这是一个可以按预期工作的简单SwiftUI列表:
struct App: View {
let items = Array(100...200)
var body: some View {
List {
ForEach(items, id: \.self) { index, item in
Text("Item \(item)")
}
}.frame(width: 200, height: 200)
}
}
但是当我尝试通过将items
替换为items.enumerated()
来枚举项目时,会出现以下错误:
在'ForEach'上引用初始化程序'init(_:id:content :)'要求'((offset:Int,element:Int)')符合'Hashable'
在'ForEach'上引用初始化程序'init(_:id:content :)'要求'EnumeratedSequence <[Int]>'符合'RandomAccessCollection'
我如何进行这项工作?
答案 0 :(得分:10)
TL; DR
警告:如果您习惯将enumerated()
与ForEach
一起使用,则有一天可能会遇到EXC_BAD_INSTRUCTION
或Fatal error: Index out of bounds
例外。这是因为并非所有集合都具有从0开始的索引。
更好的默认设置是改用zip
:
ForEach(Array(zip(items.indices, items)), id: \.0) { index, item in
// index and item are both safe to use here
}
(如果您的商品符合id: \.1
,也可以使用Identifiable
。)
从技术上讲,这不是最正确的方法。用其索引集合压缩
todos
数组会更正确,也更冗长。在这种情况下,我们是安全的,因为我们正在处理基于0的简单索引数组,但是如果我们在生产中进行此操作,则可能应该采用基于zip
的方法。
Apple的枚举函数文档也提到了这一点:
/// Returns a sequence of pairs (*n*, *x*), where *n* represents a
/// consecutive integer starting at zero and *x* represents an element of
/// the sequence.
///
/// This example enumerates the characters of the string "Swift" and prints
/// each character along with its place in the string.
///
/// for (n, c) in "Swift".enumerated() {
/// print("\(n): '\(c)'")
/// }
/// // Prints "0: 'S'"
/// // Prints "1: 'w'"
/// // Prints "2: 'i'"
/// // Prints "3: 'f'"
/// // Prints "4: 't'"
///
/// When you enumerate a collection, the integer part of each pair is a counter
/// for the enumeration, but is not necessarily the index of the paired value.
/// These counters can be used as indices only in instances of zero-based,
/// integer-indexed collections, such as `Array` and `ContiguousArray`. For
/// other collections the counters may be out of range or of the wrong type
/// to use as an index. To iterate over the elements of a collection with its
/// indices, use the `zip(_:_:)` function.
///
/// This example iterates over the indices and elements of a set, building a
/// list consisting of indices of names with five or fewer letters.
///
/// let names: Set = ["Sofia", "Camilla", "Martina", "Mateo", "Nicolás"]
/// var shorterIndices: [Set<String>.Index] = []
/// for (i, name) in zip(names.indices, names) {
/// if name.count <= 5 {
/// shorterIndices.append(i)
/// }
/// }
///
/// Now that the `shorterIndices` array holds the indices of the shorter
/// names in the `names` set, you can use those indices to access elements in
/// the set.
///
/// for i in shorterIndices {
/// print(names[i])
/// }
/// // Prints "Sofia"
/// // Prints "Mateo"
///
/// - Returns: A sequence of pairs enumerating the sequence.
///
/// - Complexity: O(1)
在特定情况下,enumerated()
可以使用,因为您使用的是基于0的索引数组,但是由于上面的详细信息,始终依靠enumerated()
可能会导致不明显错误。
以以下代码段为例:
ForEach(Array(items.enumerated()), id: \.offset) { offset, item in
Button(item, action: { store.didTapItem(at: offset) })
}
// ...
class Store {
var items: ArraySlice<String>
func didTapItem(at index: Int) {
print(items[index])
}
}
首先要注意的是,由于Button(item...
