Use defer in Swift to close the current context's scope

When writing Swift code, it’s helpful to ensure that clean-up tasks are executed even in the presence of errors or early returns. One powerful tool for achieving this is the defer statement.

The defer statement allows you to defer the execution of a block of code until the current scope exits. This can be incredibly useful for tasks like resource cleanup, closing files, or releasing memory.

Here’s a simple example to illustrate the usage of defer:

func updateData() {
    // show loading indicator to reflect loading state in UI
    loadingIndicator.isHidden = false

    // hide loading indicator regardless of how we exit this function
    defer {
        loadingIndicator.isHidden = true
    }

    // load data from server
    (...)
}

In this example, the loading indicator will be hidden from the UI regardless of how the following implementation of the function exits.

Here are a few key points to remember when using defer:

1. Execution Order: Code within defer blocks is executed in the reverse order of their appearance in the source code. The last defer block is executed first, and so on.
2. Scope Boundaries: The deferred code is executed when the current scope is exited, whether that’s due to returning from a function, throwing an error, or exiting a loop or conditional statement.
3. Resource Management: defer is commonly used for resource management tasks like closing files, releasing locks, or cleaning up temporary data.
4. Error Handling: defer is particularly useful in conjunction with error handling mechanisms like do-try-catch. You can defer clean-up tasks to ensure that resources are properly released even if an error is thrown.

By using defer effectively, you can ensure that your Swift code remains clean, concise, and robust in the face of unexpected situations.