You have a class. This class uses some other class. But by using or creating an instance of this other class directly you create a dependency on something concrete. That’s not what you want, because you might want to replace this with something else if required. For example with a mock or fake implementation in a test. ‚Dependency injection‘ allows you to declare this dependency with just an interface/protocol and have some other facility (the dependency injection framework) ‚inject‘ a concrete object at run-time. A similar thing could certainly be done by just using a constructor parameter (strategy pattern). But I think the important part here is the dependency on just an interface and not on a concrete implementation. For flexibility.
Manfred > Am 06.02.2021 um 22:27 schrieb Marco Antoniotti <marco.antonio...@unimib.it>: > > I know I could look it up in Wikipedia, but posing the question here may > probably generate more amusement. > > WTF is a “dependency injection”? > > MA > > > On Sat, 6 Feb 2021 at 22:18, Scott McKay <swmc...@gmail.com> wrote: > > Nobody in the list community ever invented a fancy pants term like > “dependency injection” > because it’s so obvious how to do this that nobody thought to give it a name. > > —Scott > > > On Feb 6, 2021, at 4:07 PM, Manfred Bergmann <manfred.bergm...@me.com> > > wrote: > > > > > > > >> Am 06.02.2021 um 21:44 schrieb Luís Oliveira <luis...@gmail.com>: > >> > >>> On Sat, 6 Feb 2021 at 20:07, Rudi Araújo <rudi.ara...@gmail.com> wrote: > >>> Class::newInstance() doesn't have any parameters (also, it's deprecated: > >>> better to use getConstructor() or getDeclaredConstructor() and call > >>> newInstance() on it). > >> > >> I guess this bit about getConstructor() explains why it'd be more > >> convenient to use a Factory, or the Factory method pattern, or some > >> dependency injection framework. > >> > > > > Yeah. Could be. > > But this constructor thingy could be hidden in a function similar as you > > would create a constructor function make-foo in Common Lisp. > > The reflection stuff is not considered a good practice in certain types of > > applications. > > > > Dependency injection is about something else IMO. Well, Abstract Factory is > > about it, too, inversion of control. > > It allows you to create something without having to know the concrete type > > and without having to have a source dependency on it. > > In Common Lisp this could be solved easily by just separating a protocol > > from the implementation, maybe in separate packages. > > > > > > > > Manfred > > -- > Marco Antoniotti, Associate Professor tel. +39 - 02 64 48 79 > 01 > DISCo, Università Milano Bicocca U14 2043 > http://bimib.disco.unimib.it > Viale Sarca 336 > I-20126 Milan (MI) ITALY
