I need to know the current view width and height, I managed to get it using the following code but it works only if I manually select the NavigationView.
I also tried to include the GeometryReader { ... } part in another view but it modifies the position of other elements.
My question to you, is there a way to call NavigationView automatically (just in order to get the view sizes) and return to the main view without manual action?
Or is there another way to get the current view width and height?
struct NavigationView : View {
#ObservedObject var myMaze = Maze.sharedInstance
var body : some View {
GeometryReader { geometry in
Path { path in
myMaze.viewWidth = geometry.size.width
myMaze.viewHeight = geometry.size.height
}
}
}
}
Give this code a try, it works in my case:
For height:
view.getMeasuredHeight()
For width:
view.getMeasuredWidth()
Note: this code won't work in onCreate because the view hasn't been drawn yet, add a listener or a timer depending on your needs.
In SwiftUI, list view by default takes up entire height of the screen and pushes other elements/views to the bottom of the screen. But I want to append some elements/views where the list items exactly end.
You can add spacer() at appropriate places in the VStack or try something like below:
VStack {
CustomView1()
List {
Section(header: HeaderView(), footer: FooterView())
{
ForEach(viewModel.permissions) { permission in
CustomeView2()
}
GeneralView()//add the views at the end of list items
}
}.listStyle(GroupedListStyle())
}
I want to use NavigationView together with the ScrollView, but I am not seeing List items.
struct ContentView: View {
var body: some View {
NavigationView {
ScrollView{
VStack {
Text("Some stuff 1")
List{
Text("one").padding()
Text("two").padding()
Text("three").padding()
}
Text("Some stuff 2")
}
}
}
}
}
All I see is the text. If I remove ScrollView I see it all, but the text is being pushed to the very bottom. I simply want to be able to add List and Views in a nice scrollable page.
The ScrollView expects dimension from content, but List expects dimension from container - as you see there is conflict, so size for list is undefined, and a result rendering engine just drop it to avoid disambiguty.
The solution is to define some size to List, depending of your needs, so ScrollView would now how to lay out it, so scroll view could scroll entire content and list could scroll internal content.
Eg.
struct ContentView: View {
#Environment(\.defaultMinListRowHeight) var minRowHeight
var body: some View {
NavigationView {
ScrollView{
VStack {
Text("Some stuff 1")
List {
Text("one").padding()
Text("two").padding()
Text("three").padding()
}.frame(minHeight: minRowHeight * 3).border(Color.red)
Text("Some stuff 2")
}
}
}
}
}
Just wanted to throw out an answer that fixed what I was seeing very similar to the original problem - I had put a Label() item ahead of my List{ ... } section, and when I deleted that Label() { } I was able to see my List content again. Possibly List is buggy with other items surrounding it (Xcode 13 Beta 5).
I find that .listRowBackground(Color.Red.Blush) can set the list row column, but in the List there are some empty row are still white. When I scroll on the List, it also show some white area. Can somebody help to slove this peoblem? It's also appreciated for telling me how to change the color with the SegmentedPickerStyle. This video shows my problem. https://youtu.be/aEPw5hHBkFE
Thank you!
struct UserNotificationCellView: View {
#ObservedObject var userNotification: UserNotification
#ObservedObject var userNotifications: UserNotifications
var body: some View {
{
......
}
.padding(.horizontal,20)
.cornerRadius(14)
.listRowBackground(Color.Red.Blush)
// .background(Color.Red.Blush)
}
}
Below is my List code
List {
ForEach(userNotifications1.userNotificationsArray, id: \.notifyId) { (userNotification) in
UserNotificationCellView(userNotification: userNotification,userNotifications: self.userNotifications1)
// .listRowBackground(Color.Red.Blush)
}.colorMultiply(Color.Red.Blush)
Spacer()
}.environment(\.defaultMinListRowHeight, 80)
I'd recommend adding a global style in your AppDelegate class. From here you can change class properties that are currently inaccessible by SwiftUI via appearance.
UITableView.appearance().backgroundColor = .clear
UISegmentedControl.appearance().....
From here you can change your list background color and any segmented control properties you need to.
