尽管List内部有元素，但List.map仍未执行(颤振) [英] List.map not executing despite List having elements inside(flutter)

问题描述

我正在为这个问题挠头，也许这很明显，但我不确定，我从firestore检索了一个querysnapshot，它返回了一个带有snapshot.docs的querydocumentsnapshot列表，但是当我尝试映射它时，什么也没有发生，如果我在for ... in循环中使用它，则效果会很好.

Hi i am scratching my head over this problem, maybe its something obvious but im not really sure, i retrieve a querysnapshot from firestore, which returns a list of querydocumentsnapshot with snapshot.docs, but when i try to map it nothing happens, if i use a for...in loop it would work perfect.

 QuerySnapshot snap =
    await FirebaseFirestore.instance.collection('Products').doc(FirebaseAuth.instance.currentUser.uid).collection('ItemDetails').get();
print(snap.docs);//returns [Instance of 'QueryDocumentSnapshot', Instance of 'QueryDocumentSnapshot', Instance of 'QueryDocumentSnapshot']

for (var item in snap.docs.toList()) {//this executes perfectly
  print(item.data());
}
snap.docs.toList().map((e) => print(e.data().toString()));//this wont execute
snap.docs.map((e) => print(e.data().toString()));//this wont execute

推荐答案

这是由于

This is because of the way map method works. According to the documentation -

map< T>方法

map<T> method

Iterable<T> map <T>(

    T f(
        E e
    )

) 

返回带有通过调用创建的元素的新的惰性Iterable按迭代顺序在此Iterable的每个元素上按f.

Returns a new lazy Iterable with elements that are created by calling f on each element of this Iterable in iteration order.

此方法返回映射元素的视图.只要返回的Iterable未迭代，提供的函数f将不被调用.转换后的元素将不会被缓存.反复进行返回的Iterable多次将调用提供的在同一个元素上多次执行f功能.

This method returns a view of the mapped elements. As long as the returned Iterable is not iterated over, the supplied function f will not be invoked. The transformed elements will not be cached. Iterating multiple times over the returned Iterable will invoke the supplied function f multiple times on the same element.

返回的iterable上的方法允许省略对任何方法的调用f不需要结果的元素.例如，elementAt可以调用f只有一次.

Methods on the returned iterable are allowed to omit calling f on any element where the result isn't needed. For example, elementAt may call f only once.

在您的示例中，因为您没有迭代生成的 Iterable ，所以它不会调用该函数.因此，如果您将代码更改为此

In your example, because you are not iterating over the resulting Iterable, it won't invoke the function. So if you change your code to this

snap.docs.map((e) => print(e.data().toString())).toList();

然后它将执行，因为现在它需要遍历生成的 Iterable 以便将其转换为 List .

then it will execute because now it needs to iterate over the resulting Iterable in order to convert it to a List.

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