Swap the rule, keep the class (Strategy and Factory)

The elif chain copied into three pages, the four rule cards that replace it, and the table that makes them.

Before you start. You need a few ideas from earlier in this track. A class is a blueprint for objects. A constructor is the code that runs once when an object is made, written __init__ in Python. Has-a beats is-a (composition over inheritance) showed an object keeping another object inside it and passing requests to it, so that part can be replaced later. Five letters, one shop (SOLID) gave the rule called open for extension and closed for editing: add new behaviour by adding code, not by changing code that is already tested. This lesson turns both into two named moves.

The idea in 30 seconds

A design pattern is a named, reusable shape for a problem programmers keep meeting, an idea Beck and Cunningham borrowed from the architect Christopher Alexander in 1987.
A long if/else chain on a “kind” is a sign that one job has several interchangeable rules inside it.
Strategy puts each rule in its own class behind one shared call, and the user of the rule holds whichever it is given.
An extension point is a place where new behaviour plugs in without editing old code, and the shared call is the extension point of a strategy.
A factory is the one place that turns a request such as the word “express” into the right object.
Dependency injection means passing the rule in from outside, so a test can hand in a fake one. Leave a two-branch if alone, because the patterns cost more lines than they save there.

Read the story

In this lesson

1987: a workshop paper about an architect’s idea

The pain: an if/else chain that only grows

The fix: a strategy, one class per rule

A new question: who picks the class?

Handing the rule in: dependency injection

Not every if is a problem

Terms it explains: design pattern, strategy, extension point, factory, dependency injection.

Index

Open everything on chalk.

Swap the rule, keep the class (Strategy and Factory)

In this lesson