Throw appropriate errors for invalid instantiation.

Throw appropriate errors if a Style Interface has not been registered or an invalid Style Interface has been registered.
Throw an appropriate error if a Theme has not been registered.

Resolves: airbnb#102

src/ThemedStyleSheet.js

@@ -12,11 +12,53 @@ function registerTheme(theme) {
   };
 }
 
+function invalidMethodError(invalidMethodName) {
+  return new TypeError(`
+    react-with-styles internal error: An invalid Style Interface has been registered.
+    typeof interface.${invalidMethodName} must be a function.
+  `);
+}
+
+function validateStyleInterfaceShape(interfaceToValidate) {
+  if (!(typeof interfaceToValidate.create === 'function')) {
+    throw invalidMethodError('create');
+  }
+
+  if (!(typeof interfaceToValidate.resolve === 'function')) {
+    throw invalidMethodError('resolve');
+  }
+}
+
 function registerInterface(interfaceToRegister) {
+  if (process.env.NODE_ENV !== 'production') {
+    validateStyleInterfaceShape(interfaceToRegister);
+  }
   styleInterface = interfaceToRegister;
 }
 
+function validateStyleInterface() {
+  if (!styleInterface) {
+    throw new ReferenceError(`
+      react-with-styles internal error: A Style Interface has not been registered.
+      You must register a valid Style Interface using ThemedStyleSheet.registerInterface.
+    `);
+  }
+}
+
+function validateStyleTheme() {
+  if (!styleTheme) {
+    throw new ReferenceError(`
+      react-with-styles internal error: A Theme has not been registered.
+      You must register a theme using ThemedStyleSheet.registerTheme.
+    `);
+  }
+}
+
 function create(makeFromTheme) {
+  if (process.env.NODE_ENV !== 'production') {
+    validateStyleInterface();
+    validateStyleTheme();
+  }
   // Get an id to associate with this stylesheet
   const id = internalId;
   internalId += 1;
@@ -34,6 +76,9 @@ function get() {
 }
 
 function resolve(...styles) {
+  if (process.env.NODE_ENV !== 'production') {
+    validateStyleInterface();
+  }
   return styleInterface.resolve(styles);
 }
 

test/ThemedStyleSheet_tests.js

@@ -0,0 +1,57 @@
+import { expect } from 'chai';
+import globalCache from 'global-cache';
+
+let ThemedStyleSheet;
+
+describe('ThemedStyleSheet', () => {
+  beforeEach(() => {
+    ThemedStyleSheet = require('../src/ThemedStyleSheet').default; // eslint-disable-line global-require
+  });
+
+  afterEach(() => {
+    delete require.cache[require.resolve('../src/ThemedStyleSheet')];
+    globalCache.clear();
+  });
+
+  describe('errors', () => {
+    it('throws a Reference Error if a theme has not been registered and create is called', () => {
+      ThemedStyleSheet.registerInterface({
+        create() {},
+        resolve() {},
+      });
+      expect(ThemedStyleSheet.create).to.throw(ReferenceError);
+    });
+
+    describe('Style Interface', () => {
+      beforeEach(() => {
+        ThemedStyleSheet.registerTheme({});
+      });
+
+      it('throws a ReferenceError if an interface has not be registered and create is called', () => {
+        expect(ThemedStyleSheet.create).to.throw(ReferenceError);
+      });
+
+      it('throws a ReferenceError if an interface has not be registered and resolve is called', () => {
+        expect(ThemedStyleSheet.resolve).to.throw(ReferenceError);
+      });
+
+      it('throws a TypeError if create is not a function when the interface is registered', () => {
+        const mockInterface = {
+          create: {},
+          resolve() {},
+        };
+        const register = () => ThemedStyleSheet.registerInterface(mockInterface);
+        expect(register).to.throw(TypeError);
+      });
+
+      it('throws a TypeError if resolve is not a function when the interface is registered', () => {
+        const mockInterface = {
+          create() {},
+          resolve: {},
+        };
+        const register = () => ThemedStyleSheet.registerInterface(mockInterface);
+        expect(register).to.throw(TypeError);
+      });
+    });
+  });
+});