Chapter 15 — Asynchronous Programming: Future, async, await

Level: intermediate Estimated duration: 6 h Prerequisites: Chapter 13 — Exceptions and Error Handling, Chapter 14 — Functional Programming on Collections What you will be able to do by the end: write a program that waits for a long-running result (loading, network, file) without ever blocking the rest of the application, using Future, async, await, Future.wait(), and Stream.

15.0 — Chapter Objectives

By the end of this chapter, you will be able to:

• explain why a long-running operation must never block the program;
• distinguish synchronous processing from asynchronous processing;
• explain what Dart's event loop is and why Dart is single-threaded;
• predict the actual output order of an asynchronous program;
• define a Future and name its three states;
• create a Future with Future.delayed(), Future.value(), and Future.error();
• use .then(), .catchError(), and .whenComplete();
• write an async function and use await;
• understand why an async function always returns a Future;
• catch an asynchronous error with try / catch / finally;
• recognize needlessly sequential await calls and parallelize them with Future.wait();
• use unawaited for a future that is intentionally not awaited;
• write an asynchronous main();
• explain the difference between a Future and a Stream;
• produce a Stream with async* and yield;
• consume a Stream with await for and with listen();
• transform a Stream with map and where;
• explain why Flutter relies entirely on asynchronous programming.