Clean up and comment

ReactNativeClient/lib/file-api-driver-local.js

@@ -26,10 +26,6 @@ class FileApiDriverLocal {
 		return output;
 	}
 
-	supportsDelta() {
-		return false;
-	}
-
 	stat(path) {
 		return new Promise((resolve, reject) => {
 			fs.stat(path, (error, s) => {

ReactNativeClient/lib/file-api-driver-memory.js

@@ -7,10 +7,6 @@ class FileApiDriverMemory {
 		this.deletedItems_ = [];
 	}
 
-	supportsDelta() {
-		return true;
-	}
-
 	itemIndexByPath(path) {
 		for (let i = 0; i < this.items_.length; i++) {
 			if (this.items_[i].path == path) return i;

ReactNativeClient/lib/file-api-driver-onedrive.js

@@ -14,10 +14,6 @@ class FileApiDriverOneDrive {
 		return this.api_;
 	}
 
-	supportsDelta() {
-		return true;
-	}
-
 	itemFilter_() {
 		return {
 			select: 'name,file,folder,fileSystemInfo,parentReference',

ReactNativeClient/lib/file-api.js

@@ -23,10 +23,6 @@ class FileApi {
 		return this.syncTargetId_;
 	}
 
-	supportsDelta() {
-		return this.driver_.supportsDelta();
-	}
-
 	setLogger(l) {
 		this.logger_ = l;
 	}

ReactNativeClient/lib/synchronizer.js

@@ -358,14 +358,14 @@ class Synchronizer {
 			while (true) {
 				if (this.cancelling()) break;
 
-				// let allIds = null;
-				// if (!this.api().supportsDelta()) {
-				// 	allIds = await BaseItem.syncedItemIds(syncTargetId);
-				// }
-
 				let listResult = await this.api().delta('', {
 					context: context,
-					// itemIds: allIds,
+
+					// allItemIdsHandler() provides a way for drivers that don't have a delta API to
+					// still provide delta functionality by comparing the items they have to the items
+					// the client has. Very inefficient but that's the only possible workaround.
+					// It's a function so that it is only called if the driver needs these IDs. For
+					// drivers with a delta functionality it's a noop.
 					allItemIdsHandler: async () => { return BaseItem.syncedItemIds(syncTargetId); }
 				});