Flutter Validation Chain of Responsibility: No If-Else
Stop nesting if-else blocks in your form validators. Build a chain-of-responsibility validation pipeline in Dart where each rule is its own class, ordering is explicit, and fail-fast vs accumulate-errors is a one-line switch.
Your signup form has six fields, each with three rules, and the validator for the email field looks like a staircase: if empty, else if bad format, else if too long, else if domain blocked… Then QA finds the expensive "is this email already registered?" network check runs *before* the cheap "is it even an email?" check, so every typo costs an API call. Then product asks for the same email rules on the invite-members screen, and you copy-paste the staircase. This is the exact mess the flutter validation chain of responsibility pattern dissolves: each rule becomes its own class, the rules link into an explicit pipeline, and ordering — including fail-fast versus accumulate-errors — becomes a visible, testable decision instead of an accident of nesting. What You'll Build By the end of this guide you will have: A reusable validator pipeline in Dart where each rule is a single-responsibility class A linked chain (not a nested conditional) that passes the value from rule to rule Fail-fast mode (stop at the first error) and accumulate-errors mode (collect every error) on a one-line switch Explicit ordering rules: cheap synchronous checks first, expensive async checks last Conditional validators that only run when relevant (business accounts, optional fields) Direct wiring into Flutter's TextFormField(validator:) with zero adapter code A decision table for when the chain earns its keep versus simpler options All code targets Dart 3.13 and Flutter 3.47 (stable versions verified October 2026). Before: Validator Chain vs Nested If-Else in Flutter Here's the baseline — the shape this guide replaces. It works, and that's the trap: String? validateEmail(String? value) { if (value == null || value.trim().isEmpty) { return 'Email is required'; } else if…