[change] introduce Jest preset

A simple Jest preset that configures module mapping and produces
human-readable styles (i.e., not converted to numeric form).

Fix #928
Fix #963

scripts/jest/serializer.js

@@ -0,0 +1,59 @@
+import React from 'react';
+import { StyleSheet } from '../../packages/react-native-web/src';
+
+function createSerializer(styleSheet) {
+  function flattenNodeStyles(node) {
+    if (node && node.props) {
+      // check for React elements in any props
+      const nextProps = Object.keys(node.props).reduce((acc, curr) => {
+        const value = node.props[curr];
+        if (React.isValidElement(value)) {
+          acc[curr] = flattenNodeStyles(value);
+        }
+        return acc;
+      }, {});
+
+      // flatten styles and avoid empty objects in snapshots
+      if (node.props.style) {
+        const style = styleSheet.flatten(node.props.style);
+        if (Object.keys(style).length > 0) {
+          nextProps.style = style;
+        } else {
+          delete nextProps.style;
+        }
+      }
+
+      const args = [node, nextProps];
+
+      // recurse over children too
+      const children = node.children || node.props.children;
+      if (children) {
+        if (Array.isArray(children)) {
+          children.forEach(child => {
+            args.push(flattenNodeStyles(child));
+          });
+        } else {
+          args.push(flattenNodeStyles(children));
+        }
+      }
+
+      return React.cloneElement.apply(React.cloneElement, args);
+    }
+
+    return node;
+  }
+
+  function test(value) {
+    return !!value && value.$$typeof === Symbol.for('react.test.json');
+  }
+
+  function print(value, serializer) {
+    return serializer(flattenNodeStyles(value));
+  }
+
+  return { test, print };
+}
+
+const serializer = createSerializer(StyleSheet);
+
+export default serializer;

scripts/jest/setupFramework.js

@@ -2,10 +2,9 @@
 
 import Adapter from 'enzyme-adapter-react-16';
 import Enzyme from 'enzyme';
-import createSerializer from '../../packages/react-native-web/jest/createSerializer';
-import { StyleSheet } from '../../packages/react-native-web/src';
-
-const serializer = createSerializer(StyleSheet);
+import serializer from './serializer';
 
 Enzyme.configure({ adapter: new Adapter() });
+
+// TODO: move off legacy serializer
 expect.addSnapshotSerializer(serializer);