WWDC20 swiftUI新增LazyVGrid和LazyHGrid两种布局方式，我们可以使用它们做网格布局。
autoFlex.png
虽然这些新组件解锁了非常强大的布局，但SwiftUI还没有提供UICollectionView那样的灵活性。
我指的是在同一个容器中有不同大小的多个视图的可能性，并在没有更多可用空间时使容器自动换行到下一行。
在本文中，让我们探索如何构建我们自己的FlexibleView，这里是最终结果的预览:
flexible.gif
介绍
从上面的预览应该很清楚我们的目标是什么，让我们看看我们的视图要怎么实现它：
- 获得水平方向的可用空间
- 获取每个元素的size
- 一种将每个元素分配到正确位置的方法
获取Size of View
这个文章将使用SwiftUI:GeometryReader一文中的扩展方法：
extension View {
func readSize(onChange: @escaping (CGSize) -> Void) -> some View {
background(
GeometryReader { geometryProxy in
Color.clear
.preference(key: SizePreferenceKey.self, value: geometryProxy.size)
}
)
.onPreferenceChange(SizePreferenceKey.self, perform: onChange)
}
}
private struct SizePreferenceKey: PreferenceKey {
static var defaultValue: CGSize = .zero
static func reduce(value: inout CGSize, nextValue: () -> CGSize) {}
}
1.获得水平方向的可用空间
FlexibleView需要的第一个信息是总水平可用空间:
用Color做个例子
var body: some View {
Color.clear
.frame(height: 1)
.readSize { size in
// the horizontal available space is size.width
}
}
因为第一个组件仅用于获取布局信息，所以我们使用Color.clear。清晰有效地，它是一个不可见的层，不会阻挡视图的其余部分。
我们也可以设置一个.frame修饰符限制Color的高为1，确保视图组件有足够的高度。
Color不是视图层次结构的一部分，我们可以用ZStack隐藏它:
var body: some View {
ZStack {
Color.clear
.frame(height: 1)
.readSize { size in
// the horizontal available space is size.width
}
// Rest of our implementation
}
}
最后，让我们利用回调从readSize存储我们的可用水平空间在FlexibleView中:
struct FlexibleView: View {
@State private var availableWidth: CGFloat = 0
var body: some View {
ZStack {
Color.clear
.frame(height: 1)
.readSize { size in
availableWidth = size.width
}
// Rest of our implementation
}
}
}
在这一点上，我们有一个视图，它填满了所有可用的水平空间，并且只在高度上取一点。我们可以进入第二步。
2.获取每个元素的size
在讨论如何获取每个元素大小之前，让我们先设置视图来接受元素。
为了简单起见，也为了后面更清楚，我们将要求:
-
Collection集合中的元素实现Hashable协议 - 一个方法，给定该集合的一个元素，该方法返回一个视图View
struct FlexibleView<Data: Collection, Content: View>: View
where Data.Element: Hashable {
let data: Data
let content: (Data.Element) -> Content
// ...
var body: some View {
// ...
}
}
让我们忘记最终的布局，只关注每个元素的大小:
struct FlexibleView<...>: View where ... {
let data: Data
let content: (Data.Element) -> Content
@State private var elementsSize: [Data.Element: CGSize] = [:]
// ...
var body: some View {
ZStack {
// ...
ForEach(data, id: \.self) { element in
content(element)
.fixedSize()
.readSize { size in
elementsSize[element] = size
}
}
}
}
}
注意我们是如何在元素视图上使用.fixedsize修饰符的，让它根据需要占用尽可能多的空间，而不管实际有多少空间可用。
这样，我们就有了每个元素的大小!是时候面对最后一步了。
3.一种将每个元素分配到正确位置的方法
这就是所有FlexibleView需要将元素视图分布到多行中:
struct FlexibleView<...>: View where ... {
// ...
func computeRows() -> [[Data.Element]] {
var rows: [[Data.Element]] = [[]]
var currentRow = 0
var remainingWidth = availableWidth
for element in data {
let elementSize = elementSizes[element, default: CGSize(width: availableWidth, height: 1)]
if remainingWidth - elementSize.width >= 0 {
rows[currentRow].append(element)
} else {
// start a new row
currentRow = currentRow + 1
rows.append([element])
remainingWidth = availableWidth
}
remainingWidth = remainingWidth - elementSize.width
}
return rows
}
}
computeRows将所有元素分布在多行中，同时保持元素的顺序，并确保每一行的宽度不超过之前获得的可用宽度。
换句话说，该函数返回一个行数组，其中每行包含该行的元素数组。
然后，我们可以将这个新函数与HStacks和Vstack结合起来，得到最终的布局:
struct FlexibleView<...>: View where ... {
// ...
var body: some View {
ZStack {
// ...
VStack {
ForEach(computeRows(), id: \.self) { rowElements in
HStack {
ForEach(rowElements, id: \.self) { element in
content(element)
.fixedSize()
.readSize { size in
elementsSize[element] = size
}
}
}
}
}
}
}
// ...