For the empty cells you can try adding an empty footer
List {
Section(footer: self.footer) {
ForEach(userNotifications1.userNotificationsArray, id: \.notifyId) { (userNotification) in
UserNotificationCellView(userNotification: userNotification,userNotifications: self.userNotifications1)
// .listRowBackground(Color.Red.Blush)
}.colorMultiply(Color.Red.Blush)
}
}.environment(\.defaultMinListRowHeight, 80)
I've been seeing some strange behavior for preference keys with ScrollView. If I put the onPreferenceChange inside the ScrollView it won't be called, but if I put it outside it does!
I've setup a width preference key as follows:
struct WidthPreferenceKey: PreferenceKey {
typealias Value = CGFloat
static var defaultValue = CGFloat(0)
static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
value = nextValue()
}
}
The following simple view does not print:
struct ContentView: View {
var body: some View {
ScrollView {
Text("Hello")
.preference(key: WidthPreferenceKey.self, value: 20)
.onPreferenceChange(WidthPreferenceKey.self) {
print($0) // Not being called, we're in a scroll view.
}
}
}
}
But this works:
struct ContentView: View {
var body: some View {
ScrollView {
Text("Hello")
.preference(key: WidthPreferenceKey.self, value: 20)
}
.onPreferenceChange(WidthPreferenceKey.self) {
print($0)
}
}
}
I know that I can use the latter approach to fix this, but sometimes I'm inside a child view that does not have access to its parent scroll view but I still want to record a preference key.
Any ideas on how to get onPreferenceChange to get called inside a ScrollView?
Note: I get Bound preference WidthPreferenceKey tried to update multiple times per frame. when I put the function inside the scroll view, which might explain what is going on but I can't figure it out.
Thanks!
I had been trying to figure out this issue for a long time and have found how to deal with it, although the way I used was just one of the workarounds.
Use onAppear to ScrollView with a flag to make its children show up.
...
#State var isShowingContent = false
...
ScrollView {
if isShowingContent {
ContentView()
}
}
.onAppear {
self.isShowingContent = true
}
Or,
Use List instead of it.
It has the scroll feature, and you can customize it with its own functionality and UITableView appearance in terms of UI. the most important is that it works as we expected.
[If you have time to read more]
Let me say my thought about that issue.
I have confirmed that onPreferenceChange isn't called at the bootstrap time of a view put inside a ScrollView. I'm not sure if it is the right behavior or not. But, I assume that it's wrong because ScrollView has to be capable of containing any views even if some of those use PreferenceKey to pass any data among views inside it. If it's the right behavior, it would be quite easy for us to get in trouble when creating our custom views.
Let's get into more detail.
I suppose that ScrollView would work slightly different from the other container views such as List, (H/V)Stack when it comes to set up its child view at the bootstrap time. In other words, ScrollView would try to draw(or lay out) children in its own way. Unfortunately, that way would affect the children's layout mechanism working incorrectly as what we've been seeing. We could guess what happened with the following message on debug view.
TestHPreferenceKey tried to update multiple times per frame.
It might be a piece of evidence to tell us that the update of children has occurred while ScrollView is doing something for its setup. At that moment, it could be guessed that the update to PreferenceKey has been ignored.
That's why I tried to put the placing child views off to onAppear.
I hope that will be useful for someone who's struggling with various issues on SwiftUI.
I think onPreferenceChange in your example is not called because it’s function is profoundly different from preference(key…)
preference(key:..) sets a preference value for the view it is used on.
whereas onPreferenceChange is a function called on a parent view – a view on a higher position in the view tree hierarchy. Its function is to go through all its children and sub-children and collect their preference(key:) values. When it found one it will use the reduce function from the PreferenceKey on this new value and all the already collected values. Once it has all the values collected and reduced them it will execute the onPreference closure on the result.
In your first example this closure is never called because the Text(“Hello”) view has no children which set the preference key value (in fact the view has no children at all). In your second example the Scroll view has a child which sets its preference value (the Text view).
All this does not explain the multiple times per frame error – which is most likely unrelated.
