Warn if 'moveSelectionForward/Backward' called with non-collapsed sel…

…ection

Summary:
Noticed that these helper methods are designed with the assumption they will
only be passed a collapsed selection, but I don't see anything enforcing that
requirement in the code. Since we will be tweaking these helpers and refactoring
things, want to make the code more sturdy and enforce this constraint.

Starting with a 'warning' in case something relies on broken behavior here, but
as a follow-up we should make this an invariant.

Reviewed By: vdurmont, mitermayer

Differential Revision: D9034617

fbshipit-source-id: 378390c308d1ab2657bea7a5cc9cae060e515658

src/component/handlers/edit/commands/moveSelectionBackward.js

@@ -15,6 +15,8 @@
 import type EditorState from 'EditorState';
 import type SelectionState from 'SelectionState';
 
+const warning = require('warning');
+
 /**
  * Given a collapsed selection, move the focus `maxDistance` backward within
  * the selected block. If the selection will go beyond the start of the block,
@@ -28,6 +30,11 @@ function moveSelectionBackward(
   maxDistance: number,
 ): SelectionState {
   const selection = editorState.getSelection();
+  // Should eventually make this an invariant
+  warning(
+    !selection.isCollapsed(),
+    'moveSelectionBackward should only be called with a collapsed SelectionState',
+  );
   const content = editorState.getCurrentContent();
   const key = selection.getStartKey();
   const offset = selection.getStartOffset();

src/component/handlers/edit/commands/moveSelectionForward.js

@@ -15,6 +15,8 @@
 import type EditorState from 'EditorState';
 import type SelectionState from 'SelectionState';
 
+const warning = require('warning');
+
 /**
  * Given a collapsed selection, move the focus `maxDistance` forward within
  * the selected block. If the selection will go beyond the end of the block,
@@ -28,6 +30,11 @@ function moveSelectionForward(
   maxDistance: number,
 ): SelectionState {
   const selection = editorState.getSelection();
+  // Should eventually make this an invariant
+  warning(
+    !selection.isCollapsed(),
+    'moveSelectionForward should only be called with a collapsed SelectionState',
+  );
   const key = selection.getStartKey();
   const offset = selection.getStartOffset();
   const content = editorState.getCurrentContent();