致命错误：在展开可选值Swift时意外发现nil

Question

So i'm now trying to convert a NSDictionary stored in NSUserdefault to my array used for table contents, but i meet many many errors including "fatal error: unexpectedly found nil while unwrapping an Optional value", (The currently one) Just wondering what's the most safe and correct way of converting it.

And here is my codes.

        let nsd:NSDictionary = NSUserDefaults.standardUserDefaults().valueForKey("storedPosts") as! NSDictionary
        for (postId, postInfo) in nsd{ //post id and the post
            let importedPost:Post = Post(postId: postId as! String, priorityLevel: postInfo.objectForKey("priorityLevel") as! String, status: postInfo.objectForKey("status") as! String, section: postInfo.objectForKey("section") as! String, userType: postInfo.objectForKey("userType") as! String, dataPosted: postInfo.objectForKey("dataPosted") as! String, lastUpdate: postInfo.objectForKey("lastUpdate") as! String, states: postInfo.objectForKey("states") as! String, personalizedToViewerData: postInfo.objectForKey("personalizedToViewerData") as! [String:Bool], content: postInfo.objectForKey("content") as! Dictionary<String, Any>)
        posts.append(importedPost)
}

Any sugguestions would be appreciated

Answer 1

When you "force-unwrap" an Optional with ! , you're telling the compiler "It's ok we can always unwrap because I know for sure there always will be a value".

But if there's actually no value, the app will crash...

So you have to first safely unwrap your Optionals.

There's a lot of techniques, the simplest for now is if let .

With "if let" you safely unwrap a value into a new constant, then you can use this new constant instead of the Optional. If you need to check the type or downcast, you can use "conditional downcasting" (the real name is "optional binding", I believe) with if let xxx = yyy as? zzz if let xxx = yyy as? zzz .

Let's see how we could do that:

if let nsd = NSUserDefaults.standardUserDefaults().valueForKey("storedPosts") as? NSDictionary {
    // use `nsd` here
}

Let's go further:

if let nsd = NSUserDefaults.standardUserDefaults().valueForKey("storedPosts") as? NSDictionary {
    for (postId, postInfo) in nsd {
        if let id = postId as? String {

        }
    }
}

It works, but we see that using an "if let" for every value would lead to a "pyramid of doom"...

A classic solution for this is to use multiple optional bindings with the same "if let":

if let nsd = NSUserDefaults.standardUserDefaults().valueForKey("storedPosts") as? NSDictionary {
    for (postId, postInfo) in nsd {
        if let id = postId as? String,
            priorityLevel = postInfo.objectForKey("priorityLevel") as? String,
            status = postInfo.objectForKey("status") as? String,
            section = postInfo.objectForKey("section") as? String {  // keep doing the same for all your values

            let importedPost = Post(postId: id, priorityLevel: priorityLevel, status: status, section: section, ...)  // etc

        }
    }
}

A nice side-effect is that you can now handle errors by adding an else branch to the "if let" conditions that need it.