[#1144] Make State Machine context aware of previous state

- ActionContext extended to hold an optional previous state value
- unit tests for the ActionContext previous state

Sources/ZcashLightClientKit/Block/Actions/Action.swift

@@ -9,6 +9,7 @@ import Foundation
 
 actor ActionContext {
     var state: CBPState
+    var prevState: CBPState?
     var syncRanges: SyncRanges
     var totalProgressRange: CompactBlockRange = 0...0
 
@@ -17,7 +18,10 @@ actor ActionContext {
         syncRanges = SyncRanges.empty
     }
 
-    func update(state: CBPState) async { self.state = state }
+    func update(state: CBPState) async {
+        prevState = self.state
+        self.state = state
+    }
     func update(syncRanges: SyncRanges) async { self.syncRanges = syncRanges }
     func update(totalProgressRange: CompactBlockRange) async { self.totalProgressRange = totalProgressRange }
 }

Tests/OfflineTests/CompactBlockProcessorActions/ActionContextStateTests.swift

@@ -0,0 +1,35 @@
+//
+//  ActionContextStateTests.swift
+//  
+//
+//  Created by Lukáš Korba on 15.06.2023.
+//
+
+import XCTest
+@testable import TestUtils
+@testable import ZcashLightClientKit
+
+final class ActionContextStateTests: XCTestCase {
+    func testPreviousState() async throws {
+        let syncContext: ActionContext = .init(state: .idle)
+        
+        await syncContext.update(state: .clearCache)
+        
+        let currentState = await syncContext.state
+        let prevState = await syncContext.prevState
+
+        XCTAssertTrue(
+            currentState == .clearCache,
+            "syncContext.state after update is expected to be .clearCache but received \(currentState)"
+        )
+
+        if let prevState {
+            XCTAssertTrue(
+                prevState == .idle,
+                "syncContext.prevState after update is expected to be .idle but received \(prevState)"
+            )
+        } else {
+            XCTFail("syncContext.prevState is not expected to be nil.")
+        }
+    }
+}