I am trying to insert a value that continuously changes into a slider. I want to see the slider move on its own as this value changes.
I have a slider structure set up and I tried inserting that value I want in it but it is not moving. I know that my variable value is changing as I want it to.
struct SliderCounter : View {
#State var timer1 : Float
var body: some View {
return Slider(value: $timer1, in : 0...100, step:1)
.padding()
}
}
SliderCounter(timer1: Float(changingValue))
I am calling this from another file but the slider is only showing up and not moving as the value changes
Since you are using SliderCounter as a reusable view, you need to user #Binding as this reusable view does not own the data.
Related
Here's what I want to do:
Have a SwiftUI view which changes a local State variable
On a button tap, pass that variable to some other part of my application
However, for some reason, even though I update the state variable, it doesn't get updated when it's passed to the next view.
Here's some sample code which shows the problem:
struct NumberView: View {
#State var number: Int = 1
#State private var showNumber = false
var body: some View {
NavigationStack {
VStack(spacing: 40) {
// Text("\(number)")
Button {
number = 99
print(number)
} label: {
Text("Change Number")
}
Button {
showNumber = true
} label: {
Text("Show Number")
}
}
.fullScreenCover(isPresented: $showNumber) {
SomeView(number: number)
}
}
}
}
struct SomeView: View {
let number: Int
var body: some View {
Text("\(number)")
}
}
If you tap on "Change Number", it updates the local state to 99. But when I create another view and pass this as a parameter, it shows 1 instead of 99. What's going on?
Some things to note:
If you uncomment Text("\(number)"), it works. But this shouldn't be necessary IMO.
It also works if you make SomeView use a binding. But for my app, this won't work. My actual use case is a 'select game options' view. Then, I will create a non-SwiftUI game view and I want to pass in these options as parameters. So, I can't have bindings all the way down my gaming code just because of this bug. I want to just capture what the user enters and create a Parameters object with that data.
It also works if you make it a navigationDestination instead of a fullScreenCover. ¯\(ツ)/¯ no idea on that one...
A View is a struct, therefore its properties are immutable, so the view can not change its own properties. This is why changing the property named number from inside the body of the view needs this property to be annotated with a #State property wrapper. Thanks to Swift and SwiftUI, transparent read and write callbacks let the value being seen changed. So you must not pass number as a parameter of SomeView() when calling fullScreenCover(), but pass a reference to number, for the callbacks to be systematically called: $number. Since you are not passing an integer anymore to construct struct SomeView, the type of the property named number in this struct can not any longer be an integer, but must be a reference to an integer (namely a binding): use the #Binding annotation for this.
So, replace SomeView(number: number) by SomeView(number: $number) and let number: Int by #Binding var number: Int to do the job.
Here is the correct source code:
import SwiftUI
struct NumberView: View {
#State var number: Int = 1
#State private var showNumber = false
var body: some View {
NavigationStack {
VStack(spacing: 40) {
// Text("\(number)")
Button {
number = 99
print(number)
} label: {
Text("Change Number")
}
Button {
showNumber = true
} label: {
Text("Show Number")
}
}
.fullScreenCover(isPresented: $showNumber) {
SomeView(number: $number)
}
}
}
}
struct SomeView: View {
#Binding var number: Int
var body: some View {
Text("\(number)")
}
}
After all that said to obtain a valid source code, their is a little trick that has not been explained up to now: if you simply replace in your source code Text("Change Number") by Text("Change Number \(number)"), without using $ reference nor #Binding keywords anywhere, you will see that the problem is also automatically solved! No need to use #binding in SomeView! This is because SwiftUI makes optimizations when building a tree of views. If it knows that the displayed view changed (not only its properties), it will compute the view with updated #State values. Adding number to the button label makes SwiftUI track changes of the number state property and it now updates its cached value to display the Text button label, therefore this new value will be correctly used to create SomeView. All of that may be considered as strange things, but is simply due to optimizations in SwiftUI. Apple does not fully explain how it implements optimizations building a tree of views, there are some informations given during WWDC events but the source code is not open. Therefore, you need to strictly follow the design pattern based on #State and #Binding to be sure that the whole thing works like it should.
