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From Marcel Offermans <marcel.offerm...@luminis.nl>
Subject Re: DependencyManager - spirit behind init,start,stop,destroy
Date Sun, 11 Nov 2012 14:33:36 GMT
Hello Bokie,

On Nov 11, 2012, at 13:40 PM, bokie <jms.cercas@gmail.com> wrote:

> When working with the DependencyManager api, there's always a little voice
> saying: where do I put this, in the init or the start, in the stop or in the
> destroy, is it really that important - so, what is the spirit behind these
> methods, what should we consider when answering that little voice :)

There is a difference, but it will depend on your use case if this actually matters.

I will try to explain this based on the "init" and "start" callbacks. The init callback is
always invoked first, and as an optional parameter you can get a reference to your own Component
(the one you created in the bundle activator). What this method allows you to do at this point
in the life cycle is to add extra dependencies dynamically. The use case for this could for
example be a component that has a dependency on some configuration. Let's say that in this
configuration there is a parameter that determines if this component should depend on another
service or not. In the init, you can be sure that all required dependencies are already injected,
so you can read your configuration. Based on what you find, you can then add an extra dependency
to your component. Now two things can happen: this extra dependency is available (or optional).
In this case the component will continue initialization and the "start" will be invoked (and
there you can rely on this extra dependency to also be injected). If the new dependency is
not available, the component will "wait" until it is. This is also why there is a flag called
"instance bound" for a dependency. You should use it here. It will ensure that this new dependency,
if not available, will not immediately destroy the component again as it normally would.

Long story, the take away is that in most cases, you can just use "init" or "start", but if
you have dynamic components, both methods play a specific role in that process.

Greetings, Marcel


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