Add Share button to the SelectableRegion toolbar on Android (#141447)

## Description

This PR adds the share button to text selection toolbar buttons on Android ~~and iOS~~ for `SelectableRegion` (and therefore `SelectionArea`).

https://github.com/flutter/flutter/pull/139479 adds this button for `EditableText` (which is used by `TextField` and `SelectableText` but not by `SelectionArea`).

**Edit**: supporting this on iOS will need more work (see https://github.com/flutter/flutter/pull/141447#issuecomment-1889942622 and https://github.com/flutter/flutter/issues/141775).

## Related Issue

Follow up for https://github.com/flutter/flutter/issues/138728

## Tests

Adds 1 test.

packages/flutter/lib/src/cupertino/adaptive_text_selection_toolbar.dart

@@ -152,6 +152,7 @@ class CupertinoAdaptiveTextSelectionToolbar extends StatelessWidget {
          selectionGeometry: selectionGeometry,
          onCopy: onCopy,
          onSelectAll: onSelectAll,
+         onShare: null, // See https://github.com/flutter/flutter/issues/141775.
        );
 
   /// {@macro flutter.material.AdaptiveTextSelectionToolbar.anchors}

packages/flutter/lib/src/material/adaptive_text_selection_toolbar.dart

@@ -150,6 +150,7 @@ class AdaptiveTextSelectionToolbar extends StatelessWidget {
     super.key,
     required VoidCallback onCopy,
     required VoidCallback onSelectAll,
+    required VoidCallback? onShare,
     required SelectionGeometry selectionGeometry,
     required this.anchors,
   }) : children = null,
@@ -157,6 +158,7 @@ class AdaptiveTextSelectionToolbar extends StatelessWidget {
          selectionGeometry: selectionGeometry,
          onCopy: onCopy,
          onSelectAll: onSelectAll,
+         onShare: onShare,
        );
 
   /// Create an instance of [AdaptiveTextSelectionToolbar] with the default

packages/flutter/lib/src/widgets/selectable_region.dart

@@ -267,9 +267,29 @@ class SelectableRegion extends StatefulWidget {
     required final SelectionGeometry selectionGeometry,
     required final VoidCallback onCopy,
     required final VoidCallback onSelectAll,
+    required final VoidCallback? onShare,
   }) {
     final bool canCopy = selectionGeometry.status == SelectionStatus.uncollapsed;
     final bool canSelectAll = selectionGeometry.hasContent;
+    final bool platformCanShare = switch (defaultTargetPlatform) {
+      TargetPlatform.android
+        => selectionGeometry.status == SelectionStatus.uncollapsed,
+      TargetPlatform.macOS
+      || TargetPlatform.fuchsia
+      || TargetPlatform.linux
+      || TargetPlatform.windows
+        => false,
+      // TODO(bleroux): the share button should be shown on iOS but the share
+      // functionality requires some changes on the engine side because, on iPad,
+      // it needs an anchor for the popup.
+      // See: https://github.com/flutter/flutter/issues/141775.
+      TargetPlatform.iOS
+        => false,
+    };
+    final bool canShare = onShare != null && platformCanShare;
+
+    // On Android, the share button is before the select all button.
+    final bool showShareBeforeSelectAll = defaultTargetPlatform == TargetPlatform.android;
 
     // Determine which buttons will appear so that the order and total number is
     // known. A button's position in the menu can slightly affect its
@@ -280,11 +300,21 @@ class SelectableRegion extends StatefulWidget {
           onPressed: onCopy,
           type: ContextMenuButtonType.copy,
         ),
+      if (canShare && showShareBeforeSelectAll)
+        ContextMenuButtonItem(
+          onPressed: onShare,
+          type: ContextMenuButtonType.share,
+        ),
       if (canSelectAll)
         ContextMenuButtonItem(
           onPressed: onSelectAll,
           type: ContextMenuButtonType.selectAll,
         ),
+      if (canShare && !showShareBeforeSelectAll)
+        ContextMenuButtonItem(
+          onPressed: onShare,
+          type: ContextMenuButtonType.share,
+        ),
     ];
   }
 
@@ -1089,6 +1119,14 @@ class SelectableRegionState extends State<SelectableRegion> with TextSelectionDe
     await Clipboard.setData(ClipboardData(text: data.plainText));
   }
 
+  Future<void> _share() async {
+    final SelectedContent? data = _selectable?.getSelectedContent();
+    if (data == null) {
+      return;
+    }
+    await SystemChannels.platform.invokeMethod('Share.invoke', data.plainText);
+  }
+
   /// {@macro flutter.widgets.EditableText.getAnchors}
   ///
   /// See also:
@@ -1191,7 +1229,7 @@ class SelectableRegionState extends State<SelectableRegion> with TextSelectionDe
       onCopy: () {
         _copy();
 
-        // In Android copy should clear the selection.
+        // On Android copy should clear the selection.
         switch (defaultTargetPlatform) {
           case TargetPlatform.android:
           case TargetPlatform.fuchsia:
@@ -1217,6 +1255,22 @@ class SelectableRegionState extends State<SelectableRegion> with TextSelectionDe
             hideToolbar();
         }
       },
+      onShare: () {
+        _share();
+
+        // On Android, share should clear the selection.
+        switch (defaultTargetPlatform) {
+          case TargetPlatform.android:
+          case TargetPlatform.fuchsia:
+            _clearSelection();
+          case TargetPlatform.iOS:
+            hideToolbar(false);
+          case TargetPlatform.linux:
+          case TargetPlatform.macOS:
+          case TargetPlatform.windows:
+            hideToolbar();
+        }
+      },
     )..addAll(_textProcessingActionButtonItems);
   }