全部博文（135）
2010年（135）
分类： Java
2010-04-13 20:32:34
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| What is the difference between a Bundle and a Parcel and a Message? What are the interactions between each. A detailed explanation would go a long way to explaining the whole of the android.os package for me. |
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| Hello! A Bundle is functionally equivalent to a standard Map. The reason we didn't just use a Map is because in the contexts where Bundle is used, the only things that are legal to put into it are primitives like Strings, ints, and so on. Because the standard Map API lets you insert arbitrary Objects, this would allow developers to put data into the Map that the system can't actually support, which would lead to weird, non-intuitive application errors. Bundle was created to replace Map with a typesafe container that makes it explicitly clear that it only supports primitives. A Parcel is similar to a Bundle, but is more sophisticated and can support more complex serialization of classes. Applications can implement the Parcelable interface to define application-specific classes that can be passed around, particularly when using Services. Parcelables can be more sophisticated than Bundles, but this comes at a cost of significantly higher overhead. Bundle and Parcel are both data serialization mechanisms, and for the most part both are used when application code is passing data across processes. However, because Parcel is much higher overhead that Bundle, Bundles are used in the more common places like the onCreate method, where overhead must be as low as possible. Parcels are most commonly used to allow applications to define Services with logical APIs that can use application-meaningful classes as method arguments and return values. If we required Bundle there, it would result in really clunky APIs. You should in general still keep your Service APIs as simple as possible, because primitives will serialize more efficiently than custom Parcelable classes. The Message class is very different. A Message is really just a way for an application to send messages between threads, and is not related to data serialization. Since accesses to Views must be single-threaded, the MessageHandler and related Message classes are commonly used to make it easier to pass messages between threads. Hope that helps! - Dan |
