(core) Comments

Summary:
First iteration for comments system for Grist.
- Comments are stored in a generic metatable `_grist_Cells`
- Each comment is connected to a particular cell (hence the generic name of the table)
- Access level works naturally for records stored in this table
-- User can add/read comments for cells he can see
-- User can't update/remove comments that he doesn't own, but he can delete them by removing cells (rows/columns)
-- Anonymous users can't see comments at all.
- Each comment can have replies (but replies can't have more replies)

Comments are hidden by default, they can be enabled by COMMENTS=true env variable.
Some things for follow-up
- Avatars, currently the user's profile image is not shown or retrieved from the server
- Virtual rendering for comments list in creator panel. Currently, there is a limit of 200 comments.

Test Plan: New and existing tests

Reviewers: georgegevoian, paulfitz

Reviewed By: georgegevoian

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3509

app/client/models/rowset.ts

@@ -26,6 +26,7 @@ import {DisposableWithEvents} from 'app/common/DisposableWithEvents';
 import {CompareFunc, sortedIndex} from 'app/common/gutil';
 import {SkippableRows} from 'app/common/TableData';
 import {RowFilterFunc} from "app/common/RowFilterFunc";
+import {Observable} from 'grainjs';
 
 /**
  * Special constant value that can be used for the `rows` array for the 'rowNotify'
@@ -390,7 +391,10 @@ class RowGroupHelper<Value> extends RowSource {
 }
 
 // ----------------------------------------------------------------------
-
+/**
+ * Helper function that does map.get(key).push(r), creating an Array for the given key if
+ * necessary.
+ */
 function _addToMapOfArrays<K, V>(map: Map<K, V[]>, key: K, r: V): void {
   let arr = map.get(key);
   if (!arr) { map.set(key, arr = []); }
@@ -437,11 +441,6 @@ export class RowGrouping<Value> extends RowListener {
 
   // Implementation of the RowListener interface.
 
-  /**
-   * Helper function that does map.get(key).push(r), creating an Array for the given key if
-   * necessary.
-   */
-
   public onAddRows(rows: RowList) {
     const groupedRows = new Map();
     for (const r of rows) {
@@ -707,6 +706,41 @@ export class SortedRowSet extends RowListener {
   }
 }
 
+type RowTester = (rowId: RowId) => boolean;
+/**
+ * RowWatcher is a RowListener that maintains an observable function that checks whether a row
+ * is in the connected RowSource.
+ */
+export class RowWatcher extends RowListener {
+  /**
+   * Observable function that returns true if the row is in the connected RowSource.
+   */
+  public rowFilter: Observable<RowTester> = Observable.create(this, () => false);
+  // We count the number of times the row is added or removed from the source.
+  // In most cases row is added and removed only once.
+  private _rowCounter: Map<RowId, number> = new Map();
+
+  public clear() {
+    this._rowCounter.clear();
+    this.rowFilter.set(() => false);
+    this.stopListening();
+  }
+
+  protected onAddRows(rows: RowList) {
+    for (const r of rows) {
+      this._rowCounter.set(r, (this._rowCounter.get(r) || 0) + 1);
+    }
+    this.rowFilter.set((row) => (this._rowCounter.get(row) ?? 0) > 0);
+  }
+
+  protected onRemoveRows(rows: RowList) {
+    for (const r of rows) {
+      this._rowCounter.set(r, (this._rowCounter.get(r) || 0) - 1);
+    }
+    this.rowFilter.set((row) => (this._rowCounter.get(row) ?? 0) > 0);
+  }
+}
+
 function isSmallChange(rows: RowList) {
   return Array.isArray(rows) && rows.length <= 2;
 }