Stream improvements

submodules/Components/HierarchyTrackingLayer/BUILD

@@ -0,0 +1,17 @@
+load("@build_bazel_rules_swift//swift:swift.bzl", "swift_library")
+
+swift_library(
+    name = "HierarchyTrackingLayer",
+    module_name = "HierarchyTrackingLayer",
+    srcs = glob([
+        "Sources/**/*.swift",
+    ]),
+    copts = [
+        "-warnings-as-errors",
+    ],
+    deps = [
+    ],
+    visibility = [
+        "//visibility:public",
+    ],
+)

submodules/Components/HierarchyTrackingLayer/Sources/HierarchyTrackingLayer.swift

@@ -0,0 +1,22 @@
+import UIKit
+
+private final class NullActionClass: NSObject, CAAction {
+    @objc public func run(forKey event: String, object anObject: Any, arguments dict: [AnyHashable : Any]?) {
+    }
+}
+
+private let nullAction = NullActionClass()
+
+open class HierarchyTrackingLayer: CALayer {
+    public var didEnterHierarchy: (() -> Void)?
+    public var didExitHierarchy: (() -> Void)?
+    
+    override open func action(forKey event: String) -> CAAction? {
+        if event == kCAOnOrderIn {
+            self.didEnterHierarchy?()
+        } else if event == kCAOnOrderOut {
+            self.didExitHierarchy?()
+        }
+        return nullAction
+    }
+}