Add a size getter, to Dict instances, to provide an easier way of checking the number of entries

This removes the need to manually call `Dict.getKeys()` and check its length.

src/core/document.js

@@ -551,7 +551,7 @@ class PDFDocument {
     // Check if a Collection dictionary is present in the document.
     try {
       const collection = this.catalog.catDict.get("Collection");
-      if (isDict(collection) && collection.getKeys().length > 0) {
+      if (isDict(collection) && collection.size > 0) {
         this.collection = collection;
       }
     } catch (ex) {

src/core/primitives.js

@@ -85,6 +85,10 @@ var Dict = (function DictClosure() {
       this.xref = newXref;
     },
 
+    get size() {
+      return Object.keys(this._map).length;
+    },
+
     // automatically dereferences Ref objects
     get(key1, key2, key3) {
       let value = this._map[key1];

test/unit/primitives_spec.js

@@ -117,6 +117,17 @@ describe("primitives", function () {
       expect(dict.xref).toEqual(xref);
     });
 
+    it("should return correct size", function () {
+      const dict = new Dict(null);
+      expect(dict.size).toEqual(0);
+
+      dict.set("Type", Name.get("Page"));
+      expect(dict.size).toEqual(1);
+
+      dict.set("Contents", Ref.get(10, 0));
+      expect(dict.size).toEqual(2);
+    });
+
     it("should return invalid values for unknown keys", function () {
       checkInvalidHasValues(emptyDict);
       checkInvalidKeyValues(emptyDict);