angularjs directive 中 scope '=?' 表达的是什么意思？
If the parent scope property doesn't exist, it will throw a NON_ASSIGNABLE_MODEL_EXPRESSION exception. You can avoid this behavior using =? or =?attr in order to flag the property as optional.
If you want to shallow watch for changes (i.e. $watchCollection instead of $watch) you can use =* or =*attr (=*? or =*?attr if the property is optional).
双向的数据绑定，你可以看看这里传送门