How do I write to a local file in Swift/XCTest?

Question

My ultimate question is about saving a screenshot from an AppleTV application using XCTest and Swift4 (running on a MacBook paired to the TV device), but I'm having trouble even writing a simple text string to a local file. If I can get this simple file-save working, I'm hoping I can resolve the screenshot issue. (Apologies for making this look like two questions but they appear to be related and resulted from my troubleshooting efforts.)

First, here's what I'm trying to do with a screenshot, based on sample code I found somewhere online:

let appshot = XCUIApplication().windows.firstMatch.screenshot()
let shotpath = FileManager.default.urls(for: .downloadsDirectory, in: .userDomainMask)[0].appendingPathComponent("appshot.png")
let shotpathUrl = URL(string: "file://\(shotpath)")
print("Saving to: \(shotpath)")

do {
    try appshot.pngRepresentation.write(to: shotpathUrl!)
} catch {
    print("Failed saving screenshot due to \(error)")
}

This gives me the following output:

Saving to: file:///var/mobile/Containers/Data/Application/77D52C66-353B-4029-97D5-48E6BAE35C92/Downloads/appshot.png
Failed saving screenshot due to Error Domain=NSCocoaErrorDomain Code=4 "The file “appshot.png” doesn’t exist." UserInfo={NSFilePath=///var/mobile/Containers/Data/Application/77D52C66-353B-4029-97D5-48E6BAE35C92/Downloads/appshot.png, NSUnderlyingError=0x1c405bc60 {Error Domain=NSPOSIXErrorDomain Code=2 "No such file or directory"}}

Of course, the file doesn't exist because that's the file I'm trying to create. But /var/mobile doesn't exist on my laptop either -- it looks like the path FileManager is building may exist on the AppleTV device, but I want it on my laptop where my test script is executing.

So I backed out to a much more simple case, and even this is giving me problems:

let str = "This is a test"
let path = "file:///Users/haljor/foo.txt"
let pathUrl = URL(string: path)!
print("Path: \(path)")
print("URL: \(pathUrl)")

do {
    try str.write(to: pathUrl, atomically: true, encoding: .utf8)
} catch {
    print("Caught error writing to \(pathUrl): \(error)")
}

And here's the output:

Path: file:///Users/haljor/foo.txt
URL: file:///Users/haljor/foo.txt
Caught error writing to file:///Users/haljor/foo.txt: Error Domain=NSCocoaErrorDomain Code=4 "The folder “foo.txt” doesn’t exist." UserInfo={NSURL=file:///Users/haljor/foo.txt, NSUserStringVariant=Folder, NSUnderlyingError=0x1c40553f0 {Error Domain=NSPOSIXErrorDomain Code=2 "No such file or directory"}}

Here, it looks like it's trying to write to a folder at the path I specified, not a file. Clearly there's something I'm not understanding in each of these cases.

I don't really have a preference for whether I use a fully-specified path or something using FileManager -- it just needs to land somewhere on my laptop (not the TV device). What am I missing?

Answer 1

You can add an attachment to the test case and save it to disk too. The problem was that Downloads folder may not exist in the container yet. The best way to handle this is via init-once property:

var downloadsFolder: URL = {
    let fm = FileManager.default
    let folder = fm.urls(for: .downloadsDirectory, in: .userDomainMask)[0]

    var isDirectory: ObjCBool = false
    if !(fm.fileExists(atPath: folder.path, isDirectory: &isDirectory) && isDirectory.boolValue) {
        try! fm.createDirectory(at: folder, withIntermediateDirectories: false, attributes: nil)
    }
    return folder
}()

func test() {
    let appshot = XCUIScreen.main.screenshot()
    let attachment = XCTAttachment(screenshot: appshot)
    attachment.lifetime = .keepAlways
    self.add(attachment)

    // Save to container
    let url = downloadsFolder.appendingPathComponent("appshot.png")
    try! appshot.pngRepresentation.write(to: url)
}

If you want to view the attachment, right-click on the test case, select Jump to Report and expand the tree. You will see the screenshot eventually: