[new feature] ImagePreview: add change event (#3155)

packages/image-preview/ImagePreview.js

@@ -201,6 +201,7 @@ export default sfc({
     onChange(active) {
       this.resetScale();
       this.active = active;
+      this.$emit('change', active);
     },
 
     resetScale() {

packages/image-preview/en-US.md

@@ -109,6 +109,7 @@ export default {
 | Event | Description | Parameters |
 |------|------|------|
 | close | Triggered when close | { index, url } |
+| change | Triggered when current image change | index: index of current image |
 
 ### onClose Parematers
 

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

@@ -37,6 +37,7 @@ test('render image', () => {
   expect(wrapper.emitted('input')).toBeFalsy();
   triggerDrag(swipe, 0, 0);
   expect(wrapper.emitted('input')[0][0]).toBeFalsy();
+  expect(wrapper.emitted('change')[0][0]).toEqual(2);
 });
 
 test('async close', () => {

packages/image-preview/zh-CN.md

@@ -125,6 +125,7 @@ export default {
 | 事件 | 说明 | 回调参数 |
 |------|------|------|
 | close | 关闭时触发 | { index: 索引, url: 图片链接 } |
+| change | 切换当前图片时触发 | index, 当前图片的索引 |
 
 ### onClose 回调参数