Mobile Application Development class 008
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This document discusses using async and await in Dart for asynchronous programming. It explains that async and await allow Dart functions to interact with asynchronous operations by suspending function execution to wait for an asynchronous task to complete. An example demonstrates defining an async function that uses await to call another asynchronous function and wait for its future value before returning. Key terms defined include async, async function, await, and future.
Mobile Application Development class 008
- 1. Mobile Application Development class 08 Dr. Mazin Alkathiri IT Department Seiyun University 2023-2024
- 2. Using await async in Dart • The async and await approaches in Dart are very similar to other languages, which makes it a comfortable topic to grasp for those who have used this pattern before. However, even if you don’t have experience with asynchronous programming using async/await, you should find it easy to learn in dart.
- 3. Why asynchronous code matters Asynchronous operations let your program complete work while waiting for another operation to finish. Here are some common asynchronous operations: • Fetching data over a network. • Writing to a database. • Reading data from a file. Such asynchronous computations usually provide their result as a Future or, if the result has multiple parts, as a Stream. These computations introduce asynchrony into a program. To accommodate that initial asynchrony, other plain Dart functions also need to become asynchronous.
- 4. • To interact with these asynchronous results, you can use the async and await keywords. Most asynchronous functions are just async Dart functions that depend, possibly deep down, on an inherently asynchronous computation.
- 5. Example: Incorrectly using an asynchronous function
- 6. Here’s why the example fails to print the value that fetchUserOrder() eventually produces: • fetchUserOrder() is an asynchronous function that, after a delay, provides a string that describes the user’s order: a “Large Latte”. • To get the user’s order, createOrderMessage() should call fetchUserOrder() and wait for it to finish. Because createOrderMessage() does not wait for fetchUserOrder() to finish, createOrderMessage() fails to get the string value that fetchUserOrder() eventually provides. • Instead, createOrderMessage() gets a representation of pending work to be done: an uncompleted future. You’ll learn more about futures in the next section. • Because createOrderMessage() fails to get the value describing the user’s order, the example fails to print “Large Latte” to the console, and instead prints “Your order is: Instance of ‘_Future<String>’”.
- 7. Key terms: • synchronous operation: A synchronous operation blocks other operations from executing until it completes. • synchronous function: A synchronous function only performs synchronous operations. • asynchronous operation: Once initiated, an asynchronous operation allows other operations to execute before it completes. • asynchronous function: An asynchronous function performs at least one asynchronous operation and can also perform synchronous operations.
- 8. What is a future? • A future (lower case “f”) is an instance of the Future (capitalized “F”) class. A future represents the result of an asynchronous operation, and can have two states: uncompleted or completed. • Uncompleted – When you call an asynchronous function, it returns an uncompleted future. That future is waiting for the function’s asynchronous operation to finish or to throw an error. • Completed – If the asynchronous operation succeeds, the future completes with a value. Otherwise, it completes with an error.
- 9. Completing with a value • A future of type Future<T> completes with a value of type T. For example, a future with type Future<String> produces a string value. If a future doesn’t produce a usable value, then the future’s type is Future<void>. • Completing with an error • If the asynchronous operation performed by the function fails for any reason, the future completes with an error.
- 10. Example: Introducing futures • In the following example, fetchUserOrder() returns a future that completes after printing to the console. Because it doesn’t return a usable value, fetchUserOrder() has the type Future<void>. Before you run the example, try to predict which will print first: “Large Latte” or “Fetching user order…”.
- 11. Example: Completing with an error • Run the following example to see how a future completes with an error. A bit later you’ll learn how to handle the error. • In this example, fetchUserOrder() completes with an error indicating that the user ID is invalid.
- 12. Working with futures: async and await • The async and await keywords provide a declarative way to define asynchronous functions and use their results. Remember these two basic guidelines when using async and await: • To define an async function, add async before the function body: • The await keyword works only in async functions.
- 13. • Here’s an example that converts main() from a synchronous to asynchronous function. – First, add the async keyword before the function body: • void main() async { ··· } – If the function has a declared return type, then update the type to be Future<T>, where T is the type of the value that the function returns. If the function doesn’t explicitly return a value, then the return type is Future<void>: • Future<void> main() async { ··· } – Now that you have an async function, you can use the await keyword to wait for a future to complete: • print(await createOrderMessage());
- 14. Future<String> createOrderMessage() async { var order = await fetchUserOrder(); return 'Your order is: $order'; } Future<String> fetchUserOrder() => // Imagine that this function is // more complex and slow. Future.delayed( const Duration(seconds: 2), () => 'Large Latte', ); Future<void> main() async { print('Fetching user order...'); print(await createOrderMessage()); } String createOrderMessage() { var order = fetchUserOrder(); return 'Your order is: $order'; } Future<String> fetchUserOrder() => // Imagine that this function is // more complex and slow. Future.delayed( const Duration(seconds: 2), () => 'Large Latte', ); void main() { print('Fetching user order...'); print(createOrderMessage()); } The asynchronous example is different in three ways: • The return type for createOrderMessage() changes from String to Future<String>. • The async keyword appears before the function bodies for createOrderMessage() and main(). • The await keyword appears before calling the asynchronous functions fetchUserOrder() and createOrderMessage().
- 15. Key terms: • async: You can use the async keyword before a function’s body to mark it as asynchronous. • async function: An async function is a function labeled with the async keyword. • await: You can use the await keyword to get the completed result of an asynchronous expression. The await keyword only works within an async function. • Execution flow with async and await – An async function runs synchronously until the first await keyword. This means that within an async function body, all synchronous code before the first await keyword executes immediately.