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//
// TrackableScrollView.swift
// ToutiaoARDemo
//
// Created by Frad LEE on 2020/6/21.
// Copyright © 2020 Frad LEE. All rights reserved.
//
import SwiftUI
/// A trackable and scrollable view. Read [this link](https://medium.com/@maxnatchanon/swiftui-how-to-get-content-offset-from-scrollview-5ce1f84603ec) for more.
///
/// The trackable scroll view displays its content within the trackable scrollable content region.
///
/**
# Usage
``` swift
struct ContentView: View {
@State private var scrollViewContentOffset = CGFloat(0) // Content offset available to use
var body: some View {
TrackableScrollView(.vertical, showIndicators: false, contentOffset: $scrollViewContentOffset) {
...
}
}
}
```
*/
struct TrackableScrollView<Content>: View where Content: View {
let axes: Axis.Set
let showIndicators: Bool
@Binding var contentOffset: CGFloat
let content: Content
/// Creates a new instance that’s scrollable in the direction of the given axis and can show indicators while scrolling.
/// - Parameters:
/// - axes: The scrollable axes of the scroll view.
/// - showIndicators: A value that indicates whether the scroll view displays the scrollable component of the content offset, in a way that’s suitable for the platform.
/// - contentOffset: A value that indicates offset of content.
/// - content: The scroll view’s content.
init(_ axes: Axis.Set = .vertical, showIndicators: Bool = true, contentOffset: Binding<CGFloat>, @ViewBuilder content: () -> Content) {
self.axes = axes
self.showIndicators = showIndicators
_contentOffset = contentOffset
self.content = content()
}
var body: some View {
GeometryReader { outsideProxy in
ScrollView(self.axes, showsIndicators: self.showIndicators) {
ZStack(alignment: self.axes == .vertical ? .top : .leading) {
GeometryReader { insideProxy in
Color.clear
.preference(key: ScrollOffsetPreferenceKey.self, value: [self.calculateContentOffset(fromOutsideProxy: outsideProxy, insideProxy: insideProxy)])
}
VStack {
self.content
}
}
}
.onPreferenceChange(ScrollOffsetPreferenceKey.self) { value in
self.contentOffset = value[0]
}
}
}
private func calculateContentOffset(fromOutsideProxy outsideProxy: GeometryProxy, insideProxy: GeometryProxy) -> CGFloat {
if axes == .vertical {
return outsideProxy.frame(in: .global).minY - insideProxy.frame(in: .global).minY
} else {
return outsideProxy.frame(in: .global).minX - insideProxy.frame(in: .global).minX
}
}
}
struct ScrollOffsetPreferenceKey: PreferenceKey {
typealias Value = [CGFloat]
static var defaultValue: [CGFloat] = [0]
static func reduce(value: inout [CGFloat], nextValue: () -> [CGFloat]) {
value.append(contentsOf: nextValue())
}
}
@imran87

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@imran87 imran87 commented Dec 24, 2020

This message is printed in Xcode console.
Bound preference ScrollOffsetPreferenceKey tried to update multiple times per frame.

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