Strategy Pattern Flutter: Kill the If-Else Chain
A nested if-else chain for payments and shipping is a bug waiting to happen. Here's how the strategy pattern kills it — with Dart 3 sealed classes for exhaustiveness.
*Last updated: October 8, 2026.* It's 2 a.m. and checkout is broken again. Somewhere in a 400-line file, fourteen nested ifs decide how each payment method charges the customer — card, wallet, cash on delivery — and the new "buy now, pay later" option just fell through every branch silently. The strategy pattern flutter if else replacement exists precisely for this moment: instead of asking "which branch am I in?" everywhere, you turn each behavior into its own class and let the caller pick one. In this guide you'll do exactly that in Dart 3, first with classic abstract classes, then with sealed classes and exhaustive switches so the compiler itself refuses to let a branch go missing. What you'll learn How to spot the if-else chain that deserves the strategy pattern (and the ones that don't) The classic strategy structure: interface, concrete strategies, context The Dart 3 upgrade: sealed classes + exhaustive switch expressions Wiring strategies into a real Flutter checkout screen A decision table for strategy vs sealed switch vs plain if-else 4 mistakes that turn strategies into a new kind of spaghetti All code targets Dart 3.13 / Flutter 3.47 (stable, October 2026) and runs as plain Dart. The checkout file that breaks at 2 a.m. Here's the code that causes the 2 a.m. incident. Every new payment method edits the same function, and every edit risks every other method: Future<void> processPayment(String method, double amount) async { if (method == 'card') { // 20 lines of card validation + charging print('Charged \$amount to card'); } else if (method == 'wallet') { // 20 lines of wallet debiting print('Debited \$amount from wallet'); } else if (method == 'cod') { // 20 lines of cash-on-delivery…