Add hasNext/PrevSnippetField getters

FEATURE: The new `hasNextSnippetField` and `hasPrevSnippetField` functions
can be used to figure out if the snippet-field-motion commands apply to a
given state.

See https://discuss.codemirror.net/t/snippet-autocompletion-fields-count/6447

src/README.md

@@ -54,8 +54,12 @@
 
 @nextSnippetField
 
+@hasNextSnippetField
+
 @prevSnippetField
 
+@hasPrevSnippetField
+
 @clearSnippet
 
 @snippetKeymap

src/index.ts

@@ -6,7 +6,8 @@ import {CompletionConfig, completionConfig} from "./config"
 import {completionPlugin, moveCompletionSelection, acceptCompletion, startCompletion, closeCompletion} from "./view"
 import {baseTheme} from "./theme"
 
-export {snippet, snippetCompletion, nextSnippetField, prevSnippetField, clearSnippet, snippetKeymap} from "./snippet"
+export {snippet, snippetCompletion, nextSnippetField, prevSnippetField,
+        hasNextSnippetField, hasPrevSnippetField, clearSnippet, snippetKeymap} from "./snippet"
 export {Completion, CompletionSection, CompletionContext, CompletionSource, CompletionResult, pickedCompletion,
         completeFromList, ifIn, ifNotIn, insertCompletionText} from "./completion"
 export {startCompletion, closeCompletion, acceptCompletion, moveCompletionSelection} from "./view"

src/snippet.ts

@@ -211,6 +211,20 @@ export const nextSnippetField = moveField(1)
 /// Move to the previous snippet field, if available.
 export const prevSnippetField = moveField(-1)
 
+/// Check if there is an active snippet with a next field for
+/// `nextSnippetField` to move to.
+export function hasNextSnippetField(state: EditorState) {
+  let active = state.field(snippetState, false)
+  return !!(active && active.ranges.some(r => r.field == active!.active + 1))
+}
+
+/// Returns true if there is an active snippet and a previous field
+/// for `prevSnippetField` to move to.
+export function hasPrevSnippetField(state: EditorState) {
+  let active = state.field(snippetState, false)
+  return !!(active && active.active > 0)
+}
+
 const defaultSnippetKeymap = [
   {key: "Tab", run: nextSnippetField, shift: prevSnippetField},
   {key: "Escape", run: clearSnippet}