Recent update (24.4.2020):
In a similar case I could induce the call of onPreferenceChange by changing the Equatable condition for the PreferenceData. PreferenceData needs to be Equatable (probably to detect a change in them). However, the Anchor type by itself is not equatable any longer. To extract the values enclosed in an Anchor type a GeometryProxy is required. You get a GeometryProxy via a GeometryReader. For not disturbing the design of views by enclosing some of them into a GeometryReader I generated one in the equatable function of the PreferenceData struct:
struct ParagraphSizeData: Equatable {
let paragraphRect: Anchor<CGRect>?
static func == (value1: ParagraphSizeData, value2: ParagraphSizeData) -> Bool {
var theResult : Bool = false
let _ = GeometryReader { geometry in
generateView(geometry:geometry, equality:&theResult)
}
func generateView(geometry: GeometryProxy, equality: inout Bool) -> Rectangle {
let paragraphSize1, paragraphSize2: NSSize
if let anAnchor = value1.paragraphRect { paragraphSize1 = geometry[anAnchor].size }
else {paragraphSize1 = NSZeroSize }
if let anAnchor = value2.paragraphRect { paragraphSize2 = geometry[anAnchor].size }
else {paragraphSize2 = NSZeroSize }
equality = (paragraphSize1 == paragraphSize2)
return Rectangle()
}
return theResult
}
}
With kind regards
It seems like the issue is not necessarily with ScrollView, but with your usage of PreferenceKey. For instance, here is a sample struct in which a PreferenceKey is set according to the width of a Rectangle, and then printed using .onPreferenceChange(), all inside of a ScrollView. As you drag the Slider to change the width, the key is updated and the print closure is executed.
struct ContentView: View {
#State private var width: CGFloat = 100
var body: some View {
VStack {
Slider(value: $width, in: 100...200)
ScrollView(.vertical) {
Rectangle()
.background(WidthPreferenceKeyReader())
.onPreferenceChange(WidthPreferenceKey.self) {
print($0)
}
}
.frame(width: self.width)
}
}
}
struct WidthPreferenceKeyReader: View {
var body: some View {
GeometryReader { geometry in
Rectangle()
.fill(Color.clear)
.preference(key: WidthPreferenceKey.self, value: geometry.size.width)
}
}
}
As you noted, the first time the key tries to set, the console prints "Bound preference WidthPreferenceKey tried to update multiple times per frame," but a real value is immediately set afterward, and it continues to update dynamically.
What value are you actually trying to set, and what are you trying to do in .onPreferenceChange()?
I think this is because you implemented reduce() incorrectly.
You can find the details here:
https://stackoverflow.com/a/73300115/4366470
TL;DR: Replace value = nextValue() in reduce() with value += nextValue().
You may only read it in superView, but you can change it with transformPreference after you set it .
struct ContentView: View {
var body: some View {
ScrollView {
VStack{
Text("Hello")
.preference(key: WidthPreferenceKey.self, value: 20)
}.transformPreference(WidthPreferenceKey.self, {
$0 = 30})
}.onPreferenceChange(WidthPreferenceKey.self) {
print($0)
}
}
}
The last value is 30 now. Hope it is what you want.
You can read from other layer:
ScrollView {
Text("Hello").preference(key: WidthPreferenceKey.self, value: CGFloat(40.0))
.backgroundPreferenceValue(WidthPreferenceKey.self) { x -> Color in
print(x)
return Color.clear
}
}
The problem here is actually not in ScrollView but in usage - this mechanism allow to transfer data up in viewTree:
A view with multiple children automatically combines its values for a
given preference into a single value visible to its ancestors.
source
The keywords here - with multiple children. This mean that u can pass it in viewTree from child to parent.
Let's review u'r code:
struct ContentView: View {
var body: some View {
ScrollView {
Text("Hello")
.preference(key: WidthPreferenceKey.self, value: 20)
.onPreferenceChange(WidthPreferenceKey.self) {
print($0) // Not being called, we're in a scroll view.
}
}
}
}
As u can see now - child pass value to itself, and not to parent - so this don't want to work, as per design.
And working case:
struct ContentView: View {
var body: some View {
ScrollView {
Text("Hello")
.preference(key: WidthPreferenceKey.self, value: 20)
}
.onPreferenceChange(WidthPreferenceKey.self) {
print($0)
}
}
}
Here, ScrollView is parent and Text is child, and child talk to parent - everything works as expected.
So, as I sad in the beginning the problem here not in ScrollView but in usage and in Apple documentation (u need to read it few times as always).
And regarding this:
Bound preference WidthPreferenceKey tried to update multiple times per
frame.
This is because u may change multiply values in same time and View can't be rendered, try to .receive(on:) or DispatchQueue.main.async as workaround (I guess this may be a bug)