为什么值突然变成零？

Question

I'm working on a project where I want to get an image for a particular identifier. I have the following function:

func imageForIdentifier(identifier: String?) -> UIImage? {
        if identifier == nil || identifier! == "" {
            return nil
        }
        let directory = directoryForID(identifier!)
        if let img = imgInCache.objectForKey(identifier!) as? UIImage {
            return img
        }
        if let img = NSData(contentsOfFile: directory) {
            return UIImage(data: img)
        }
        return nil
    }

...and the directoryForID function...

func directoryForID(identifier: String) -> String {
        let directoryPath: NSURL = NSFileManager.defaultManager().URLsForDirectory(NSSearchPathDirectory.DocumentationDirectory, inDomains: NSSearchPathDomainMask.UserDomainMask).first!
        return (directoryPath.URLByAppendingPathComponent(identifier).path!)
    }

If I pass an image identifier it should create a directory and in the last if-Statement it should return a UIImage of the data from this directory. However it skips all of the if-Statements and returns nil. How is this possible even though I pass a correct identifier?

Thank you in advance.

Answer 1

The reason is how you are testing the nil-ability or emptiness of the string object identifier :

identifier.length can be helpful to validate the content of the identifier...

Answer 2

Edited answer.

func imageForIdentifier(identifier: String?) -> UIImage? {

    guard let identifier else { return }

    let directory = directoryForID(identifier)
    if let img = imgInCache.objectForKey(identifier) as? UIImage {
        return img
    }
    if let img = NSData(contentsOfFile: directory) {
        return UIImage(data: img)
    }
    return nil
}