Singletons

Singletons let you define a resource only once per application.

This example shows a pattern for how to implement singletons in Wing.

As an example, say we want to maintain a central bucket that can be accessed from any part of the application:

main.w

bring cloud;

class SingletonBucket {
  pub static of(scope: std.IResource): cloud.Bucket {
    let uid = "SingletonBucket";
    let root = nodeof(scope).root;
    let rootNode = nodeof(root);
    return unsafeCast(rootNode.tryFindChild(uid)) ?? new cloud.Bucket() as uid in root;
  }
}

The SingletonBucket.of() static method uses std.Node.of(scope).root to find the root node of the app. Then, if there is already a child with the identifier SingletonBucket at that level, it returns it or otherwise it creates a new bucket with this id (as uid) under the root node (in root).

unsafeCast() is needed here to cast the returned object from tryFindChild() to cloud.Bucket.

Now, in order to access our bucket from anywhere within the app, I can just use:

class MyFoo {
  new() {
    let bucket = SingletonBucket.of(this);
  }
}

And we will always get the same bucket.

NOTE: if you use this in an entrypoint (outside of a class declaration), its type is going to be Construct (known issue: #6049), so you can use unsafeCast as a workaround:

let bucket = SingletonBucket.of(unsafeCast(this));