'auth', 'parse',
@@ -42,7 +43,7 @@ describe('app', function() {
       app.middleware('routes:after', namedHandler('routes:after'));
       app.use(namedHandler('main'));
 
-      executeHandlers(function(err) {
+      executeMiddlewareHandlers(app, function(err) {
         if (err) return done(err);
         expect(steps).to.eql(['routes:before', 'main', 'routes:after']);
         done();
@@ -64,7 +65,7 @@ describe('app', function() {
         next();
       });
 
-      executeHandlers(function(err) {
+      executeMiddlewareHandlers(app, function(err) {
         if (err) return done(err);
         expect(steps).to.eql(['initial', 'error']);
         done();
@@ -78,7 +79,7 @@ describe('app', function() {
         next(expectedError);
       });
 
-      executeHandlers(function(err) {
+      executeMiddlewareHandlers(app, function(err) {
         expect(err).to.equal(expectedError);
         done();
       });
@@ -90,18 +91,59 @@ describe('app', function() {
         next();
       };
     }
-
-    function executeHandlers(callback) {
-      var server = http.createServer(function(req, res) {
-        app.handle(req, res, callback);
   });
 
-      request(server)
-        .get('/test/url')
-        .end(function(err) {
-          if (err) return callback(err);
+  describe.onServer('.middlewareFromConfig', function() {
+    it('provides API for loading middleware from JSON config', function(done) {
+      var steps = [];
+      var expectedConfig = { key: 'value' };
+
+      var handlerFactory = function() {
+        var args = Array.prototype.slice.apply(arguments);
+        return function(req, res, next) {
+          steps.push(args);
+          next();
+        };
+      };
+
+      // Config as an object (single arg)
+      app.middlewareFromConfig(handlerFactory, {
+        enabled: true,
+        phase: 'session',
+        config: expectedConfig
+      });
+
+      // Config as a value (single arg)
+      app.middlewareFromConfig(handlerFactory, {
+        enabled: true,
+        phase: 'session:before',
+        config: 'before'
+      });
+
+      // Config as a list of args
+      app.middlewareFromConfig(handlerFactory, {
+        enabled: true,
+        phase: 'session:after',
+        config: ['after', 2]
+      });
+
+      // Disabled by configuration
+      app.middlewareFromConfig(handlerFactory, {
+        enabled: false,
+        phase: 'initial',
+        config: null
+      });
+
+      executeMiddlewareHandlers(app, function(err) {
+        if (err) return done(err);
+        expect(steps).to.eql([
+          ['before'],
+          [expectedConfig],
+          ['after', 2]
+        ]);
+        done();
+      });
     });
-    }
   });
 
   describe('app.model(Model)', function() {
@@ -470,3 +512,15 @@ describe('app', function() {
     });
   });
 });
+
+function executeMiddlewareHandlers(app, callback) {
+  var server = http.createServer(function(req, res) {
+    app.handle(req, res, callback);
+  });
+
+  request(server)
+    .get('/test/url')
+    .end(function(err) {
+      if (err) return callback(err);
+    });
+}