Resolve ${var} values in middleware.json

App variables can now be specified in middleware.json using the ${var}
format. The value of var is looked up using app.get().

This allows loopback.rest and other loopback middleware which require
app variables, to be loaded declaratively using middleware.json.

lib/executor.js

@@ -316,10 +316,50 @@ function setupMiddleware(app, instructions) {
     }
     assert(typeof factory === 'function',
       'Middleware factory must be a function');
+    data.config = getUpdatedConfigObject(data.config, app);
     app.middlewareFromConfig(factory, data.config);
   });
 }
 
+function getUpdatedConfigObject(config, app) {
+
+  var DYNAMIC_CONFIG_PARAM = /\$\{(\w+)\}$/;
+
+  function getConfigVariable(param) {
+    var configVariable = param;
+    var match = configVariable.match(DYNAMIC_CONFIG_PARAM);
+    if (match) {
+      var appValue = app.get(match[1]);
+      if (appValue !== undefined) {
+        configVariable = appValue;
+      } else {
+        console.warn('%s does not resolve to a valid value. ' +
+        '"%s" must be resolvable by app.get().', param, match[1]);
+      }
+    }
+    return configVariable;
+  }
+
+  function interpolateVariables(config) {
+    var interpolated = {};
+    Object.keys(config).forEach(function(configKey) {
+      var value = config[configKey];
+      if (Array.isArray(value)) {
+        interpolated[configKey] = value.map(getConfigVariable);
+      } else if (typeof value === 'string') {
+        interpolated[configKey] = getConfigVariable(value);
+      } else if (typeof value === 'object' && Object.keys(value).length) {
+        interpolated[configKey] = interpolateVariables(value);
+      } else {
+        interpolated[configKey] = value;
+      }
+    });
+    return interpolated;
+  }
+
+  return interpolateVariables(config);
+}
+
 function setupComponents(app, instructions) {
   instructions.components.forEach(function(data) {
     debug('Configuring component %j', data.sourceFile);

test/executor.test.js

@@ -292,7 +292,7 @@ describe('executor', function() {
       });
     });
 
-    describe ('for mixins', function() {
+    describe('for mixins', function() {
       var options;
       beforeEach(function() {
         appdir.writeFileSync('custom-mixins/example.js',
@@ -425,6 +425,77 @@ describe('executor', function() {
     });
   });
 
+  describe('with middleware.json', function() {
+
+    it('should parse a simple config variable', function(done) {
+      boot.execute(app, simpleMiddlewareConfig('routes',
+        { path: '${restApiRoot}' }
+      ));
+
+      supertest(app).get('/').end(function(err, res) {
+        if (err) return done(err);
+        expect(res.body.path).to.equal(app.get('restApiRoot'));
+        done();
+      });
+    });
+
+    it('should parse multiple config variables', function(done) {
+      boot.execute(app, simpleMiddlewareConfig('routes',
+        { path: '${restApiRoot}', env: '${env}' }
+      ));
+
+      supertest(app).get('/').end(function(err, res) {
+        if (err) return done(err);
+        expect(res.body.path).to.equal(app.get('restApiRoot'));
+        expect(res.body.env).to.equal(app.get('env'));
+        done();
+      });
+    });
+
+    it('should parse config variables in an array', function(done) {
+      boot.execute(app, simpleMiddlewareConfig('routes',
+        { paths: ['${restApiRoot}'] }
+      ));
+
+      supertest(app).get('/').end(function(err, res) {
+        if (err) return done(err);
+        expect(res.body.paths).to.eql(
+          [app.get('restApiRoot')]
+          );
+        done();
+      });
+    });
+
+    it('should parse config variables in an object', function(done) {
+      boot.execute(app, simpleMiddlewareConfig('routes',
+        { info: { path: '${restApiRoot}' } }
+      ));
+
+      supertest(app).get('/').end(function(err, res) {
+        if (err) return done(err);
+        expect(res.body.info).to.eql({
+          path: app.get('restApiRoot')
+        });
+        done();
+      });
+    });
+
+    it('should parse config variables in a nested object', function(done) {
+      boot.execute(app, simpleMiddlewareConfig('routes',
+        { nested: { info: { path: '${restApiRoot}' } } }
+      ));
+
+      supertest(app).get('/').end(function(err, res) {
+        if (err) return done(err);
+        expect(res.body.nested).to.eql({
+          info: { path: app.get('restApiRoot') }
+        });
+        done();
+      });
+    });
+
+  });
+
   it('calls function exported by boot/init.js', function() {
     var file = appdir.writeFileSync('boot/init.js',
       'module.exports = function(app) { app.fnCalled = true; };');
@@ -583,6 +654,23 @@ describe('executor', function() {
   });
 });
 
+function simpleMiddlewareConfig(phase, params) {
+  return someInstructions({
+    middleware: {
+      phases: [phase],
+      middleware: [
+        {
+          sourceFile: path.join(__dirname, './fixtures/simple-middleware.js'),
+          config: {
+            phase: phase,
+            params: params
+          }
+        }
+      ]
+    }
+  });
+}
+
 function assertValidDataSource(dataSource) {
   // has methods
   assert.isFunc(dataSource, 'createModel');

test/fixtures/simple-middleware.js

@@ -0,0 +1,5 @@
+module.exports = function(params) {
+  return function(req, res, next) {
+    res.send(params);
+  };
+};