Humble Object pattern
A pattern for testing hard-to-test behavior
Basic idea
- Problem: some behavior (like how data is represented on a screen) is hard to test
- Solution: split the behavior into two parts
- Humble Object containing the hard-to-test stuff, stripped down to the bare essence
- Other part which contains everything stripped from the Humble Object
Example: showing data in a UI
- Create a view model that describes as much as possible about how the data will be shown
- Dates already converted to correct format
- Flags describing if elements should be disabled, hidden, ...
- Humble Object is a view which does nothing more than showing contents of view model on the screen
Values as boundaries
See this talk: Boundaries
Same idea as Humble Object pattern, but taken to a more architectural level:
- Core: all decision logic sits here, no mutation, input and output are just values (see below)
- Pure functions: output depends on input only, not on some other kind of state
- Very easy and efficient to unit test
- Shell: layer around the core, holds dependencies, takes care of interaction with the outside world (DB, web, screen, keyboard, ...)
- Contains the slow code that can fail
- Hard to test, but there shouldn't be much to test (a few integration or end-to-end tests should give you all the confidence you need)
- System can consist of multiple shells (each with their own core) that communicate with each other
The definition of a value in this context: anything immutable
- Could be a simple integer
- Could be an object with immutable state, plus methods whose result depends on that state (and thus doesn't change either)
- Every mutating operation should return a new object rather than mutating an existing object's state
- Example: Java's String class
Additional benefit of these values (in addition to testability): values like these are easy to pass as messages. This way, values can be used as boundaries between classes, subsystems, processes and even different machines.
Resources
- Clean Architecture (book by Robert C. Martin)
- TDD Patterns: Humble Object