ioc(Inversion of Control)
ioc
--以上来自百度--
Inversion of Control
When these containers talk about how they are so useful because they implement "Inversion of Control" I end up very puzzled. Inversion of control(http://martinfowler.com/bliki/InversionOfControl.html) is a common characteristic of frameworks, so saying that these lightweight containers are special because they use inversion of control is like saying my car is special because it has wheels.
The question, is what aspect of control are they inverting? When I first ran into inversion of control, it was in the main control of a user interface. Early user interfaces were controlled by the application program. You would have a sequence of commands like "Enter name", "enter address"; your program would drive the prompts and pick up a response to each one. With graphical (or even screen based) UIs the UI framework would contain this main loop and your program instead provided event handlers for the various fields on the screen. The main control of the program was inverted, moved away from you to the framework.
For this new breed of containers the inversion is about how they lookup a plugin implementation. In my naive example the lister looked up the finder implementation by directly instantiating it. This stops the finder from being a plugin. The approach that these containers use is to ensure that any user of a plugin follows some convention that allows a separate assembler module to inject the implementation into the lister.
As a result I think we need a more specific name for this pattern. Inversion of Control is too generic a term, and thus people find it confusing. As a result with a lot of discussion with various IoC advocates we settled on the name Dependency Injection.
I'm going to start by talking about the various forms of dependency injection, but I'll point out now that that's not the only way of removing the dependency from the application class to the plugin implementation. The other pattern you can use to do this is Service Locator, and I'll discuss that after I'm done with explaining Dependency Injection.