Dependency Injection ("DI") and the IEclipseContext
DI (aka 'Inversion of Concerns") is a recent development in development. It's based on the premise that it's better for a component to describe what it needs and having those requirements met through the use of an appropriate 'context' Rather than requiring the component to rely on external frameworks through which it can satisfy its own requirements.
In order for the component to expose its needs it uses a set of java annotations, some generic (@Inject, @PostConstruct...) as well as domain (e4) specific (@Focus, @CanExecute...).
In order to be useful there are a number of things that must be possible:
- Create a new instance of a class given the class spec and a context
- Inject an existing instance with the values in a context
- Find and execute specifically annotated methods within an existing instance
The Eclipse Context
In order for DI to work there must be some container from which injectable values can be retrieved. In e4 this is the IEclipseContext. In reality these contexts are defined in a tree and the values are retrieved using a lookup strategy (1. local value, 2. result if it's an IContextFunction, 3. delegate to parent).
The package org.eclipse.e4.core.contexts provides the API to make, inject or uninject or to invoke specifically annotated methods on an Object.
|Active||This defines an injection annotation that we don'd appear to use and should be deleted|
|ContextFunction||Defines an abstract class with a single abstract method 'compute' that is called whenever the value is retrieved from an IEclipseContext.|
|ContextInjectionFactory||Defines the 'inject', 'make' and 'uninject' methods used to manipulate objects through injection. It also contains a 'setDefault' method that should not be API.|
|IContextFunction||An interface containing a single method 'compute'. ContextFuncton implements this interface but since we expect that clients will be implementing instances of this we should likely remove (deprecate?) this interface in favor of supplying the base class.|
|IEclipseContext|| The interface through which context information is accessed. There are far many methods here...are they all really needed ?
modify(*)/declareModifiable(*) - it looks like it can be used to set a value up a chain of contexts to the parent chain. The other methods all have uses in the SDK
An abstract class containing a single abstract mehtod 'changed' and a utility method 'runExternalCode' (should this be final?)
The package org.eclipse.e4.core.di.annotations contains a variety of generally useful anotations:
- CanExecute - commands/handlers are headless
- Execute - commands/handlers are headless
We should make org.eclipse.e4.core.di.suppliers provisional for now.
- This section is used to provide extra annotations used for injection or invocation, like org.eclipse.e4.core.di.extensions.Preference Pwebster.ca.ibm.com
The package org.eclipse.e4.ul.di contains a variety of ui specific annotations:
- PersistState (How is this different from 'Persist' ?) It saves the UI state of a Part, Persists works together with Dirty to save the data model Lars.Vogel@gmail.com
- UISynchronize (How is this used?) It is used as a replacement for Display.asyncExec and Display.asyncExec Lars.Vogel@gmail.com
We may want to add some other life-cycle annotations here as well but they needn't be declared API for Kepler