First of all, thank you for your contribution! 

Please makes sure that these checkboxes are checked before submitting your PR, thank you!

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CheckList:

**if** *isBugFix* **:**

  * [ ] Make sure that you add at least one unit test for the bug which you had fixed.

**elif** *isNewFeature* **:**

  * [ ] Update API docs for the component.
  * [ ] Update/Add demo to demonstrate new feature.
  * [ ] Update test scripts for the feature.
  * [ ] Add unit tests for the feature.

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Notes: Weex will move into Apache Software Foundation (ASF) on Feb 24 2017.

Our new GitHub repo is https://github.com/apache/incubator-weex

After Feb 24 2017, we only accept pull requests from https://github.com/apache/incubator-weex

Thank you for your support.

----

注意：Weex 将于 2017-02-24 迁移至 Apache 基金会

届时我们会使用新的 GitHub 仓库：https://github.com/apache/incubator-weex 并在那里继续接受大家的 pull request。

更多详情请关注：https://github.com/weexteam/article/issues/130

感谢理解和支持

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It's ***RECOMMENDED*** to submit typo fix, new demo, tiny bugfix and large feature to `master` branch.
    
See [Branch Strategy](https://github.com/alibaba/weex/blob/dev/CONTRIBUTING.md#branch-management) for more detail.

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错别字修改、新 demo、较小的 bugfix、甚至较大的功能都可以直接提到 `master` 分支；

查看完整的[分支策略 (英文)](https://github.com/alibaba/weex/blob/dev/CONTRIBUTING.md#branch-management)。

-->

<!-- First of all, thank you for your contribution! 

All PRs should be submitted to master branch -->

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* If you are going to fix a bug of Weex, check whether it already exists in [Github Issue](https://github.com/apache/incubator-weex/issues). If it exists, make sure to write down the link to the corresponding Github issue in the PR you are going to create.
* If you are going to add a feature for weex, reference the following recommend procedure:
    1. Writing a email to [mailing list](https://weex.io/guide/contribute/how-to-contribute.html#mailing-list) to talk about what you'd like to do.
    1. Write the corresponding [document](https://weex.io/guide/contribute/how-to-contribute.html#contribute-code-or-document) -->


# Brief Description of the PR

# Checklist
* [] Demo
* [] Document

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