From af94f1c5d09c3c8953b7f52b03b41bbf313b69c0 Mon Sep 17 00:00:00 2001
From: Cyrus Najmabadi <cyrusn@microsoft.com>
Date: Mon, 6 Jul 2015 14:04:42 -0700
Subject: [PATCH] Throttle how often we call into the host side to check for
 cancellation.

---
 src/services/services.ts |  1 -
 src/services/shims.ts    | 26 +++++++++++++++++++++++++-
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/src/services/services.ts b/src/services/services.ts
index 19dd5288721..1e98e29d4cf 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -1618,7 +1618,6 @@ namespace ts {
     export class OperationCanceledException { }
 
     export class CancellationTokenObject {
-
         public static None: CancellationTokenObject = new CancellationTokenObject(null)
 
         constructor(private cancellationToken: CancellationToken) {
diff --git a/src/services/shims.ts b/src/services/shims.ts
index 2e8b3eb774d..22c0d3f6065 100644
--- a/src/services/shims.ts
+++ b/src/services/shims.ts
@@ -327,7 +327,8 @@ namespace ts {
         }
 
         public getCancellationToken(): CancellationToken {
-            return this.shimHost.getCancellationToken();
+            var hostCancellationToken = this.shimHost.getCancellationToken();
+            return new ThrottledCancellationToken(hostCancellationToken);
         }
 
         public getCurrentDirectory(): string {
@@ -346,6 +347,29 @@ namespace ts {
         }
     }
 
+    /** A cancellation that throttles calls to the host */
+    class ThrottledCancellationToken implements CancellationToken {
+        // Store when we last tried to cancel.  Checking cancellation can be expensive (as we have
+        // to marshall over to the host layer).  So we only bother actually checking once enough
+        // time has passed.
+        private lastCancellationCheckTime = 0;
+
+        constructor(private hostCancellationToken: CancellationToken) {
+        }
+
+        public isCancellationRequested(): boolean {
+            var time = Date.now();
+            var duration = Math.abs(time - this.lastCancellationCheckTime);
+            if (duration > 10) {
+                // Check no more than once every 10 ms.
+                this.lastCancellationCheckTime = time;
+                return this.hostCancellationToken.isCancellationRequested();
+            }
+
+            return false;
+        }
+    }
+
     export class CoreServicesShimHostAdapter implements ParseConfigHost {
 
         constructor(private shimHost: CoreServicesShimHost) {