}
在这一点上，
FlexibleView将只采取这个VStack的高度
有了这个，我们就结束了!最终的项目还处理了元素之间的间距和不同的排列:一旦理解了上面的基本原理，添加这些特性就变得很简单了。
完整代码：
//ContentView.swift
import SwiftUI
class ContentViewModel: ObservableObject {
@Published var originalItems = [
"Here’s", "to", "the", "crazy", "ones", "the", "misfits", "the", "rebels", "the", "troublemakers", "the", "round", "pegs", "in", "the", "square", "holes", "the", "ones", "who", "see", "things", "differently", "they’re", "not", "fond", "of", "rules", "You", "can", "quote", "them", "disagree", "with", "them", "glorify", "or", "vilify", "them", "but", "the", "only", "thing", "you", "can’t", "do", "is", "ignore", "them", "because", "they", "change", "things", "they", "push", "the", "human", "race", "forward", "and", "while", "some", "may", "see", "them", "as", "the", "crazy", "ones", "we", "see", "genius", "because", "the", "ones", "who", "are", "crazy", "enough", "to", "think", "that", "they", "can", "change", "the", "world", "are", "the", "ones", "who", "do"
]
@Published var spacing: CGFloat = 8
@Published var padding: CGFloat = 8
@Published var wordCount: Int = 75
@Published var alignmentIndex = 0
var words: [String] {
Array(originalItems.prefix(wordCount))
}
let alignments: [HorizontalAlignment] = [.leading, .center, .trailing]
var alignment: HorizontalAlignment {
alignments[alignmentIndex]
}
}
struct ContentView: View {
@StateObject var model = ContentViewModel()
var body: some View {
ScrollView {
FlexibleView(
data: model.words,
spacing: model.spacing,
alignment: model.alignment
) { item in
Text(verbatim: item)
.padding(8)
.background(
RoundedRectangle(cornerRadius: 8)
.fill(Color.gray.opacity(0.2))
)
}
.padding(.horizontal, model.padding)
}
.overlay(Settings(model: model), alignment: .bottom)
}
}
struct Settings: View {
@ObservedObject var model: ContentViewModel
let alignmentName: [String] = ["leading", "center", "trailing"]
var body: some View {
VStack {
Stepper(value: $model.wordCount, in: 0...model.originalItems.count) {
Text("\(model.wordCount) words")
}
HStack {
Text("Padding")
Slider(value: $model.padding, in: 0...60) { Text("") }
}
HStack {
Text("Spacing")
Slider(value: $model.spacing, in: 0...40) { Text("") }
}
HStack {
Text("Alignment")
Picker("Choose alignment", selection: $model.alignmentIndex) {
ForEach(0..<model.alignments.count) {
Text(alignmentName[$0])
}
}
.pickerStyle(SegmentedPickerStyle())
}
Button {
model.originalItems.shuffle()
} label: {
Text("Shuffle")
}
}
.padding()
.background(Color(UIColor.systemBackground))
.clipShape(RoundedRectangle(cornerRadius: 20))
.overlay(
RoundedRectangle(cornerRadius: 20)
.stroke(Color.primary, lineWidth: 4)
)
.padding()
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
//FlexibleView.swift
struct FlexibleView<Data: Collection, Content: View>: View where Data.Element: Hashable {
let data: Data
let spacing: CGFloat
let alignment: HorizontalAlignment
let content: (Data.Element) -> Content
@State private var availableWidth: CGFloat = 0
var body: some View {
ZStack(alignment: Alignment(horizontal: alignment, vertical: .center)) {
Color.clear
.frame(height: 1)
.readSize { size in
availableWidth = size.width
}
_FlexibleView(
availableWidth: availableWidth,
data: data,
spacing: spacing,
alignment: alignment,
content: content
)
}
}
}
//_FlexibleView.swift
struct _FlexibleView<Data: Collection, Content: View>: View where Data.Element: Hashable {
let availableWidth: CGFloat
let data: Data
let spacing: CGFloat
let alignment: HorizontalAlignment
let content: (Data.Element) -> Content
@State var elementsSize: [Data.Element: CGSize] = [:]
var body : some View {
VStack(alignment: alignment, spacing: spacing) {
ForEach(computeRows(), id: \.self) { rowElements in
HStack(spacing: spacing) {
ForEach(rowElements, id: \.self) { element in
content(element)
.fixedSize()
.readSize { size in
elementsSize[element] = size
}
}
}
}
}
}
func computeRows() -> [[Data.Element]] {
var rows: [[Data.Element]] = [[]]
var currentRow = 0
var remainingWidth = availableWidth
for element in data {
let elementSize = elementsSize[element, default: CGSize(width: availableWidth, height: 1)]
if remainingWidth - (elementSize.width + spacing) >= 0 {
rows[currentRow].append(element)
} else {
currentRow = currentRow + 1
rows.append([element])
remainingWidth = availableWidth
}
remainingWidth = remainingWidth - (elementSize.width + spacing)
}
return rows
}
}
//SizeReader.swift
extension View {
func readSize(onChange: @escaping (CGSize) -> Void) -> some View {
background(
GeometryReader { geometryProxy in
Color.clear
.preference(key: SizePreferenceKey.self, value: geometryProxy.size)
}
)
.onPreferenceChange(SizePreferenceKey.self, perform: onChange)
}
}
private struct SizePreferenceKey: PreferenceKey {
static var defaultValue: CGSize = .zero
static func reduce(value: inout CGSize, nextValue: () -> CGSize) {}
}
