Add a new delegate method.

Pod/Classes/MWGridCell.m

@@ -158,6 +158,14 @@ - (void)setIsSelected:(BOOL)isSelected {
 }
 
 - (void)selectionButtonPressed {
+
+    if ([_gridController.browser.delegate respondsToSelector:@selector(photoBrowser:shouldSelectPhotoAtIndex:)]) {
+        BOOL shouldSelect = [_gridController.browser.delegate photoBrowser:_gridController.browser shouldSelectPhotoAtIndex:_index];
+        if (!shouldSelect && !_selectedButton.isSelected) {
+            return;
+        }
+    }
+
     _selectedButton.selected = !_selectedButton.selected;
     [_gridController.browser setPhotoSelected:_selectedButton.selected atIndex:_index];
 }

Pod/Classes/MWPhotoBrowser.h

@@ -35,6 +35,7 @@
 - (BOOL)photoBrowser:(MWPhotoBrowser *)photoBrowser isPhotoSelectedAtIndex:(NSUInteger)index;
 - (void)photoBrowser:(MWPhotoBrowser *)photoBrowser photoAtIndex:(NSUInteger)index selectedChanged:(BOOL)selected;
 - (void)photoBrowserDidFinishModalPresentation:(MWPhotoBrowser *)photoBrowser;
+- (BOOL)photoBrowser:(MWPhotoBrowser *)photoBrowser shouldSelectPhotoAtIndex:(NSUInteger)index;
 
 @end
 

Pod/Classes/MWPhotoBrowser.m

@@ -1161,6 +1161,12 @@ - (void)selectedButtonTapped:(id)sender {
         }
     }
     if (index != NSUIntegerMax) {
+        if ([self.delegate respondsToSelector:@selector(photoBrowser:shouldSelectPhotoAtIndex:)]) {
+            BOOL shouldSelect = [self.delegate photoBrowser:self shouldSelectPhotoAtIndex:index];
+            if (!shouldSelect && !selectedButton.isSelected) {
+                return;
+            }
+        }
         [self setPhotoSelected:selectedButton.selected atIndex:index];
     }
 }