update react refs to use callback instead of strings

src/Calendar.jsx

@@ -227,7 +227,6 @@ const Calendar = createReactClass({
     }) : null;
     const dateInputElement = props.showDateInput ? (
       <DateInput
-        ref="dateInput"
         format={this.getFormat()}
         key="date-input"
         value={value}

src/RangeCalendar.js

@@ -473,7 +473,7 @@ const RangeCalendar = createReactClass({
             nextMonthOfStart.month() === endValue.month();
     return (
       <div
-        ref="root"
+        ref={this.saveRoot}
         className={classes}
         style={props.style}
         tabIndex="0"

src/date/DateInput.js

@@ -96,7 +96,13 @@ const DateInput = createReactClass({
   },
 
   focus() {
-    this.refs.dateInput.focus();
+    if (this.dateInputInstance) {
+      this.dateInputInstance.focus();
+    }
+  },
+
+  saveDateInput(dateInput) {
+    this.dateInputInstance = dateInput;
   },
 
   render() {
@@ -107,7 +113,7 @@ const DateInput = createReactClass({
     return (<div className={`${prefixCls}-input-wrap`}>
       <div className={`${prefixCls}-date-input-wrap`}>
         <input
-          ref="dateInput"
+          ref={this.saveDateInput}
           className={`${prefixCls}-input ${invalidClass}`}
           value={str}
           disabled={props.disabled}

src/mixin/CalendarMixin.js

@@ -79,7 +79,7 @@ const CalendarMixin = {
 
     return (
       <div
-        ref="root"
+        ref={this.saveRoot}
         className={`${classnames(className)}`}
         style={this.props.style}
         tabIndex="0"

src/mixin/CommonMixin.js

@@ -53,8 +53,12 @@ export default {
   },
 
   focus() {
-    if (this.refs.root) {
-      this.refs.root.focus();
+    if (this.rootInstance) {
+      this.rootInstance.focus();
     }
   },
+
+  saveRoot(root) {
+    this.rootInstance = root;
+  },
 };