Merge pull request #212 from pluma2/smarter-cursors

Allow cursor onChange function to intercept the cursor's new value

contrib/cursor/__tests__/Cursor.ts

@@ -109,6 +109,36 @@ describe('Cursor', () => {
     expect(onChange.mock.calls.length).toBe(3);
   });
 
+  it('updates with the return value of onChange', () => {
+    var onChange = jest.genMockFunction();
+
+    var data = Immutable.fromJS(json);
+    var deepCursor = Cursor.from(data, ['a', 'b', 'c'], onChange);
+
+    onChange.mockReturnValueOnce(undefined);
+    // onChange returning undefined has no effect
+    var newCursor = deepCursor.update(x => x + 1);
+    expect(newCursor.deref()).toBe(2);
+    expect(onChange).lastCalledWith(
+      Immutable.fromJS({a:{b:{c:2}}}),
+      data,
+      ['a', 'b', 'c']
+    );
+
+    onChange.mockReturnValueOnce(Immutable.fromJS({a:{b:{c:11}}}));
+    // onChange returning something else has an effect
+    newCursor = newCursor.update(x => 999);
+    expect(newCursor.deref()).toBe(11);
+    expect(onChange).lastCalledWith(
+      Immutable.fromJS({a:{b:{c:999}}}),
+      Immutable.fromJS({a:{b:{c:2}}}),
+      ['a', 'b', 'c']
+    );
+
+    // and update has been called exactly twice
+    expect(onChange.mock.calls.length).toBe(2);
+  });
+
   it('has map API for update shorthand', () => {
     var onChange = jest.genMockFunction();
 

contrib/cursor/index.d.ts

@@ -43,17 +43,17 @@ declare module 'immutable/contrib/cursor' {
 
   export function from(
     collection: Immutable.Collection<any, any>,
-    onChange?: (newValue: any, oldValue?: any, keyPath?: Array<any>) => void
+    onChange?: (newValue: any, oldValue?: any, keyPath?: Array<any>) => any
   ): Cursor;
   export function from(
     collection: Immutable.Collection<any, any>,
     keyPath: Array<any>,
-    onChange?: (newValue: any, oldValue?: any, keyPath?: Array<any>) => void
+    onChange?: (newValue: any, oldValue?: any, keyPath?: Array<any>) => any
   ): Cursor;
   export function from(
     collection: Immutable.Collection<any, any>,
     key: any,
-    onChange?: (newValue: any, oldValue?: any, keyPath?: Array<any>) => void
+    onChange?: (newValue: any, oldValue?: any, keyPath?: Array<any>) => any
   ): Cursor;
 
 

contrib/cursor/index.js

@@ -199,12 +199,15 @@ function updateCursor(cursor, changeFn, changeKey) {
     changeFn
   );
   var keyPath = cursor._keyPath || [];
-  cursor._onChange && cursor._onChange.call(
+  var result = cursor._onChange && cursor._onChange.call(
     undefined,
     newRootData,
     cursor._rootData,
     changeKey ? keyPath.concat(changeKey) : keyPath
   );
+  if (result !== undefined) {
+    newRootData = result;
+  }
   return makeCursor(newRootData, cursor._keyPath, cursor._onChange);
 }