SwiftUI Firebase - Remembering Variable

You have two ways to handle this, both ways require you to think about all possible cases. Option #1, you handle it through Firebase alone. Option #2, you handle it locally and push to Firebase. You already mentioned an issue with Option #2 where uninstalling will cause it to lose data. The obvious, best solution is Option #1, where we let Firebase handle it.

Implementation of Option #1

Let's think about this issue logically, a user likes an item, firebase is updated with a "Reference" to that "Post ID" (I'm assuming). If the app is closed, and re-opened, there is no data indicating that a post is liked.

To fix that, we need to add a check to grab that data that has been saved to firebase. If the data exists, then set it to "Liked" if it doesn't set it to "Default"

In your firebase you might need a document structure like this.

user/profiles/<someProfileId>/likedPosts : [ someID, someID2, someID3 ] 

Each time the app launches, firebase checks this particular document and grabs the related dictionary, at run-time. The purpose for this is #1, it's simple data to grab, #2 it prevents unnecessary Firebase queries. Then, you'll need to store that in some object. I recommend a singleton as you'll be needing that data all over the place. It might look something like this.

class FirebaseDataManager {
     static var shared = FirebaseDataManager()
     var likedPosts: [String] //String or Int ID's to the posts themselves.      

     init() {
          //Fetch our latest liked posts
          super.init()
          likedPosts = fetchRecentLikedPosts()
     }
  
     func fetchRecentLikedPosts() -> [String] {
          //Do whatever logic you need to here. 
          return [String]()
     }

     func addLikedPost(postID: String) {
          //Add to the likedPosts [String] & Firebase
     }
}

Doing it this way will allow you to have a "Source of truth" being the Firebase, which eliminates any problems you might have with de-sync data or with constant queries to the firebase. To boot you can access it from anywhere. Further diving into it, each time a post is shown, check the ID against the FirebaseDataManager.shared.likedPosts array and if there is a match, you know it's liked, set accordingly.

Usage Example

struct someView: View {
  
    let firManager = FirebaseDataManager.shared

    var body: some View { 
        //You should handle the case when there is nothing. 
        //This is merely an example. 
        Text("\(firManager.likedPosts.first())"
    }

}

Documentation: https://firebase.google.com/docs/firestore/query-data/get-data

I encourage you to checkout firebase documentation on some of the concepts I'm mentioning here. Their documentation will help you tremendously. Let me know if you have any issues and I'll try my best to help you out.