[add] jest snapshot serializer

Flatten style objects in snapshots

jest/serializer.js

@@ -0,0 +1,61 @@
+const React = require('react');
+const { StyleSheet } = require('../dist');
+
+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);
+createSerializer.test = serializer.test;
+createSerializer.print = serializer.print;
+
+module.exports = createSerializer;