writeToFile how to tell when it is completed

The method writeToFile:atomically is "synchronous". It will write the file and then return YES or NO depending on wether the file was successfully written or not.

That means that the operation is complete as soon as the method returns.

BOOL success = [imageData writeToFile:fullPathToFile atomically:YES];
// Now, the operation is complete
// success indicates whether the file was successfully written or not

Swift 5:

do {
    try data.write(to: path)
    // here's when write operation is completed without errors thrown
} catch {
    Swift.print(error)
}