为什么地图不起作用，而foreach却起作用? [英] Why is map not working but foreach is?

问题描述

此 forEach 正常运行

var newMarkers = new List<Marker>();
providers.forEach((p) {
  var marker = markerFrom(p);
  newMarkers.add(marker);
  print("got one");
});

_markers = newMarkers;

，但是将此 map 放置在与 forEach 完全相同的位置时，永远不会被调用:

but this map doesn't ever get called when placed in the exact same place as the forEach:

_markers = providers.map((p) => markerFrom(p));

此外，这是 markerFrom 方法:

  Marker markerFrom(FoodProvider provider) {
  var marker = new Marker(new MarkerOptions()
    ..map = _map
    ..position = new LatLng(provider.latitude, provider.longitude)
    ..title = provider.name
    ..icon = 'http://maps.google.com/mapfiles/ms/icons/red-dot.png'
  );

  var infoWindow = new InfoWindow(new InfoWindowOptions()..content = marker.title);

  marker.onClick.listen((e) {
    infoWindow.open(_map, marker);
  });

  return marker;
}

推荐答案

Iterable 上的 map 函数是 lazy .它只是在原始的 Iterable 周围创建了一个包装器，但是直到您开始迭代时，它实际上才做任何事情.

The map function on Iterable is lazy. It just creates a wrapper around the original Iterable, but it doesn't actually do anything until you start iterating.

这是一件好事，即使您习惯于使用其他语言的急切的 map 函数，这也可能令人惊讶.这意味着您可以结合对可迭代对象的操作，例如:

That's a good thing, even if it can be surprising if you are used to an eager map function from other languages. It means that you can combine operations on iterables like:

listOfStrings
    .map((x) => complicatedOperationOnString(x))
    .take(2)
    .forEach(print);

这仅对前两个字符串执行复杂的操作.

This only does the complicated operation on the first two strings.

如果需要标记列表，则需要在 map 之后调用 toList :

If you want a list of the markers, you need to call toList after the map:

_markers = providers.map(markerFrom).toList();

通常，当传递给 map 的函数具有只希望发生一次的副作用时，应格外小心.如果多次迭代 map 的结果，则每次都会发生这种效果，因此，如果这样做:

In general, you should be very careful when the function passed to map has side-effects that you want to only happen once. If you iterate the result of map more than once, that effect will happen each time, so if you did:

_markers = providers.map(markerFrom);
int count = _markers.length;
_markers.forEach((marker) { doSomething(marker); });

您会冒风险发生两次副作用，一次是发生在 length 上，一次是发生在 forEach 上，这都会迭代 _markers .它并不总是会发生(某些可迭代对象在没有实际迭代的情况下知道它们的 length )，但这总是有风险的.在这些情况下，如果仅是副作用，则使用 forEach ，或者立即执行 toList 强制执行所有操作，然后仅查看之后的结果列表.

you would risk the side effects happening twice, once for length and once for forEach, which both iterate _markers. It doesn't always happen (some iterables know their length without actually iterating), but it is always a risk. In those cases, either use forEach if the side-effect is the only thing you are after, or do an immediate toList to force all the operations, and then only look at the resulting list after that.

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