previewBuilder property Null safety

ContextMenuPreviewBuilder? previewBuilder
final

A function that returns an alternative widget to show when the CupertinoContextMenu is open.

If not specified, child will be shown.

The preview is often used to show a slight variation of the child. For example, the child could be given rounded corners in the preview but have sharp corners when in the page.

In addition to the current BuildContext, the function is also called with an Animation and the child. The animation goes from 0 to 1 when the CupertinoContextMenu opens, and from 1 to 0 when it closes, and it can be used to animate the preview in sync with this opening and closing. The child parameter provides access to the child displayed when the CupertinoContextMenu is closed.

link
Below is an example of using previewBuilder to show an image tile that's similar to each tile in the iOS iPhoto app's context menu. Several of these could be used in a GridView for a similar effect.

When opened, the child animates to show its full aspect ratio and has rounded corners. The larger size of the open CupertinoContextMenu allows the FittedBox to fit the entire image, even when it has a very tall or wide aspect ratio compared to the square of a GridView, so this animates into view as the CupertinoContextMenu is opened. The preview is swapped in right when the open animation begins, which includes the rounded corners. 

CupertinoContextMenu(
  // The FittedBox in the preview here allows the image to animate its
  // aspect ratio when the CupertinoContextMenu is animating its preview
  // widget open and closed.
  previewBuilder: (BuildContext context, Animation<double> animation, Widget child) {
    return FittedBox(
      fit: BoxFit.cover,
      // This ClipRRect rounds the corners of the image when the
      // CupertinoContextMenu is open, even though it's not rounded when
      // it's closed. It uses the given animation to animate the corners
      // in sync with the opening animation.
      child: ClipRRect(
        borderRadius: BorderRadius.circular(64.0 * animation.value),
        child: Image.asset('assets/photo.jpg'),
      ),
    );
  },
  actions: <Widget>[
    CupertinoContextMenuAction(
      child: const Text('Action one'),
      onPressed: () {},
    ),
  ],
  child: FittedBox(
    fit: BoxFit.cover,
    child: Image.asset('assets/photo.jpg'),
  ),
)

Implementation

final ContextMenuPreviewBuilder? previewBuilder;