diff --git a/app/common/DocActions.ts b/app/common/DocActions.ts
index 1b137f4a..894c84c9 100644
--- a/app/common/DocActions.ts
+++ b/app/common/DocActions.ts
@@ -2,6 +2,10 @@
  * This mirrors action definitions from sandbox/grist/actions.py
  */
 
+// Some definitions have moved to be part of plugin API.
+import { CellValue } from 'app/plugin/GristData';
+export { CellValue, RowRecord } from 'app/plugin/GristData';
+
 import map = require('lodash/map');
 
 export type AddRecord = ['AddRecord', string, number, ColValues];
@@ -83,7 +87,6 @@ export function getTableId(action: DocAction): string {
 
 // Helper types used in the definitions above.
 
-export type CellValue = number|string|boolean|null|[string, any?];
 export interface ColValues { [colId: string]: CellValue; }
 export interface BulkColValues { [colId: string]: CellValue[]; }
 export interface ColInfoMap { [colId: string]: ColInfo; }
@@ -98,11 +101,6 @@ export interface ColInfoWithId extends ColInfo {
   id: string;
 }
 
-export interface RowRecord {
-  id: number;
-  [colId: string]: CellValue;
-}
-
 // Multiple records in column-oriented format, i.e. same as BulkColValues but with a mandatory
 // 'id' column. This is preferred over TableDataAction in external APIs.
 export interface TableColValues {
diff --git a/app/plugin/GristAPI-ti.ts b/app/plugin/GristAPI-ti.ts
index 096d0135..c1a599bf 100644
--- a/app/plugin/GristAPI-ti.ts
+++ b/app/plugin/GristAPI-ti.ts
@@ -23,6 +23,7 @@ export const GristDocAPI = t.iface([], {
 
 export const GristView = t.iface([], {
   "fetchSelectedTable": t.func("any"),
+  "fetchSelectedRecord": t.func("any", t.param("rowId", "number")),
 });
 
 const exportedTypeSuite: t.ITypeSuite = {
diff --git a/app/plugin/GristAPI.ts b/app/plugin/GristAPI.ts
index 7560d613..405cef60 100644
--- a/app/plugin/GristAPI.ts
+++ b/app/plugin/GristAPI.ts
@@ -96,4 +96,7 @@ export interface GristView {
   // TODO: return type is Promise{[colId: string]: CellValue[]}> but cannot be specified because
   // ts-interface-builder does not properly support index-signature.
   fetchSelectedTable(): Promise<any>;
+
+  // Similar TODO to fetchSelectedTable for return type.
+  fetchSelectedRecord(rowId: number): Promise<any>;
 }
diff --git a/app/plugin/GristData.ts b/app/plugin/GristData.ts
new file mode 100644
index 00000000..c161ecb7
--- /dev/null
+++ b/app/plugin/GristData.ts
@@ -0,0 +1,6 @@
+export type CellValue = number|string|boolean|null|[string, any?];
+
+export interface RowRecord {
+  id: number;
+  [colId: string]: CellValue;
+}
diff --git a/app/plugin/GristTable-ti.ts b/app/plugin/GristTable-ti.ts
index e47f2103..237d94dc 100644
--- a/app/plugin/GristTable-ti.ts
+++ b/app/plugin/GristTable-ti.ts
@@ -1,3 +1,6 @@
+/**
+ * This module was automatically generated by `ts-interface-builder`
+ */
 import * as t from "ts-interface-checker";
 // tslint:disable:object-literal-key-quotes
 
@@ -16,9 +19,17 @@ export const GristColumn = t.iface([], {
   "type": "string",
 });
 
+export const APIType = t.enumtype({
+  "ImportSourceAPI": 0,
+  "ImportProcessorAPI": 1,
+  "ParseOptionsAPI": 2,
+  "ParseFileAPI": 3,
+});
+
 const exportedTypeSuite: t.ITypeSuite = {
   GristTable,
   GristTables,
   GristColumn,
+  APIType,
 };
 export default exportedTypeSuite;
diff --git a/app/plugin/grist-plugin-api.ts b/app/plugin/grist-plugin-api.ts
index be168ca3..62d7f295 100644
--- a/app/plugin/grist-plugin-api.ts
+++ b/app/plugin/grist-plugin-api.ts
@@ -19,6 +19,7 @@
 // tslint:disable:no-console
 
 import { GristAPI, GristDocAPI, GristView, RPC_GRISTAPI_INTERFACE } from './GristAPI';
+import { RowRecord } from './GristData';
 import { ImportSource, ImportSourceAPI, InternalImportSourceAPI } from './InternalImportSourceAPI';
 import { RenderOptions, RenderTarget } from './RenderOptions';
 import { checkers } from './TypeCheckers';
@@ -45,6 +46,40 @@ export const docApi = {
 
 export const on = rpc.on.bind(rpc);
 
+// For custom widgets, add a handler that will be called whenever the
+// row with the cursor changes - either by switching to a different row, or
+// by some value within the row potentially changing.  Handler may
+// in the future be called with null if the cursor moves away from
+// any row.
+// TODO: currently this will be called even if the content of a different row
+// changes.
+export function onRecord(callback: (data: RowRecord | null) => unknown) {
+  on('message', async function(msg) {
+    if (!msg.tableId || !msg.rowId) { return; }
+    const rec = await docApi.fetchSelectedRecord(msg.rowId);
+    callback(rec);
+  });
+}
+
+// For custom widgets, add a handler that will be called whenever the
+// selected records change.  Handler will be called with a list of records.
+export function onRecords(callback: (data: RowRecord[]) => unknown) {
+  on('message', async function(msg) {
+    if (!msg.tableId || !msg.dataChange) { return; }
+    const data = await docApi.fetchSelectedTable();
+    if (!data.id) { return; }
+    const rows: RowRecord[] = [];
+    for (let i = 0; i < data.id.length; i++) {
+      const row: RowRecord = {id: data.id[i]};
+      for (const key of Object.keys(data)) {
+        row[key] = data[key][i];
+      }
+      rows.push(row);
+    }
+    callback(rows);
+  });
+}
+
 /**
  * Calling `addImporter(...)` adds a safeBrowser importer. It is a short-hand for forwarding calls
  * to an `ImportSourceAPI` implementation registered in the file at `path`. It takes care of