### What problem does this feature solve? 只提供前后改变的值,没有提供改变的索引?? ### What does the proposed API look like? vue.watch <!-- generated by vue-issues. DO NOT REMOVE --> 这样做审核类似的页面,做不下去了,都不知道是哪一行的索引改动了,难道每一行定制一个函数?