fix: Ensure column cell values update when reordered

Fixes #808
Fixes #820

The idea behind this is basically the same as #567.

The value of `cellValue` is computed by grabbing the `columnValue.valuePath`
and then grabbing the value of the corresponding key on `rowValue`.
Basically something like `rowValue.${columnValue.valuePath}`, which is
not possible using any of the built in computed macros.

This basically uses an observer to watch for changes to
`columnValue.valuePath`, and defines/redefines `cellValue` as a computed
alias for the property at that path on `rowValue`. This ensures the
`cellValue` is correct if `columnValue.valuePath` or the actual value at
that path on `rowValue` changes. Observers aren't recommended but this
was already using one.

Previously this was solved by creating a more generic `dynamicAlias`
computed macro in #567. To be
honest, I was having a little trouble wrapping my head around all that
was happening in there but I think the changes in this PR accomplish the
same idea.

I'm not totally sure what the issue was with the other implementation
but, since the `dynamicAlias` macro was only used in this one place, it
feels OK to have a more simple implementation that is hard coded
specifically for this case.

addon/components/-private/row-wrapper.js

@@ -1,14 +1,23 @@
 import Component from '@ember/component';
 import hbs from 'htmlbars-inline-precompile';
 
-import EmberObject, { get, setProperties, computed } from '@ember/object';
+import EmberObject, { get, setProperties, computed, defineProperty } from '@ember/object';
+import { alias } from '@ember/object/computed';
 import { A as emberA } from '@ember/array';
 
+import { notifyPropertyChange } from '../../-private/utils/ember';
 import { objectAt } from '../../-private/utils/array';
-import { dynamicAlias } from '../../-private/utils/computed';
+import { observer } from '../../-private/utils/observer';
 
 const CellWrapper = EmberObject.extend({
-  cellValue: dynamicAlias('rowValue', 'columnValue.valuePath'),
+  /* eslint-disable-next-line ember/no-observers, ember-best-practices/no-observers */
+  columnValueValuePathDidChange: observer('columnValue.valuePath', function() {
+    let columnValuePath = get(this, 'columnValue.valuePath');
+    let cellValue = columnValuePath ? alias(`rowValue.${columnValuePath}`) : null;
+
+    defineProperty(this, 'cellValue', cellValue);
+    notifyPropertyChange(this, 'cellValue');
+  }),
 
   cellMeta: computed('rowMeta', 'columnValue', function() {
     let rowMeta = get(this, 'rowMeta');

tests/integration/components/headers/reorder-test.js

@@ -69,12 +69,40 @@ module('Integration | headers | reorder', function() {
       await generateTable(this);
 
       await table.headers.objectAt(0).reorderBy(1);
-      assert.equal(table.headers.objectAt(0).text.trim(), 'B', 'First column is swapped forward');
-      assert.equal(table.headers.objectAt(1).text.trim(), 'A', 'Second column is swapped backward');
+      assert.equal(
+        table.headers.objectAt(0).text.trim(),
+        'B',
+        'First column header is swapped forward'
+      );
+      assert.equal(table.getCell(0, 0).text.trim(), '0B', 'First column cells are swapped forward');
+      assert.equal(
+        table.headers.objectAt(1).text.trim(),
+        'A',
+        'Second column header is swapped backward'
+      );
+      assert.equal(
+        table.getCell(0, 1).text.trim(),
+        '0A',
+        'Second column cells are swapped forward'
+      );
 
       await table.headers.objectAt(1).reorderBy(-1);
-      assert.equal(table.headers.objectAt(1).text.trim(), 'B', 'Second column is swapped backward');
-      assert.equal(table.headers.objectAt(0).text.trim(), 'A', 'First column is swapped forward');
+      assert.equal(
+        table.headers.objectAt(1).text.trim(),
+        'B',
+        'Second column header is swapped backward'
+      );
+      assert.equal(
+        table.getCell(0, 1).text.trim(),
+        '0B',
+        'Second column cells are swapped forward'
+      );
+      assert.equal(
+        table.headers.objectAt(0).text.trim(),
+        'A',
+        'First column header is swapped forward'
+      );
+      assert.equal(table.getCell(0, 0).text.trim(), '0A', 'First column cells are swapped forward');
     });
 
     test('column reorder action is sent up to controller', async function(assert) {