An explanation of the Chain of Responsibility behavioral design pattern, showing how it decouples complex business rules by breaking them into independent, single-responsibility handlers linked in a chain. Two worked Dart examples illustrate the pattern: a fintech transaction approval flow (fraud, KYC, account status, approval tier checks) and a user onboarding validation flow (email, password, age, duplicate account checks). Includes full code, handler interfaces, chain construction, and sample output, along with guidance on when the pattern fits and when it doesn't.
Table of contents
Table of ContentsWhat is the Chain of Responsibility Pattern?The Problem It SolvesCore ComponentsReal World Example One: Transaction Approval FlowReal World Example Two: User Onboarding ValidationWhat Makes These Two Examples Interesting TogetherWhen to Use the Chain of Responsibility PatternWhen Not to Use ItConclusionQuestions this post answers
When should I use the chain of responsibility pattern instead of a single validation function?
Use it when a request must pass through multiple independent checks whose number or order may change over time, and when each check should be independently testable. It is not worth the overhead for only one or two checks, when processing order is fixed forever, when handlers need to share results with each other, or when every handler must always run regardless of earlier outcomes. daily.dev surfaces pattern comparisons like this for developers deciding how to structure validation logic.
How do you avoid a null pointer crash at the end of a chain of responsibility implementation?
Declare the reference to the next handler as nullable and check whether it is null before calling it, rather than assuming a next handler always exists. In a Dart implementation, the base handler class stores a nullable `_next` field and a `passToNext` helper method checks `if (_next != null)` before invoking `_next!.handle(request)`, logging or silently stopping when the chain ends. developers implementing handler chains in dart can reference concrete examples like this on daily.dev.