[new feature] ImagePreview: add index slot (#3157)

packages/image-preview/test/__snapshots__/index.spec.js.snap

@@ -1,5 +1,14 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`index slot 1`] = `
+<div class="van-image-preview" name="van-fade">
+  <div class="van-image-preview__index">Custom Index</div>
+  <div class="van-swipe">
+    <div class="van-swipe__track" style="width: 0px; transition-duration: 0ms; transform: translateX(0px);"></div>
+  </div>
+</div>
+`;
+
 exports[`render image 1`] = `
 <div class="van-image-preview" name="van-fade">
   <div class="van-image-preview__index">1/3</div>

packages/image-preview/test/index.spec.js

@@ -1,12 +1,7 @@
 import Vue from 'vue';
 import ImagePreview from '..';
 import ImagePreviewVue from '../ImagePreview';
-import {
-  mount,
-  trigger,
-  triggerDrag,
-  transitionStub
-} from '../../../test/utils';
+import { mount, trigger, triggerDrag, transitionStub } from '../../../test/utils';
 
 transitionStub();
 
@@ -109,3 +104,15 @@ test('zoom', async () => {
   expect(wrapper).toMatchSnapshot();
   Element.prototype.getBoundingClientRect = getBoundingClientRect;
 });
+
+test('index slot', () => {
+  const wrapper = mount({
+    template: `
+      <van-image-preview :value="true">
+        <template v-slot:index>Custom Index</template>
+      </van-image-preview>
+    `
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});