Remove support for the scope parameter in the MessageHandler.on method

At this point in time it's easy to convert the `MessageHandler.on` call-sites to use arrow functions, and thus let the JavaScript engine handle scopes for us, rather than having to manually keep references to the relevant scopes in `MessageHandler`.[1]
An additional benefit of this is that a couple of `Function.prototype.call()` instances can now be converted into "normal" function calls, which should be a tiny bit more efficient.

All in all, I don't see any compelling reason why it'd be necessary to keep supporting custom `scope`s in the `MessageHandler` implementation.

---
[1] In the event that a custom scope is ever needed, simply using `bind` on the handler function when calling `MessageHandler.on` ought to work as well.

src/display/api.js

@@ -1875,7 +1875,7 @@ class WorkerTransport {
   setupMessageHandler() {
     const { messageHandler, loadingTask, } = this;
 
-    messageHandler.on('GetReader', function(data, sink) {
+    messageHandler.on('GetReader', (data, sink) => {
       assert(this._networkStream);
       this._fullReader = this._networkStream.getFullReader();
       this._fullReader.onProgress = (evt) => {
@@ -1902,9 +1902,9 @@ class WorkerTransport {
       sink.onCancel = (reason) => {
         this._fullReader.cancel(reason);
       };
-    }, this);
+    });
 
-    messageHandler.on('ReaderHeadersReady', function(data) {
+    messageHandler.on('ReaderHeadersReady', (data) => {
       const headersCapability = createPromiseCapability();
       const fullReader = this._fullReader;
       fullReader.headersReady.then(() => {
@@ -1932,9 +1932,9 @@ class WorkerTransport {
       }, headersCapability.reject);
 
       return headersCapability.promise;
-    }, this);
+    });
 
-    messageHandler.on('GetRangeReader', function(data, sink) {
+    messageHandler.on('GetRangeReader', (data, sink) => {
       assert(this._networkStream);
       const rangeReader =
         this._networkStream.getRangeReader(data.begin, data.end);
@@ -1970,14 +1970,14 @@ class WorkerTransport {
       sink.onCancel = (reason) => {
         rangeReader.cancel(reason);
       };
-    }, this);
+    });
 
-    messageHandler.on('GetDoc', function({ pdfInfo, }) {
+    messageHandler.on('GetDoc', ({ pdfInfo, }) => {
       this._numPages = pdfInfo.numPages;
       loadingTask._capability.resolve(new PDFDocumentProxy(pdfInfo, this));
-    }, this);
+    });
 
-    messageHandler.on('PasswordRequest', function(exception) {
+    messageHandler.on('PasswordRequest', (exception) => {
       this._passwordCapability = createPromiseCapability();
 
       if (loadingTask.onPassword) {
@@ -1996,34 +1996,34 @@ class WorkerTransport {
           new PasswordException(exception.message, exception.code));
       }
       return this._passwordCapability.promise;
-    }, this);
+    });
 
     messageHandler.on('PasswordException', function(exception) {
       loadingTask._capability.reject(
         new PasswordException(exception.message, exception.code));
-    }, this);
+    });
 
     messageHandler.on('InvalidPDF', function(exception) {
       loadingTask._capability.reject(
         new InvalidPDFException(exception.message));
-    }, this);
+    });
 
     messageHandler.on('MissingPDF', function(exception) {
       loadingTask._capability.reject(
         new MissingPDFException(exception.message));
-    }, this);
+    });
 
     messageHandler.on('UnexpectedResponse', function(exception) {
       loadingTask._capability.reject(
         new UnexpectedResponseException(exception.message, exception.status));
-    }, this);
+    });
 
     messageHandler.on('UnknownError', function(exception) {
       loadingTask._capability.reject(
         new UnknownErrorException(exception.message, exception.details));
-    }, this);
+    });
 
-    messageHandler.on('DataLoaded', function(data) {
+    messageHandler.on('DataLoaded', (data) => {
       // For consistency: Ensure that progress is always reported when the
       // entire PDF file has been loaded, regardless of how it was fetched.
       if (loadingTask.onProgress) {
@@ -2033,9 +2033,9 @@ class WorkerTransport {
         });
       }
       this.downloadInfoCapability.resolve(data);
-    }, this);
+    });
 
-    messageHandler.on('StartRenderPage', function(data) {
+    messageHandler.on('StartRenderPage', (data) => {
       if (this.destroyed) {
         return; // Ignore any pending requests if the worker was terminated.
       }
@@ -2043,9 +2043,9 @@ class WorkerTransport {
       const page = this.pageCache[data.pageIndex];
       page._stats.timeEnd('Page Request');
       page._startRenderPage(data.transparency, data.intent);
-    }, this);
+    });
 