已保证可以直接访问enumerated()
而不会引起异常,因此我们在item
处添加了项目符号。但是,如果我们使用item
代替items[offset]
,则很容易引发异常。
最后,由于索引(实际上是偏移量)可能超出范围,因此print(items[index])
行很容易导致异常。
因此,一种更安全的方法是始终使用本文顶部提到的zip
方法。
更喜欢zip
的另一个原因是,如果您尝试将相同的代码用于不同的Collection(例如Set),则在索引到类型(items[index]
)时可能会遇到以下语法错误:< / p>
无法将“ Int”类型的值转换为预期的参数类型“ Set.Index”
通过使用基于zip
的方法,您仍然可以索引到集合中。
如果您打算经常使用它,也可以create an extension on collection。
您可以在Playground中进行全部测试:
import PlaygroundSupport
import SwiftUI
// MARK: - Array
let array = ["a", "b", "c"]
Array(array.enumerated()) // [(offset 0, element "a"), (offset 1, element "b"), (offset 2, element "c")]
Array(zip(array.indices, array)) // [(.0 0, .1 "a"), (.0 1, .1 "b"), (.0 2, .1 "c")]
let arrayView = Group {
ForEach(Array(array.enumerated()), id: \.offset) { offset, element in
PrintView("offset: \(offset), element: \(element)")
Text("value: \(array[offset])")
}
// offset: 0, element: a
// offset: 1, element: b
// offset: 2, element: c
ForEach(Array(zip(array.indices, array)), id: \.0) { index, element in
PrintView("index: \(index), element: \(element)")
Text("value: \(array[index])")
}
// index: 0, element: a
// index: 1, element: b
// index: 2, element: c
}
// MARK: - Array Slice
let arraySlice = array[1...2] // ["b", "c"]
Array(arraySlice.enumerated()) // [(offset 0, element "b"), (offset 1, element "c")]
Array(zip(arraySlice.indices, arraySlice)) // [(.0 1, .1 "b"), (.0 2, .1 "c")]
// arraySlice[0] // ❌ EXC_BAD_INSTRUCTION
arraySlice[1] // "b"
arraySlice[2] // "c"
let arraySliceView = Group {
ForEach(Array(arraySlice.enumerated()), id: \.offset) { offset, element in
PrintView("offset: \(offset), element: \(element)")
// Text("value: \(arraySlice[offset])") ❌ Fatal error: Index out of bounds
}
// offset: 0, element: b
// offset: 1, element: c
ForEach(Array(zip(arraySlice.indices, arraySlice)), id: \.0) { index, element in
PrintView("index: \(index), element: \(element)")
Text("value: \(arraySlice[index])")
}
// index: 1, element: b
// index: 2, element: c
}
// MARK: - Set
let set: Set = ["a", "b", "c"]
Array(set.enumerated()) // [(offset 0, element "b"), (offset 1, element "c"), (offset 2, element "a")]
Array(zip(set.indices, set)) // [({…}, .1 "a"), ({…}, .1 "b"), ({…}, .1 "c")]
let setView = Group {
ForEach(Array(set.enumerated()), id: \.offset) { offset, element in
PrintView("offset: \(offset), element: \(element)")
// Text("value: \(set[offset])") // ❌ Syntax error: Cannot convert value of type 'Int' to expected argument type 'Set<String>.Index'
}
// offset: 0, element: a
// offset: 1, element: b
// offset: 2, element: c
ForEach(Array(zip(set.indices, set)), id: \.0) { index, element in
PrintView("index: \(index), element: \(element)")
Text("value: \(set[index])")
}
// index: Index(_variant: Swift.Set<Swift.String>.Index._Variant.native(Swift._HashTable.Index(bucket: Swift._HashTable.Bucket(offset: 0), age: -481854246))), element: a
// index: Index(_variant: Swift.Set<Swift.String>.Index._Variant.native(Swift._HashTable.Index(bucket: Swift._HashTable.Bucket(offset: 2), age: -481854246))), element: b
// index: Index(_variant: Swift.Set<Swift.String>.Index._Variant.native(Swift._HashTable.Index(bucket: Swift._HashTable.Bucket(offset: 3), age: -481854246))), element: c
}
// MARK: -
struct PrintView: View {
init(_ string: String) {
print(string)
self.string = string
}
var string: String
var body: some View {
Text(string)
}
}
let allViews = Group {
arrayView
arraySliceView
setView
}
PlaygroundPage.current.setLiveView(allViews)
答案 1 :(得分:3)
Apple SwiftUI 示例中的一个 enumerated() 是在 Array 内部使用,然后您可以添加偏移量作为 id,这在您枚举 Array 时是唯一的。
ForEach(Array(data.enumerated()), id: \.offset) { index, observation in
答案 2 :(得分:2)
枚举此集合时,枚举中的每个元素都是一个类型的touple:
(offset: Int, element: Int)
因此id参数应从id: \.self
更改为id: \.element
。
ForEach(items.enumerated(), id: \.element) { ...
但是,在进行此更改后,您仍然会收到错误消息:
在'ForEach'上引用初始化程序'init(_:id:content :)'要求'EnumeratedSequence <[Int]>'符合'RandomAccessCollection'
因为ForEach
需要对数据进行随机访问,但是枚举仅允许按顺序访问。要解决此问题,请将枚举转换为数组。
ForEach(Array(items.enumerated()), id: \.element) { ...
这是一个扩展名,您可以使用它来简化一些操作:
extension Collection {
func enumeratedArray() -> Array<(offset: Int, element: Self.Element)> {
return Array(self.enumerated())
}
}
以及可以在(macos)Xcode游乐场中运行的示例:
import AppKit
import PlaygroundSupport
import SwiftUI
extension Collection {
func enumeratedArray() -> Array<(offset: Int, element: Self.Element)> {
return Array(self.enumerated())
}
}
struct App: View {
let items = 100...200
var body: some View {
List {
ForEach(items.enumeratedArray(), id: \.element) { index, item in
Text("\(index): Item \(item)")
}
}.frame(width: 200, height: 200)
}
}
PlaygroundPage.current.liveView = NSHostingView(rootView: App())
答案 3 :(得分:1)
在大多数情况下,您不需要enumerate
,因为它有点慢。
struct App: View {
let items = Array(100...200)
var body: some View {
List {
ForEach(items.indices, id: \.self) { index in
Text("Item \(self.items[index])")
}
}.id(items).frame(width: 200, height: 200)
}
}