[fix] fix preload weird behavior bug

closed[https://github.com/Chimeejs/chimee/issues/71](https://github.com/Chimeejs/chimee/issues/71)

Author: toxic-johann

__tests__/dispatcher/plugin.js

@@ -810,6 +810,13 @@ describe('dispatcher/plugin => $watch', () => {
     let index = 0;
     player.__dispatcher.videoConfig._realDomAttr.forEach(key => {
       if ([ 'src' ].indexOf(key) > -1) return;
+      if (key === 'preload') {
+        const origin = player[key];
+        player[key] = '123';
+        expect(fn).toHaveBeenCalledTimes(++index);
+        expect(fn).lastCalledWith('none', origin);
+        return;
+      }
       if (isBoolean(player[key])) {
         player[key] = !player[key];
         expect(fn).toHaveBeenCalledTimes(++index);

__tests__/index.js

@@ -316,8 +316,15 @@ describe('Chimee', () => {
       expect(player.preload).toBe('auto');
       expect(videoElement.preload).toBe('auto');
       player.preload = null;
-      expect(player.preload).toBe('');
-      expect(videoElement.preload).toBe('');
+      expect(player.preload).toBe('none');
+      expect(videoElement.preload).toBe('none');
+      player.preload = 'metadata';
+      expect(player.preload).toBe('metadata');
+      expect(videoElement.preload).toBe('metadata');
+      // need to run in browser
+      // player.preload = '';
+      // expect(player.preload).toBe('auto');
+      // expect(videoElement.preload).toBe('auto');
     });
     describe('width', () => {
       test('player set', () => {

doc/zh-cn/chimee-api.md

@@ -221,7 +221,7 @@ const chimee = new Chimee({
 | crossOrigin             | 是否跨域                           | boolean          | undefined   |                                          |
 | loop                    | 是否循环                           | boolean          | false       |                                          |
 | muted                   | 是否静音                           | boolean          | false       |                                          |
-| preload                 | 是否预加载                          | boolean          | undefined   |                                          |
+| preload                 | 是否预加载                          | string         | 'auto'  |                                          |
 | poster                  | 封面                             | string           | ''          |                                          |
 | playsInline             | 是否内联                           | boolean          | false       | 我们会为此添加 `playsinle webkit-playsinline x5-playsinline` |
 | xWebkitAirplay          | 是否添加 `x-webkit-airplay`        | boolean          | false       |                                          |
@@ -683,9 +683,10 @@ const player = new Chimee({
 
 ### preload
 
-- 类型：`string | void`
+- 类型：`string`
 - 含义：视频的预加载策略
-- 默认：`undefined`
+- 默认：`auto`
+- 可选项： `'auto'`, `'metadata'`, `'none'`, `''`
 
 ### poster
 

doc/zh-cn/plugin-api.md

@@ -765,9 +765,10 @@ const player = new Chimee({
 
 ### preload
 
-- 类型：`string | void`
+- 类型：`string`
 - 含义：视频的预加载策略
-- 默认：`undefined`
+- 默认：`auto`
+- 可选项： `'auto'`, `'metadata'`, `'none'`, `''`
 
 ### poster
 
@@ -1271,7 +1272,7 @@ player.$del(test, 'bar'); // {foo: 2}, {foo: 2}
 | crossOrigin             | 是否跨域                           | boolean          | undefined |                                          |
 | loop                    | 是否循环                           | boolean          | false     |                                          |
 | muted                   | 是否静音                           | boolean          | false     |                                          |
-| preload                 | 是否预加载                          | boolean          | auto      |                                          |
+| preload                 | 是否预加载                          | string         | auto      |                                          |
 | poster                  | 封面                             | string           | ''        |                                          |
 | playsInline             | 是否内联                           | boolean          | false     | 我们会为此添加 `playsinle webkit-playsinline x5-playsinline` |
 | xWebkitAirplay          | 是否添加 `x-webkit-airplay`        | boolean          | false     |                                          |

src/config/video.js

@@ -47,6 +47,8 @@ function accessorVideoAttribute(attribute: string | {set: string, get: string, i
       this.dom.setAttr('video', set, val);
       return value;
     },
+  }, {
+    preSet: false,
   });
 }
 
@@ -151,7 +153,16 @@ const accessorMap = {
     accessorVideoProperty('muted'),
   ],
   preload: [
-    accessor({ set: stringOrVoid }),
+    accessor({
+      set(value) {
+        const options = [ 'none', 'auto', 'metadata', '' ];
+        return options.indexOf(value) > -1
+          ? value
+          : 'none';
+      },
+    }, {
+      preSet: true,
+    }),
     accessorVideoAttribute('preload'),
   ],
   poster: [