Show a toast when you've gone over the max media selection.

res/values/strings.xml

@@ -440,6 +440,10 @@
 
     <!-- MediaPickerItemFragment -->
     <string name="MediaPickerItemFragment_tap_to_select">Tap to select</string>
+    <plurals name="MediaPickerItemFragment_cant_share_more_than_n_items">
+        <item quantity="one">You can\'t share more than %d item.</item>
+        <item quantity="other">You can\'t share more than %d items.</item>
+    </plurals>
 
     <!-- MediaSendActivity -->
     <string name="MediaSendActivity_add_a_caption">Add a caption...</string>

src/org/thoughtcrime/securesms/mediasend/MediaPickerItemAdapter.java

@@ -134,6 +134,8 @@ public class MediaPickerItemAdapter extends RecyclerView.Adapter<MediaPickerItem
           if (selected.size() < maxSelection) {
             selected.add(media);
             eventListener.onMediaSelectionChanged(new ArrayList<>(selected));
+          } else {
+            eventListener.onMediaSelectionOverflow(maxSelection);
           }
         });
       }
@@ -147,5 +149,6 @@ public class MediaPickerItemAdapter extends RecyclerView.Adapter<MediaPickerItem
   interface EventListener {
     void onMediaChosen(@NonNull Media media);
     void onMediaSelectionChanged(@NonNull List<Media> media);
+    void onMediaSelectionOverflow(int maxSelection);
   }
 }

src/org/thoughtcrime/securesms/mediasend/MediaPickerItemFragment.java

@@ -22,6 +22,7 @@ import android.view.View;
 import android.view.ViewGroup;
 import android.view.Window;
 import android.view.WindowManager;
+import android.widget.Toast;
 
 import org.thoughtcrime.securesms.R;
 import org.thoughtcrime.securesms.mms.GlideApp;
@@ -167,6 +168,11 @@ public class MediaPickerItemFragment extends Fragment implements MediaPickerItem
     viewModel.onSelectedMediaChanged(selected);
   }
 
+  @Override
+  public void onMediaSelectionOverflow(int maxSelection) {
+    Toast.makeText(requireContext(), getResources().getQuantityString(R.plurals.MediaPickerItemFragment_cant_share_more_than_n_items, maxSelection, maxSelection), Toast.LENGTH_SHORT).show();
+  }
+
   private void initToolbar(Toolbar toolbar) {
     ((AppCompatActivity) requireActivity()).setSupportActionBar(toolbar);
     ((AppCompatActivity) requireActivity()).getSupportActionBar().setDisplayHomeAsUpEnabled(true);