Merge pull request ReactiveX#1493 from sue71/fix/improve-performance-subscribe

Fix bad performance caused by stacktrace

Author: devxoul

RxSwift/ObservableType+Extensions.swift

@@ -50,7 +50,7 @@ extension ObservableType {
 
                 let synchronizationTracker = SynchronizationTracker()
 
-                let callStack = Thread.callStackSymbols
+                let callStack = Hooks.recordCallStackOnError ? Thread.callStackSymbols : []
 
                 let observer = AnonymousObserver<E> { event in
 

RxSwift/Rx.swift

@@ -135,5 +135,8 @@ func decrementChecked(_ i: inout Int) throws -> Int {
 
 /// RxSwift global hooks
 public enum Hooks {
+    
+    // Should capture call stack
+    public static var recordCallStackOnError: Bool = false
 
 }