Throttle how often we call into the host side to check for cancellation.

Author: CyrusNajmabadi

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) {

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) {