feat: add mixin for form.

components/form/ValueMixin.jsx

@@ -0,0 +1,32 @@
+function merge() {
+  const ret = {};
+  const args = [].slice.call(arguments, 0);
+  args.forEach((a)=> {
+    Object.keys(a).forEach((k)=> {
+      ret[k] = a[k];
+    });
+  });
+  return ret;
+}
+
+const ValueMixin = {
+  setValue(field, e) {
+    let v = e;
+    const target = e && e.target;
+    if (target) {
+      if ((target.nodeName + '').toLowerCase() === 'input' &&
+        target.type === 'checkbox') {
+        v = target.checked;
+      } else {
+        v = e.target.value;
+      }
+    }
+    const newFormData = {};
+    newFormData[field] = v;
+    this.setState({
+      formData: merge(this.state.formData, newFormData),
+    });
+  },
+};
+
+export default ValueMixin;