-    messageHandler.on('commonobj', function(data) {
+    messageHandler.on('commonobj', (data) => {
       if (this.destroyed) {
         return; // Ignore any pending requests if the worker was terminated.
       }
@@ -2100,9 +2100,9 @@ class WorkerTransport {
         default:
           throw new Error(`Got unknown common object type ${type}`);
       }
-    }, this);
+    });
 
-    messageHandler.on('obj', function(data) {
+    messageHandler.on('obj', (data) => {
       if (this.destroyed) {
         // Ignore any pending requests if the worker was terminated.
         return undefined;
@@ -2149,9 +2149,9 @@ class WorkerTransport {
           throw new Error(`Got unknown object type ${type}`);
       }
       return undefined;
-    }, this);
+    });
 
-    messageHandler.on('DocProgress', function(data) {
+    messageHandler.on('DocProgress', (data) => {
       if (this.destroyed) {
         return; // Ignore any pending requests if the worker was terminated.
       }
@@ -2162,11 +2162,12 @@ class WorkerTransport {
           total: data.total,
         });
       }
-    }, this);
+    });
 
-    messageHandler.on('UnsupportedFeature', this._onUnsupportedFeature, this);
+    messageHandler.on('UnsupportedFeature',
+                      this._onUnsupportedFeature.bind(this));
 
-    messageHandler.on('JpegDecode', function(data) {
+    messageHandler.on('JpegDecode', (data) => {
       if (this.destroyed) {
         return Promise.reject(new Error('Worker was destroyed'));
       }
@@ -2227,16 +2228,14 @@ class WorkerTransport {
         };
         img.src = imageUrl;
       });
-    }, this);
+    });
 
-    messageHandler.on('FetchBuiltInCMap', function(data) {
+    messageHandler.on('FetchBuiltInCMap', (data) => {
       if (this.destroyed) {
         return Promise.reject(new Error('Worker was destroyed'));
       }
-      return this.CMapReaderFactory.fetch({
+      return this.CMapReaderFactory.fetch(data);
-        name: data.name,
+    });
-      });
-    }, this);
   }
 
   _onUnsupportedFeature({ featureId, }) {

src/shared/message_handler.js

@@ -30,11 +30,11 @@ const StreamKind = {
   START_COMPLETE: 8,
 };
 
-async function resolveCall(fn, args, thisArg = null) {
+async function resolveCall(fn, args) {
   if (!fn) {
     return undefined;
   }
-  return fn.apply(thisArg, args);
+  return fn.apply(null, args);
 }
 
 function wrapReason(reason) {
@@ -100,8 +100,8 @@ function MessageHandler(sourceName, targetName, comObj) {
       if (data.callbackId) {
         let sourceName = this.sourceName;
         let targetName = data.sourceName;
-        Promise.resolve().then(function () {
+        Promise.resolve().then(function() {
-          return action[0].call(action[1], data.data);
+          return action(data.data);
         }).then((result) => {
           comObj.postMessage({
             sourceName,
@@ -122,7 +122,7 @@ function MessageHandler(sourceName, targetName, comObj) {
       } else if (data.streamId) {
         this._createStreamSink(data);
       } else {
-        action[0].call(action[1], data.data);
+        action(data.data);
       }
     } else {
       throw new Error(`Unknown action from worker: ${data.action}`);
@@ -132,12 +132,12 @@ function MessageHandler(sourceName, targetName, comObj) {
 }
 
 MessageHandler.prototype = {
-  on(actionName, handler, scope) {
+  on(actionName, handler) {
     var ah = this.actionHandler;
     if (ah[actionName]) {
       throw new Error(`There is already an actionName called "${actionName}"`);
     }
-    ah[actionName] = [handler, scope];
+    ah[actionName] = handler;
   },
   /**
    * Sends a message to the comObj to invoke the action with the supplied data.
@@ -318,7 +318,7 @@ MessageHandler.prototype = {
     streamSink.sinkCapability.resolve();
     streamSink.ready = streamSink.sinkCapability.promise;
     this.streamSinks[streamId] = streamSink;
-    resolveCall(action[0], [data.data, streamSink], action[1]).then(() => {
+    resolveCall(action, [data.data, streamSink]).then(() => {
       comObj.postMessage({
         sourceName,
         targetName,