文档中写道这个 key 要么是对象的键，要么是 *this 关键字，但二维数组这俩都不满足，咋写？尬住了啊...
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第一维：wx:key='index'
第二维：wx:key='idx' wx:for-index='idx'
如果搞不懂，一律写 wx:key='null'，完事。
wx:key="index" 是用索引给 key 赋值的意思吗？
key 看你数组啊 如果又唯一key就用，确保复用没问题了就
个人习惯 所有都是 wx:key="key"
想写什么写什么，一般没啥影响