Prioritizes app refresh order

Tries to refresh apps that are about to expire first, and then always refreshes AltStore itself last, since refreshing AltStore means that the app will quit.

AltStore/Operations/OperationGroup.swift

@@ -0,0 +1,63 @@
+//
+//  OperationGroup.swift
+//  AltStore
+//
+//  Created by Riley Testut on 6/20/19.
+//  Copyright © 2019 Riley Testut. All rights reserved.
+//
+
+import Foundation
+import CoreData
+
+import AltSign
+
+class OperationGroup
+{
+    let progress = Progress.discreteProgress(totalUnitCount: 0)
+    
+    var completionHandler: ((Result<[String: Result<InstalledApp, Error>], Error>) -> Void)?
+    var beginInstallationHandler: ((InstalledApp) -> Void)?
+    
+    var server: Server?
+    var signer: ALTSigner?
+    
+    var error: Error?
+    
+    var results = [String: Result<InstalledApp, Error>]()
+    
+    private let operationQueue = OperationQueue()
+    private let installOperationQueue = OperationQueue()
+    
+    init()
+    {
+        // Enforce only one installation at a time.
+        self.installOperationQueue.maxConcurrentOperationCount = 1
+    }
+    
+    func cancel()
+    {
+        self.operationQueue.cancelAllOperations()
+        self.installOperationQueue.cancelAllOperations()
+    }
+    
+    func addOperations(_ operations: [Operation])
+    {
+        for operation in operations
+        {
+            if let installOperation = operation as? InstallAppOperation
+            {
+                if let previousOperation = self.installOperationQueue.operations.last
+                {
+                    // Ensures they execute in the order they're added, since isReady is still false at this point.
+                    installOperation.addDependency(previousOperation)
+                }
+                
+                self.installOperationQueue.addOperation(installOperation)
+            }
+            else
+            {
+                self.operationQueue.addOperation(operation)
+            }
+        }
+    }
+}