Use UIScheduler instead safetyAccessUI

Author: iwecon

Sources/StackKit/UIView+StackKit/UIView+Combine.swift

@@ -8,16 +8,68 @@
 import UIKit
 import Combine
 
-fileprivate func safetyAccessUI(_ closure: @escaping () -> Void) {
-    if Thread.isMainThread {
-        closure()
-    } else {
-        DispatchQueue.main.async {
-            closure()
+/// Safety Access UI.
+///
+/// A scheduler that performs all work on the main queue, as soon as possible.
+///
+/// If the caller is already running on the main queue when an action is
+/// scheduled, it may be run synchronously. However, ordering between actions
+/// will always be preserved.
+fileprivate final class UIScheduler {
+    private static let dispatchSpecificKey = DispatchSpecificKey<UInt8>()
+    private static let dispatchSpecificValue = UInt8.max
+    private static var __once: () = {
+            DispatchQueue.main.setSpecific(key: UIScheduler.dispatchSpecificKey,
+                                           value: dispatchSpecificValue)
+    }()
+    
+    private let queueLength: UnsafeMutablePointer<Int32> = {
+        let memory = UnsafeMutablePointer<Int32>.allocate(capacity: 1)
+        memory.initialize(to: 0)
+        return memory
+    }()
+    
+    deinit {
+        queueLength.deinitialize(count: 1)
+        queueLength.deallocate()
+    }
+    
+    init() {
+        /// This call is to ensure the main queue has been setup appropriately
+        /// for `UIScheduler`. It is only called once during the application
+        /// lifetime, since Swift has a `dispatch_once` like mechanism to
+        /// lazily initialize global variables and static variables.
+        _ = UIScheduler.__once
+    }
+    
+    /// Queues an action to be performed on main queue. If the action is called
+    /// on the main thread and no work is queued, no scheduling takes place and
+    /// the action is called instantly.
+    func schedule(_ action: @escaping () -> Void) {
+        let positionInQueue = enqueue()
+        
+        // If we're already running on the main queue, and there isn't work
+        // already enqueued, we can skip scheduling and just execute directly.
+        if positionInQueue == 1, DispatchQueue.getSpecific(key: UIScheduler.dispatchSpecificKey) == UIScheduler.dispatchSpecificValue {
+            action()
+            dequeue()
+        } else {
+            DispatchQueue.main.async {
+                defer { self.dequeue() }
+                action()
+            }
         }
     }
+    
+    private func dequeue() {
+        OSAtomicDecrement32(queueLength)
+    }
+    private func enqueue() -> Int32 {
+        OSAtomicIncrement32(queueLength)
+    }
 }
 
+
 @available(iOS 13.0, *)
 extension StackKitCompatible where Base: UIView {
 
@@ -58,7 +110,7 @@ extension StackKitCompatible where Base: UIView {
     ) -> Self
     {
         receive(publisher: publisher, storeIn: &cancellables) { view, output in
-            safetyAccessUI {
+            UIScheduler().schedule {
                 view.isHidden = output
             }
         }
@@ -72,27 +124,14 @@ extension StackKitCompatible where Base: UIView {
     ) -> Self
     {
         receive(publisher: publisher, cancellable: &cancellable) { view, output in
-            safetyAccessUI {
+            UIScheduler().schedule {
                 view.isHidden = output
             }
         }
         return self
     }
-    
 }
 
-/**
- 这里由于设计逻辑，会有个问题
- 
- 系统提供的 receive(on: DispatchQueue.main) 虽然也可以
-    ⚠️ 但是：DispatchQueue 是个调度器
- 任务添加后需要等到下一个 loop cycle 才会执行
- 这样就会导致一个问题：
-    ❌ 在主线程中修改值，并触发 `container.setNeedsLayout()` 的时候，
- `setNeedsLayout` 会先执行，而 `publisher` 会将任务派发到下一个 loop cycle (也就是 setNeedsLayout 和 receive 先后执行的问题)
- 所以这里采用 `safetyAccessUI` 来处理线程问题
- */
-
 @available(iOS 13.0, *)
 extension StackKitCompatible where Base: UILabel {
 
@@ -103,7 +142,7 @@ extension StackKitCompatible where Base: UILabel {
     ) -> Self
     {
         receive(publisher: publisher, storeIn: &cancellables) { view, output in
-            safetyAccessUI {
+            UIScheduler().schedule {
                 view.text = output
             }
         }
@@ -116,7 +155,7 @@ extension StackKitCompatible where Base: UILabel {
     ) -> Self
     {
         receive(publisher: publisher, cancellable: &cancellable) { view, output in
-            safetyAccessUI {
+            UIScheduler().schedule {
                 view.text = output
             }
         }
@@ -130,7 +169,7 @@ extension StackKitCompatible where Base: UILabel {
     ) -> Self
     {
         receive(publisher: publisher, storeIn: &cancellables) { view, output in
-            safetyAccessUI {
+            UIScheduler().schedule {
                 view.text = output
             }
         }
@@ -143,7 +182,7 @@ extension StackKitCompatible where Base: UILabel {
     ) -> Self
     {
         receive(publisher: publisher, cancellable: &cancellable) { view, output in
-            safetyAccessUI {
+            UIScheduler().schedule {
                 view.text = output
             }
         }
@@ -156,7 +195,7 @@ extension StackKitCompatible where Base: UILabel {
     ) -> Self
     {
         receive(publisher: publisher, storeIn: &cancellables) { view, output in
-            safetyAccessUI {
+            UIScheduler().schedule {
                 view.attributedText = output
             }
         }
@@ -169,7 +208,7 @@ extension StackKitCompatible where Base: UILabel {
     ) -> Self
     {
         receive(publisher: publisher, cancellable: &cancellable) { view, output in
-            safetyAccessUI {
+            UIScheduler().schedule {
                 view.attributedText = output
             }
         }
@@ -182,7 +221,7 @@ extension StackKitCompatible where Base: UILabel {
     ) -> Self
     {
         receive(publisher: publisher, storeIn: &cancellables) { view, output in
-            safetyAccessUI {
+            UIScheduler().schedule {
                 view.attributedText = output
             }
         }
@@ -195,7 +234,7 @@ extension StackKitCompatible where Base: UILabel {
     ) -> Self
     {
         receive(publisher: publisher, cancellable: &cancellable) { view, output in
-            safetyAccessUI {
+            UIScheduler().schedule {
                 view.attributedText = output
             }
         }