更改var值的Kotlin數據類複製

Question

爲什麼不能複製具有var值更改的數據類對象？

data class AppState(var list: List<Image> = ArrayList<Image>(), 
       val uiState: UIState = UIState.LIST, 
       val isFetching: Boolean = false) 

... 

val list = state!!.list.sublist(0,1); 
state.copy(list = list)  // No change 
state.copy(isFetching = true) // Works like a charm 

Answer 1

copy方法不改變原始對象。它返回一個新的對象與更改的值。我想你的例子，這兩個複印件的工作，如預期時，我捕捉到一個新val返回的對象：

val list = state!!.list.subList(0,1) 

// Creates a new object with the new list. 
val withNewList = state.copy(list = list) 

// Creates a new object with the new isFetching. 
val withNewIsFetching = withNewList.copy(isFetching = true) 

如果你想你的突變state -object的列表值，你可以做這個：

val list = state!!.list.subList(0,1) 
state.list = list