android-contentresolver - 内容解析器的使用

Question

我是安卓领域的新手，正处于学习阶段。我有几个疑问:

每个应用程序是否有一个 ContentResolver 对象？
它是一个单例对象吗？
谁管理这个对象生命周期？
如果是单例，它如何处理多个查询 ContentProvider 的请求？

Answer 1

来自 Alex Lockwood 的博客 -
http://www.androiddesignpatterns.com/2012/06/content-resolvers-and-content-providers.html

What is the Content Resolver?

The Content Resolver is the single, global instance in your application that provides access to your (and other applications') content providers. The Content Resolver behaves exactly as its name implies: it accepts requests from clients, and resolves these requests by directing them to the content provider with a distinct authority. To do this, the Content Resolver stores a mapping from authorities to Content Providers. This design is important, as it allows a simple and secure means of accessing other applications' Content Providers.

The Content Resolver includes the CRUD (create, read, update, delete) methods corresponding to the abstract methods (insert, delete, query, update) in the Content Provider class. The Content Resolver does not know the implementation of the Content Providers it is interacting with (nor does it need to know); each method is passed an URI that specifies the Content Provider to interact with.

What is a Content Provider?

Whereas the Content Resolver provides an abstraction from the application's Content Providers, Content Providers provides an abstraction from the underlying data source (i.e. a SQLite database). They provide mechanisms for defining data security (i.e. by enforcing read/write permissions) and offer a standard interface that connects data in one process with code running in another process.

Content Providers provide an interface for publishing and consuming data, based around a simple URI addressing model using the content:// schema. They enable you to decouble your application layers from the underlying data layers, making your application data-source agnostic by abstracting the underlying data source.

The Life of a Query

So what exactly is the step-by-step process behind a simple query? As described above, when you query data from your database via the content provider, you don't communicate with the provider directly. Instead, you use the Content Resolver object to communicate with the provider. The specific sequence of events that occurs when a query is made is given below:

• A call to getContentResolver().query(Uri, String, String, String, String) is made. The call invokes the Content Resolver's query
  method, not the ContentProvider's.

• When the query method is invoked, the Content Resolver parses the uri argument and extracts its authority.

• The Content Resolver directs the request to the content provider registered with the (unique) authority. This is done by calling the
  Content Provider's query method.

• When the Content Provider's query method is invoked, the query is performed and a Cursor is returned (or an exception is thrown). The
  resulting behavior depends entirely on the Content Provider's
  implementation.