iOS: add undo/redo support

Enable the undo/redo buttons on the keyboard, as well as the
Cmd-Z/Cmd-Shift-Z shortcuts.

For UIKit to support this, we need to add undo actions to
first responders undo manager. Since we don't know if Qt
has anything to undo/redo, we just enable the shortcuts
all the time. To do that, we trick NSUndoManager to always
have both an undo operation and a redo operation in the
stack.

Change-Id: I3294a962cc24f56585e7e515856142f3dda56d0a
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>

src/plugins/platforms/ios/qiostextresponder.mm

@@ -209,6 +209,9 @@
     if (UIView *accessoryView = static_cast<UIView *>(platformData.value(kImePlatformDataInputAccessoryView).value<void *>()))
         self.inputAccessoryView = [[[WrapperView alloc] initWithView:accessoryView] autorelease];
 
+    self.undoManager.groupsByEvent = NO;
+    [self rebuildUndoStack];
+
     return self;
 }
 
@@ -380,6 +383,56 @@
 
 // -------------------------------------------------------------------------
 
+- (void)undo
+{
+    [self sendKeyPressRelease:Qt::Key_Z modifiers:Qt::ControlModifier];
+    [self rebuildUndoStack];
+}
+
+- (void)redo
+{
+    [self sendKeyPressRelease:Qt::Key_Z modifiers:Qt::ControlModifier|Qt::ShiftModifier];
+    [self rebuildUndoStack];
+}
+
+- (void)registerRedo
+{
+    NSUndoManager *undoMgr = self.undoManager;
+    [undoMgr beginUndoGrouping];
+    [undoMgr registerUndoWithTarget:self selector:@selector(redo) object:nil];
+    [undoMgr endUndoGrouping];
+}
+
+- (void)rebuildUndoStack
+{
+    dispatch_async(dispatch_get_main_queue (), ^{
+        // Register dummy undo/redo operations to enable Cmd-Z and Cmd-Shift-Z
+        // Ensure we do this outside any undo/redo callback since NSUndoManager
+        // will treat registerUndoWithTarget as registering a redo when called
+        // from within a undo callback.
+        NSUndoManager *undoMgr = self.undoManager;
+        [undoMgr removeAllActions];
+        [undoMgr beginUndoGrouping];
+        [undoMgr registerUndoWithTarget:self selector:@selector(undo) object:nil];
+        [undoMgr endUndoGrouping];
+
+        // Schedule an operation that we immediately pop off to be able to schedule a redo
+        [undoMgr beginUndoGrouping];
+        [undoMgr registerUndoWithTarget:self selector:@selector(registerRedo) object:nil];
+        [undoMgr endUndoGrouping];
+        [undoMgr undo];
+
+        // Note that, perhaps because of a bug in UIKit, the buttons on the shortcuts bar ends up
+        // disabled if a undo/redo callback doesn't lead to a [UITextInputDelegate textDidChange].
+        // And we only call that method if Qt made changes to the text. The effect is that the buttons
+        // become disabled when there is nothing more to undo (Qt didn't change anything upon receiving
+        // an undo request). This seems to be OK behavior, so we let it stay like that unless it shows
+        // to cause problems.
+    });
+}
+
+// -------------------------------------------------------------------------
+
 - (void)keyCommandTriggered:(UIKeyCommand *)keyCommand
 {
     Qt::Key key = Qt::Key_unknown;