All of that said again, one could argue that Apple says that you do not have to use #Binding to pass a value to a child view if this child view only wants to access the value: share the state with any child views that also need access, either directly for read-only access, or as a binding for read-write access (https://developer.apple.com/documentation/swiftui/state). This is right, but Apple says in the same article that you need to place [state] in the highest view in the view hierarchy that needs access to the value. With Apple, needing to access a value means that you need it to display the view, not only to do other computations that have no impact on the screen. This is this interpretation that lets Apple optimize the computation of the state property when it needs to update NumberView, for instance when computing the content of the Text("Change Number \(number)") line. You could find it really tricky. But there is a way to understand that: take the initial code you wrote, remove the #State in front of var number: Int = 1. To compile it, you need to move this line from inside the struct to outside, for instance at the very first line of your source file, just after the import declaration. And you will see that it works! This is because you do not need this value to display NumberView. And thus, it is perfectly legal to put the value higher, to build the view named SomeView. Be careful, here you do not want to update SomeView, so there is no border effects. But it would not work if you had to update SomeView.
Here is the code for this last trick:
import SwiftUI
// number is declared outside the views!
var number: Int = 1
struct NumberView: View {
// no more state variable named number!
// No more modification: the following code is exactly yours!
#State private var showNumber = false
var body: some View {
NavigationStack {
VStack(spacing: 40) {
// Text("\(number)")
Button {
number = 99
print(number)
} label: {
Text("Change Number")
}
Button {
showNumber = true
} label: {
Text("Show Number")
}
}
.fullScreenCover(isPresented: $showNumber) {
SomeView(number: number)
}
}
}
}
struct SomeView: View {
let number: Int
var body: some View {
Text("\(number)")
}
}
This is why you should definitely follow the #State and #Binding design pattern, taking into account that if you declare a state in a view that does not use it to display its content, you should declare this state as a #Binding in child views even if those children do not need to make changes to this state. The best way to use #State is to declare it in the highest view that needs it to display something: never forget that #State must be declared in the view that owns this variable; creating a view that owns a variable but that does not have to use it to display its content is an anti-pattern.
Since number isn't read in body, SwiftUI's dependency tracking detect it. You can give it a nudge like this:
.fullScreenCover(isPresented: $showNumber) { [number] in
Now a new closure will be created with the updated number value whenever number changes. Fyi the [number] in syntax is called a "capture list", read about it here.
Nathan Tannar gave me this explanation via another channel which I think gets to the crux of my problem. It does seem that this is a SwiftUI weirdness caused by knowing when and how it updates views based on state. Thanks Nathan!
It’s because the number isn’t “read” in the body of the view. SwiftUI is smart in that it only triggers view updates when a dependency of the view changes. Why this causes issues with the fullScreenCover modifier is because it captures an #escaping closure for the body. Which means it’s not read until the cover is presented. Since its not read the view body will not be re-evaluated when the #State changes, you can validate this by setting a breakpoint in the view body. Because the view body is not re-evaluated, the #escaping closure is never re-captured and thus it will hold a copy of the original value.
As a side note, you’ll find that once you present the cover for the first time and then dismiss, subsequent presentations will update correctly.
Arguably this seems like a SwiftUI bug, the fullScreenCover probably shouldn’t be #escaping. You can workaround by reading the number within the body, or wrapping the modifier with something like this, since here destination is not #escaping captured so the number will be read in the views body evaluation.
struct FullScreenModifier<Destination: View>: ViewModifier {
#Binding var isPresented: Bool
#ViewBuilder var destination: Destination
func body(content: Content) -> some View {
content
.fullScreenCover(isPresented: $isPresented) {
destination
}
}
}
So I'm doing something sort of jank, but I need to do it for a class that I am taking
I need to update the view after every step of a while loop that is inside of a button's action: callback, in order to create a really simple "animation"
is there any way to get a swift UI view to drop everything and redraw immediately?
code for my button:
Button(action: {
pass_again = true;
while(pass_again){
(right_text, pass_again) = twos.collapse(dir: Direction.right)
cur_id+=1
usleep(1000000)
// I need to redraw the UI here
}
twos.spawn()
}, label: {
Text(right_text)
}).padding(0.5)
You can bind an #State variable to any SwiftUI view i.e. Text(self.yourvar), so any time you change the #State value it will automatically update the text or any other property bonded with it.
About the animation you can use .withAnimation modifier to create a simple animation that swiftUI will perform
withAnimation {
your state var updates goes here
}
I have a card view which contains a list of TextFields that are drawn from CoreData, something like:
var body: some View {
ZStack {
ForEach(textsArray, id:\.self) { text in
TextFieldView(textBlock: text, editing: editing)
.onTapGesture(count: 1) {
selected.selectedText = text
}
}
}
}
The textfield stores its contents and color in CoreData.
This card is displayed by an Edit view. The edit view also contains a ColorPicker, which should allow you to modify the color of the selected text field. So, if the user taps on a text field and starts editing it, a color picker will appear in the corner of their screen to allow them to modify the color of that field.
I attempted to create an observable object to keep track of the selected text field:
class Selection:ObservableObject {
#Published var selectedText : TextBlock?
