Last updated at 2023-07-10

How to Write the Content of Data to a File in Swift

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Jul 10, 2023 04:22 PM
Jul 4, 2023
This article is an alternative on how to write the content of String to a file in Swift. If you haven’t yet read that post, you may want to read it, as it gives you the same idea of how to write to a file.
Writing data to a file has many use cases, such as if you are saving sessions or trying to cache the data for future use. There are some cases when you have large amounts of data, but you just show a window of it, so you don’t need all of it to bloat your memory. Although memory has paging, you may need to write that data to ensure availability or for your own use case.
Regardless of the use case, Swift already provides APIs for writing data to a file on disk. This is how.
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Just like the String tutorial counterpart, you need nothing to be installed because the API is already provided by the Foundation module. All you need to do is import that module, and you are good to go to use the API.
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Write to a File using Swift

To write to a file in Swift, you can use the write method in Data. The write method can be used to write content to a file synchronously. Here's an example:
import Foundation print("Start Write") let content = "My name is Mozzlog".data(using: .utf8)! let url = URL(filePath: "file.txt") do { try content.write(to: url) print("End Write") } catch { print("Error: \\(error)") }
Just like in String.write, when you use Data.write you will overwrite the existing file if it exists or create a new one if it does not.
The code above is almost identical to how you write a content of String to a file in Swift. You use a similar write method to write it. But there are some differences with String.write method as follows:
  1. The Data.write method requires you to pass a URL instance instead of String instance
  1. The Data.write API does not pass String filepath

Write to a File using Swift Asynchronously

Well, like in the String write to file tutorial, you will use DispatchQueue because the write API is only available as a synchronous API. Foundation doesn’t provide you with an asynchronous write API.
It is easy to implement
import Foundation func writeFileAsync() { print("Start Write") let content = "My name is Mozzlog" let fileURL = URL(fileURLWithPath: "file.txt") DispatchQueue.global(qos: .background).async { do { try content.write(to: fileURL, atomically: true, encoding: .utf8) DispatchQueue.main.async { print("Finish Write") print("Continue Working") } } catch { print("Error: \\(error)") } } }
The above code will produce output in the log like this:
Start Write Finish Write Continue Working
Actually, you don’t need to write in DispatchQueue.global(qos:.background)because as long as you call the async method from a dispatch queue, then it will be executed in a concurrent manner.
Writing the file as a background task allows you to block the current thread so the program can do a much more meaningful task. Your program doesn’t need to wait for the writing logic to finish before continuing with the next task.
Dispatching the write operation in the background queue and calling the callback ensures your file writing is done asynchronously.


Using the Foundation framework's file system capabilities, it is straightforward to write synchronously or asynchronously to a file in Swift. This enables you to efficiently manage file operations and maintain a seamless user experience in your applications.
Swift already provides the API necessary for synchronous and asynchronous writing. Every API has unique advantages and considerations. Asynchronous programming allows you to optimize the performance of your program, whereas synchronous programming allows you to design your code from the top down.
Happy Writing Your File!

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