Content provider

在ANDROID所有的应用程序数据(包括文件)是私有的。

ANDROID提供了一个标准的方式(对于本应用程序)去暴露他的私有数据给其他的application-通过contentprovider

不管你是否想要暴露你的application数据,你需要一种方式去存储。Preferences , Files , Databases , and Network .

 

You cannot share preferences across applications (except by using a content provider).

Each database is private to the application that creates it.


We do recommend including an autoincrement value key field that can be used as a unique ID to quickly find a record. This is not required for private data, but if you implement a content provider, you must include such a unique ID field.

 

contentprovider是唯一的方式去跨越application共享数据

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Android 为常见数据类型(音频,视频,图像,个人联系人信息,等等)装载了很多内容提供器。你可以看到在 android.provider 包里列举了一些。你还能查询这些提供器包含了什么数据(尽管,对某些提供器,你必须获取合适的权限来读取数据)。

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如果你想公开你自己的数据,你有两个选择:你可以创建你自己的内容提供器(一个 ContentProvider 子类)或者你可以给已有的提供器添加数据 - 如果存在一个控制同样类型数据的内容提供器且你拥有写的权限。

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你可以使用 ContentResolver.query() 方法或者 Activity.managedQuery() 方法来查询一个内容提供器。两种方法使用相同的参数序列,而且都返回一个 Cursor 对象。不过, managedQuery() 使得活动需要管理这个游标的生命周期。 一个被管理的游标处理所有的细节,比如当活动暂停时卸载自身,而活动重新启动时重新查询它自己。你可以让一个活动开始管理一个尚未被管理的游标对象,通过如下调用: Activity.startManagingCursor()

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如果一个查询可能返回二进制数据,比如一个图像或声音,这个数据可能直接被输入到表格或表格条目中也可能是一个 content: URI 的字符串可用来获取这个数据,一般而言,较小的数据(例如, 20 50K 或更小)最可能被直接存放到表格中,可以通过调用 Cursor.getBlob() 来获取。 它返回一个字节数组。

如果这个表格条目是一个 content: URI ,你不该试图直接打开和读取该文件(会因为权限问题而失败)。相反,你应该调用 ContentResolver.openInputStream() 来得到一个 InputStream 对象,你可以使用它来读取数据。

 

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这些内容提供器的方法可以从不同的进程和线程的各个 ContentResolver 对象中调用,所以它们必须以线程安全的方式来实现。

周到起见,当数据被修改时,你可能还需要调用 ContentResolver.notifyChange() 方法来通知侦听者。


 

 

 

 

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