Adds Streams API in getTextContent to stream data.

This patch adds Streams API support in getTextContent
so that we can stream data in chunks instead of fetching
whole data from worker thread to main thread. This patch
supports Streams API without changing the core functionality
of getTextContent.

Enqueue textContent directly at getTextContent in partialEvaluator.

Adds desiredSize and ready property in streamSink.

src/core/evaluator.js

@@ -1176,7 +1176,8 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
     },
 
     getTextContent({ stream, task, resources, stateManager = null,
-                     normalizeWhitespace = false, combineTextItems = false, }) {
+                     normalizeWhitespace = false, combineTextItems = false,
+                     sink, seenStyles = Object.create(null), }) {
       // Ensure that `resources`/`stateManager` is correctly initialized,
       // even if the provided parameter is e.g. `null`.
       resources = resources || Dict.empty;
@@ -1214,7 +1215,7 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
 
       // The xobj is parsed iff it's needed, e.g. if there is a `DO` cmd.
       var xobjs = null;
-      var xobjsCache = Object.create(null);
+      var skipEmptyXObjs = Object.create(null);
 
       var preprocessor = new EvaluatorPreprocessor(stream, xref, stateManager);
 
@@ -1225,7 +1226,8 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
           return textContentItem;
         }
         var font = textState.font;
-        if (!(font.loadedName in textContent.styles)) {
+        if (!(font.loadedName in seenStyles)) {
+          seenStyles[font.loadedName] = true;
           textContent.styles[font.loadedName] = {
             fontFamily: font.fallbackName,
             ascent: font.ascent,
@@ -1416,11 +1418,21 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
         textContentItem.str.length = 0;
       }
 
+      function enqueueChunk() {
+        let length = textContent.items.length;
+        if (length > 0) {
+          sink.enqueue(textContent, length);
+          textContent.items = [];
+          textContent.styles = Object.create(null);
+        }
+      }
+
       var timeSlotManager = new TimeSlotManager();
 
       return new Promise(function promiseBody(resolve, reject) {
-        var next = function (promise) {
-          promise.then(function () {
+        let next = function (promise) {
+          enqueueChunk();
+          Promise.all([promise, sink.ready]).then(function () {
             try {
               promiseBody(resolve, reject);
             } catch (ex) {
@@ -1615,11 +1627,7 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
               }
 
               var name = args[0].name;
-              if (xobjsCache.key === name) {
-                if (xobjsCache.texts) {
-                  Util.appendToArray(textContent.items, xobjsCache.texts.items);
-                  Util.extendObj(textContent.styles, xobjsCache.texts.styles);
-                }
+              if (name in skipEmptyXObjs) {
                 break;
               }
 
@@ -1633,8 +1641,7 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
               assert(isName(type), 'XObject should have a Name subtype');
 
               if (type.name !== 'Form') {
-                xobjsCache.key = name;
-                xobjsCache.texts = null;
+                skipEmptyXObjs[name] = true;
                 break;
               }
 
@@ -1650,6 +1657,26 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
                 xObjStateManager.transform(matrix);
               }
 
+              // Enqueue the `textContent` chunk before parsing the /Form
+              // XObject.
+              enqueueChunk();
+              let sinkWrapper = {
+                enqueueInvoked: false,
+
+                enqueue(chunk, size) {
+                  this.enqueueInvoked = true;
+                  sink.enqueue(chunk, size);
+                },
+
+                get desiredSize() {
+                  return sink.desiredSize;
+                },
+
+                get ready() {
+                  return sink.ready;
+                },
+              };
+
               next(self.getTextContent({
                 stream: xobj,
                 task,
@@ -1657,12 +1684,12 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
                 stateManager: xObjStateManager,
                 normalizeWhitespace,
                 combineTextItems,
-              }).then(function (formTextContent) {
-                Util.appendToArray(textContent.items, formTextContent.items);
-                Util.extendObj(textContent.styles, formTextContent.styles);
-
-                xobjsCache.key = name;
-                xobjsCache.texts = formTextContent;
+                sink: sinkWrapper,
+                seenStyles,
+              }).then(function() {
+                if (!sinkWrapper.enqueueInvoked) {
+                  skipEmptyXObjs[name] = true;
+                }
               }));
               return;
             case OPS.setGState:
@@ -1686,20 +1713,27 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
               }
               break;
           } // switch
+          if (textContent.items.length >= sink.desiredSize) {
+            // Wait for ready, if we reach highWaterMark.
+            stop = true;
+            break;
+          }
         } // while
         if (stop) {
           next(deferred);
           return;
         }
         flushTextContentItem();
-        resolve(textContent);
+        enqueueChunk();
+        resolve();
       }).catch((reason) => {
         if (this.options.ignoreErrors) {
           // Error(s) in the TextContent -- allow text-extraction to continue.
           warn('getTextContent - ignoring errors during task: ' + task.name);
 
           flushTextContentItem();
-          return textContent;
+          enqueueChunk();
+          return;
         }
         throw reason;
       });