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SwiftPamphletApp/Resource/Guide/SwiftUI/数据集合组件/ForEach(ap).md
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| ## 使用 | ||
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| 在 SwiftUI 中，`ForEach` 是一个结构体，它可以创建一组视图，每个视图都有一个与数据集中的元素相对应的唯一标识符。这对于在列表或其他集合视图中显示数据非常有用。 | ||
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| 以下视图集会用到 ForEach： | ||
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| - List | ||
| - ScrollView | ||
| - LazyVStack / LazyHStack | ||
| - Picker | ||
| - Grids (LazyVGrid / LazyHGrid) | ||
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| 例如，如果你有一个 `BookmarkModel` 的数组，并且你想为每个书签创建一个文本视图，你可以这样做： | ||
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| ```swift | ||
| struct ContentView: View { | ||
| var bookmarks: [BookmarkModel] | ||
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| var body: some View { | ||
| List { | ||
| ForEach(bookmarks) { bookmark in | ||
| Text(bookmark.name) | ||
| } | ||
| } | ||
| } | ||
| } | ||
| ``` | ||
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| `ForEach` 遍历 `bookmarks` 数组，并为每个 `BookmarkModel` 对象创建一个 `Text` 视图。`bookmark` 参数是当前遍历的 `BookmarkModel` 对象。 | ||
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| `BookmarkModel` 必须遵循 `Identifiable` 协议，这样 SwiftUI 才能知道如何唯一地标识每个视图。在你的代码中，`BookmarkModel` 已经有一个 `id` 属性，所以你只需要让 `BookmarkModel` 遵循 `Identifiable` 协议即可： | ||
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| ```swift | ||
| final class BookmarkModel: Identifiable { | ||
| // your code here | ||
| } | ||
| ``` | ||
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| ## 使用索引范围进行编号 | ||
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| 你可以使用 `ForEach` 结构体的另一个版本，它接受一个范围作为其数据源。这个范围可以是一个索引范围，这样你就可以为每个项目编号。 | ||
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| 例如，如果你有一个 `BookmarkModel` 的数组，并且你想为每个书签创建一个文本视图，并在前面添加一个编号，你可以这样做： | ||
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| ```swift | ||
| struct ContentView: View { | ||
| var bookmarks: [BookmarkModel] | ||
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| var body: some View { | ||
| List { | ||
| ForEach(bookmarks.indices, id: \.self) { index in | ||
| Text("\(index + 1). \(bookmarks[index].name)") | ||
| } | ||
| } | ||
| } | ||
| } | ||
| ``` | ||
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| 在这个例子中，`ForEach` 遍历 `bookmarks` 数组的索引，并为每个 `BookmarkModel` 对象创建一个 `Text` 视图。`index` 参数是当前遍历的索引。我们使用 `\(index + 1). \(bookmarks[index].name)` 来创建一个带有编号的文本视图。请注意，我们使用 `index + 1` 而不是 `index`，因为数组的索引是从 0 开始的，但我们通常希望编号是从 1 开始的。 | ||
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| ## 使用 enumerated 编号 | ||
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| `enumerated()` | ||
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| 以下是一个例子： | ||
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| ```swift | ||
| struct ContentView: View { | ||
| var bookmarks: [BookmarkModel] | ||
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| var body: some View { | ||
| List { | ||
| ForEach(Array(bookmarks.enumerated()), id: \.element.id) { index, bookmark in | ||
| Text("\(index). \(bookmark.name)") | ||
| } | ||
| } | ||
| } | ||
| } | ||
| ``` | ||
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| 我们使用 `Array(bookmarks.enumerated())` 来创建一个元组数组，每个元组包含一个索引和一个 `BookmarkModel` 对象。然后，我们使用 `ForEach` 遍历这个元组数组，并为每个元组创建一个 `Text` 视图。`index` 参数是当前遍历的索引，`bookmark` 参数是当前遍历的 `BookmarkModel` 对象。 | ||
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| ## 使用 zip 编号 | ||
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| `zip(_:_:)` 函数可以将两个序列合并为一个元组序列。你可以使用这个函数和 `ForEach` 结构体来为数组中的每个元素添加一个编号。 | ||
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| 例如，如果你有一个 `BookmarkModel` 的数组，并且你想为每个书签创建一个文本视图，并在前面添加一个编号，你可以这样做： | ||
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| ```swift | ||
| struct ContentView: View { | ||
| var bookmarks: [BookmarkModel] | ||
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| var body: some View { | ||
| List { | ||
| ForEach(Array(zip(1..., bookmarks)), id: \.1.id) { index, bookmark in | ||
| Text("\(index). \(bookmark.name)") | ||
| } | ||
| } | ||
| } | ||
| } | ||
| ``` | ||
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| 写出扩展，方便调用 | ||
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| ```swift | ||
| @dynamicMemberLookup | ||
| struct Numbered<Element> { | ||
| var number: Int | ||
| var element: Element | ||
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| subscript<T>(dynamicMember keyPath: WritableKeyPath<Element, T>) -> T { | ||
| get { element[keyPath: keyPath] } | ||
| set { element[keyPath: keyPath] = newValue } | ||
| } | ||
| } | ||
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| extension Sequence { | ||
| func numbered(startingAt start: Int = 1) -> [Numbered<Element>] { | ||
| zip(start..., self) | ||
| .map { Numbered(number: $0.0, element: $0.1) } | ||
| } | ||
| } | ||
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| extension Numbered: Identifiable where Element: Identifiable { | ||
| var id: Element.ID { element.id } | ||
| } | ||
| ``` | ||
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| 使用： | ||
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| ```swift | ||
| ForEach(bookmark.numbered()) { numberedBookmark in | ||
| Text("\(numberedBookmark.number). \(numberedBookmark.name)") | ||
| } | ||
| ``` |
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