bitmap vm
http://pernghh.pixnet.net/blog/post/35164141-%E8%A7%A3%E6%B1%BA%E5%9C%A8-android-%E4%B8%AD%E4%BD%BF%E7%94%A8-bitmap-%E9%80%A0%E6%88%90-out-of-memory-%E7%9A%84
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http://www.android123.com.cn/kaifafaq/594.html? 临时空间
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http://www.cnblogs.com/TerryBlog/archive/2010/08/04/1791934.html? 弱引用imageview
