Is my understanding of Dart's Future correct?

Question

I'm learning Dart's Future, and have read some articles about the Future.

It says Dart is single-thread, and we can use Future to make some expensive functions run later, e.g. reading files.

Suppose reading a file will cost 10 seconds, and I have 3 files to read.

My dart code:

main() {
  readFile("aaa.txt");
  readFile("bbb.txt");
  readFile("ccc.txt");
  print("Will print the content of the files later");
}

readFile(String filename) {
  File file = new File(filename);
  file.readAsString().then((content) {
    print("File content:\n");
    print(content);
  });
}

Since reading a file will cost 10 seconds, so the above code will cost at least 30 seconds, right? Using futures to read files just to make the expensive tasks run later one by one, without blocking current code, but won't reduce the total cost?

If in java, I can make a thread pool, and make 3 future tasks running in parallel, the total cost will be between 10 and 20 seconds.

Is it possible to do the same in Dart? Is using Dart's isolate the only solution?

Answer 1

I would expect that this could take 10 seconds, as it will start three reads, each of which will queue an callback to the "then" function when the read is complete. It is entirely possible that the three files will load in parallel and all complete after 10 seconds. The callbacks will be called on the main thread sequentially though.

Although the user code in dart is single threaded (assuming you don't use isolates or web workers), nothing says that the implementation can't create threads or use the operating system's asynchronous loading to perform tasks in parallel as long as the future's run sequentially in the main thread.