Overriding loadTiles in Mapkit Swift-CodePudding

I am trying to use custom tiles in MapKit with SwiftUI. I have found examples of how to do this if I have URLs for the tile, but I would like the tile to be dynamically and locally generated by the app. To do so, Apple's documentation seems to suggest I would need to override the loadTile function in MKTileOverlay. Per the documentation at https://developer.apple.com/documentation/mapkit/mktileoverlay/1452445-loadtile I have started defining my class as follows:

class TileOverlay: MKTileOverlay{
    override func loadTile(at path: MKTileOverlayPath, result: @escaping (Data?, Error?) -> Void){
        //Now what?
    }
}

If I understand correctly this function would eventually return a tile, which is a 256x256 image. But how do I actually return this image? It appears that I have to return a closure that returns the image as a Data? object, but I was unsuccessful in finding how to convert a 256x256 image to such an object, nor was I able to determine how such a closure should work.

Any help is greatly appreciated!

CodePudding user response：

This is what you need to do:

Retrieve raw image data related to the path
Call at some point result(theRawImageData, nil) or result(nil, theErrorYouGot)

Then the caller of that method will get the closure parameters, and if there is no error, try do to: let image = UIImage(data: theRawImageData), and render it where it's needed.

If you already have an image in RAM, you can call jpegData(compressionQuality:), pngData() to get Data. But as you can see, you'll go from UIImage -> Data -> UIImage, there is a little extra work in the way. So try to keep Data instead of UIImage if you do some RAM caching (with NSCache for instance).

But, if you have a local image on disk (or on a distant URL), you can use Data(contentsOf:). Yet, it's not recommended to use that method, because it's synchrone (ie, blocks the current threads). Prefer using URLSession to do that.

override func loadTile(at path: MKTileOverlayPath, result: @escaping (Data?, Error?) -> Void){

    //If you use caching:
    if let imageDataInCache = ... {
        result(imageDataInCache, nil)
    } else {
        //Create an `URL` or a `URLRequest` from the `path`:
        let urlOrURLRequest = ...
        //Let's "fetch" the image:
        let task = URLSession.dataTask(with: urlOrURLRequest) { data, response, error in
            //Do additional work if needed, like checking `response`, etc.
            result(data, error)
        }
        task.resume()
    }
}