Warn when returning repo.state, allow viewmodel function

docs/api/presenter.md

@@ -60,6 +60,24 @@ planets from the Microcosm instance provided to it via the `repo`
 prop. This is available as state, which the Presenter can send into a
 child component.
 
+### Listening to all state changes
+
+While we do not recommend it for large projects, some times it's simply easier
+to subscribe to all repo changes. `viewModel` can also return a function, which
+will be called with state:
+
+```javascript
+class PlanetsPresenter extends Presenter {
+  viewModel(props) {
+    return state => state
+  }
+
+  render() {
+    return <p>{ this.state.planets.join(', ') }</p>
+  }
+}
+```
+
 ## Receiving Intents
 
 Though explicit, passing event callbacks down through a deep component

src/addons/presenter.js

@@ -107,7 +107,14 @@ export default class Presenter extends Component {
    * @private
    */
   _updatePropMap (props) {
-    this.propMap = this.viewModel(props)
+    this.propMap = this.viewModel ? this.viewModel(props) : {}
+
+    if (this.repo && this.propMap === this.repo.state) {
+      console.warn("The view model for this presenter returned repo.state. " +
+                   "This method onlys run when a presenter is given new properties. " +
+                   "If you would like to subscribe to all state changes, return a " +
+                   "function like: `(state) => state`.")
+    }
   }
 
   /**
@@ -125,9 +132,14 @@ export default class Presenter extends Component {
    * @private
    */
   _getState () {
-    const nextState = {}
     const repoState = this._getRepoState()
 
+    if (typeof this.propMap === 'function') {
+      return this.propMap(repoState)
+    }
+
+    const nextState = {}
+
     for (let key in this.propMap) {
       const entry = this.propMap[key]
 

test/addons/presenter.test.js

@@ -240,3 +240,37 @@ test('calling setState in setup does not raise a warning', t => {
 
   console.restore()
 })
+
+test('warns when setState in setup does not raise a warning', t => {
+  class MyPresenter extends Presenter {
+    viewModel() {
+      return this.repo.state
+    }
+    render() {
+      return <p>Test</p>
+    }
+  }
+
+  console.record()
+
+  mount(<MyPresenter repo={ new Microcosm() } />)
+
+  t.regex(console.last('warn'), /The view model for this presenter returned repo\.state/i)
+
+  console.restore()
+})
+
+test('allows functions to return from viewModel', t => {
+  class MyPresenter extends Presenter {
+    viewModel() {
+      return state => state
+    }
+    render() {
+      return <p>Test</p>
+    }
+  }
+
+  const el = mount(<MyPresenter repo={new Microcosm()} />)
+
+  t.is(el.state(), el.instance().repo.state)
+})

test/helpers/console.js

@@ -14,10 +14,14 @@ export default {
   record() {
     for (let key in messages) {
       originals[key] = console[key]
-      console[key] = (...args) => messages[key].push([args])
+      console[key] = (...args) => messages[key].push([args.join(' ')])
     }
   },
 
+  last(key) {
+    return messages[key][messages[key].length - 1]
+  },
+
   count(key) {
     if (key in messages === false) {
       throw new Error(`console.${key} is not tracked. Please add it to "${ __filename }."`)