I have a love/hate relationship with Design Patterns, which is actually captured in the first two sentences of the Wikipedia article I just linked to.
I love them because they provide a general reusable solution–a rule of thumb, so to speak–on how to solve some fairly common problems.
For example, if you are developing a user interface application, then the Model-View-Controller pattern or the Model-View-Presenter pattern is definitely a lifesaver in helping you design a well-organized application.
(The difference between the two have to do with the ability of view components having direct access to the model data: MVC allows the connection as a rule of thumb, while MVP denies it. For most applications–such as simple iOS applications which access remote data from a database and present the results on a screen–strictly separating views from the model object makes a tremendous amount of sense. However, there are certain types of applications where the user interaction of the view with the data is so involved, you need to allow the view insight into the model data directly. Usually this is reserved for complex editing applications, like a text editor or a CAD system, where the logic for handling key or mouse events makes more sense to contain in a text editor view or a CAD drawing view than allow the raw events to percolate to an already over-loaded controller module.)
And having a good working knowledge of various design patterns helps you have a standard bag of tricks to apply to problems you may see in the field: singletons, proxies and modules are excellent ways to handle organization of conceptual functions. Flyweights are helpful when creating a complex page of preferences in a dialog. Object pools help with reducing the overhead of allocation. And so forth.
I hate them, though.
And I hate them for the very simple reason that to an inexperienced developer equipped with a hammer, the entire world becomes full of nails.
Design patterns are, as defined in Wikipedia, “general reusable solutions to commonly occurring problems.”
And there are three problems with that.
First, it requires some degree of understanding of the solution and how to apply it. If you don’t understand the ‘chain of responsibility’ pattern, for example, then you can really shoot yourself in the foot if you implement it poorly–as I’ve seen done on a number of occasions, with events in a custom event chain that just “disappeared”.
Second, it requires some degree of understanding what the problem was the solution is attempting to solve. One developer I had a conversation with a long time ago insisted on using a decorator pattern to allow a view to both have presentation and editing functionality, when either a flyweight or simple inheritance could have solved the problem far more gracefully.
And third, many developers labor under the presumption that these are the only solutions–that there are no other solutions out there. The whole world becomes full of nothing but nails, and by God if my hammer doesn’t work the problem is not solvable by us “mere mortals.”
To me, design patterns are just solutions to common problems. There are other potential solutions to other problems not quite as common: for example, I once wrote a networking stack for a small microprocessor by creating a state machine: a large switch statement where each ‘case’ ended by changing the variable controlling the state machine to the next ‘case’ that was to be executed.
Emulating multi-threading was achieved by having each network connection hold its own state–and on exit of each case, a pointer pointed to the next active network object and picked up with whatever state that network object was in.
(I’ve successfully used state machines to solve a number of esoteric problems, including building my own LISP interpreter, and my own virtual machine interpreter. And state machines are built by YACC for building a parser; they’re far more flexible than recursive-descent parsers.)
And there are many solutions I’ve come across which solve common problems related to multi-threading that I’ve yet to see written down as an “approved” Design Pattern, such as the solution I provide here, to solve the problem of anonymous inner blocks which may be deallocated by a user action prior to the completion of an asynchronous task.
Principally I hate design patterns for the same reason I hate orthodoxy: it’s just an excuse to stop thinking now that all of the approved answers have been derived for us.