feat: add connection previewer interface (#7637)

* feat: add connection previewer interface

* chore: PR comments

core/interfaces/i_connection_previewer.ts

@@ -0,0 +1,50 @@
+/**
+ * @license
+ * Copyright 2023 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type {BlockSvg} from '../block_svg';
+import type {RenderedConnection} from '../rendered_connection';
+
+/**
+ * Displays visual "previews" of where a block will be connected if it is
+ * dropped.
+ */
+export interface IConnectionPreviewer {
+  /**
+   * Display a connection preview where the draggedCon connects to the
+   * staticCon, replacing the replacedBlock (currently connected to the
+   * staticCon).
+   *
+   * @param draggedCon The connection on the block stack being dragged.
+   * @param staticCon The connection not being dragged that we are
+   *     connecting to.
+   * @param replacedBlock The block currently connected to the staticCon that
+   *     is being replaced.
+   */
+  previewReplacement(
+    draggedConn: RenderedConnection,
+    staticConn: RenderedConnection,
+    replacedBlock: BlockSvg,
+  ): void;
+
+  /**
+   * Display a connection preview where the draggedCon connects to the
+   * staticCon, and no block is being relaced.
+   *
+   * @param draggedCon The connection on the block stack being dragged.
+   * @param staticCon The connection not being dragged that we are
+   *     connecting to.
+   */
+  previewConnection(
+    draggedConn: RenderedConnection,
+    staticConn: RenderedConnection,
+  ): void;
+
+  /** Hide any previews that are currently displayed. */
+  hidePreview(): void;
+
+  /** Dispose of any references held by this connection previewer. */
+  dispose(): void;
+}