[Widgets]: [WIP]: Implement Pagination in ActiveAppWidget based on user interaction

AltWidget/Widgets/PaginationViewModel.swift

@@ -0,0 +1,105 @@
+//
+//  PaginationViewModel.swift
+//  AltStore
+//
+//  Created by Magesh K on 09/01/25.
+//  Copyright © 2025 SideStore. All rights reserved.
+//
+
+import Foundation
+import Combine
+
+class PaginationViewModel: ObservableObject {
+    
+    private static var instances: [String:PaginationViewModel] = [:]
+    
+    public static let MAX_ROWS_PER_PAGE = 3
+    
+    @Published public var sliding_window: [AppSnapshot] = []
+    @Published public var refreshed: Bool = false
+    
+    private init(){}
+    
+    private var _widgetID: String?
+    public lazy var widgetID: String = {
+        return _widgetID!
+    }()
+    
+    public static func getNewInstance(_ widgetID: String) -> PaginationViewModel {
+        let instance = PaginationViewModel()
+        PaginationViewModel.instances[widgetID] = instance
+        instance._widgetID = widgetID
+        return instance
+    }
+
+    public static func instance(_ widgetID: String) -> PaginationViewModel? {
+        return PaginationViewModel.instances[widgetID]
+    }
+    
+    public private(set) var backup_entries: [AppSnapshot] = []
+    
+    private var r_queue: [AppSnapshot] = []
+    private var l_queue: [AppSnapshot] = []
+    
+    private var lastIndex: Int { r_queue.count - 1 }
+
+    public func setEntries(_ entries: [AppSnapshot]) {
+        r_queue = entries
+        backup_entries = entries
+    }
+    
+    public func handlePagination(_ direction: Direction) {
+        
+        var sliding_window = Array(sliding_window)
+        var l_queue = Array(l_queue)
+        var r_queue = Array(r_queue)
+        
+        // If entries is empty, do nothing
+        guard !backup_entries.isEmpty else {
+            sliding_window.removeAll()
+            return
+        }
+        
+        switch direction {
+        case .up:
+            // move window contents to left-q since we are moving right side
+            if !sliding_window.isEmpty {
+                // take the window as-is and put it to right of l_queue
+                l_queue.append(contentsOf: sliding_window)
+            }
+            
+            // clear the window
+            sliding_window.removeAll()
+        
+            let size = min(r_queue.count, Self.MAX_ROWS_PER_PAGE)
+            for _ in 0..<size {
+                if !r_queue.isEmpty {
+                    sliding_window.append(r_queue.removeFirst())
+                }
+            }
+            
+        case .down:
+            // move window contents to right-q since we are moving left side
+            if !sliding_window.isEmpty {
+                // take the window as-is and put it to left of r_queue
+                r_queue.insert(contentsOf: sliding_window, at: 0)
+            }
+            
+            // clear the window
+            sliding_window.removeAll()
+            
+            let size = min(l_queue.count, Self.MAX_ROWS_PER_PAGE)
+            for _ in 0..<size {
+                sliding_window.insert(l_queue.removeLast(), at: 0)
+            }
+        }
+        
+        if !sliding_window.isEmpty {
+            // commit
+            self.sliding_window = sliding_window
+            self.l_queue = l_queue
+            self.r_queue = r_queue
+            self.refreshed = true
+        }
+    }
+}