}
Then my edit view would simply keep track of the selected text:
#ObservedObject var selected : Selection = Selection()
It also passes it down into the card view.
The issue is that the ColorPicker view requires a binding to a CGColor. I'm not sure how to pass this binding to the ColorPicker: I tried this:
ColorPicker("", selection: self.selected.selectedText.$color)
But XCode tells me there's no member of selected text called $color, which I guess is because color is #NSManaged rather than a #State property.
How can I pass a binding of the Color property to the color picker? Am I even approaching this the right way? I'm brand new to iOS development so I have no idea what the idiomatic ways of doing things are.
You can separate it into other view and call like
if self.selected.selectedText != nil {
MyColorPicker(selection: self.selected.selectedText!)
}
where
struct MyColorPicker: View {
#ObservedObject var selection: TextBlock
var body: some View {
ColorPicker("", selection: self.$selection.color)
}
}
Trying to build a simple MacOS app using SwiftUI. I have a View that contains a Picker bound to a State var. As a sanity check I have added a Text Views (the dwarves one and the volumes...itemName) that should also change with the Picker changes. And they do, but the View I want to rerender (the FileList) does not.
I suspect it has something to do with how I am trying to pass the new FileSystemItem (internal class) to the FileList. Like when the FilePanel rerenders the volumeSelection is back to 0 and the state is applied afterwards. So my problem is that I seem to be missing a fundamental concept on how this data is supposed to flow. I have gone through the WWDC info again and been reading other articles but I am not seeing the answer.
The desired behavior is changing the selection on the picker should cause a new FileSystemItem to be displayed in the FileList view. What is the right way to get this to happen? To state it more generically, how to you get a child view to display new data when a Picker selection changes?
struct FilePanel: View
{
#State var volumeSelection = 0
#State var pathSelection = 0
var volumes = VolumesModel() //should be passed in?
var dwarves = ["Schlumpy","Wheezy","Poxxy","Slimy","Pervy","Drooly"]
var body: some View {
VStack {
Picker(selection: $volumeSelection, label:
Text("Volume")
, content: {
ForEach (0 ..< volumes.count()) {
Text(self.volumes.volumeAtIndex(index: $0).itemName).tag($0)
}
})
FileList(item:volumes.volumeAtIndex(index: volumeSelection)).frame(minWidth: CGFloat(100.0), maxHeight: .infinity)
Text(dwarves[volumeSelection])
Text(volumes.volumeAtIndex(index: volumeSelection).itemName)
}
}
}
struct FileList: View {
#State var item : FileSystemItem
var body: some View {
VStack {
List(item.childItems){fsi in
FileCell(expanded:false, item: fsi)
}
Text(item.itemName)
}
}
}
#State is a state private to to the owning view, FileList will never see any change.
If VolumesModel was a simple struct turning FileList.item into a binding (in-out-state) may already work (the caller still needs to turn it's #State into a binding when passing it to the dependent using the $):
struct FileList: View {
#Binding var item : FileSystemItem
...
}
However, it feels as if VolumesModel was a more complex object having an array of members.
If this is the case the above will not suffice:
VolumesModel needs to be a class adopting ObservableObject
VolumesModel's important members need a bindable wrapper (#Published)
FileList.item should be turned into #ObservedObject (instead of #State or #Binding)
FilePanel.volumes also to be #ObservedObject wrapped
Hope that helps or at least points you into the right direction.
I have a SwiftUI View where I declare a condition like this
#State var condition = UserDefaults.standard.bool(forKey: "JustKey")
When I first push this view into the navigation stack condition variable is getting the correct value. Then when I pop this View I change the value in UserDefaults but when I push this screen again condition variable remembers the old value which it got first time.
How to find a workaround for this because I want to reinitialize my condition variable each time I enter my custom view where I declared it?
In this case, #State is behaving exactly like it is supposed to: as a persistent store for the component it's attached to.
Fundamentally, pushing a view with a NavigationLink is like any other component in the view hierarchy. Whether or not the screen is actually visible is an implementation detail. While SwiftUI is not actually rendering your hidden UI elements after closing a screen, it does hold on to the View tree.
You can force a view to be completely thrown away with the .id(_:) modifier, for example:
struct ContentView: View {
#State var i = 0
var body: some View {
NavigationView {
VStack {
NavigationLink(destination: DetailView().id(i)) {
Text("Show Detail View")
}
Button("Toggle") {
self.i += 1
UserDefaults.standard.set(
!UserDefaults.standard.bool(forKey: "JustKey"),
forKey: "JustKey")
}
}
}
}
}
The toggle button both modifies the value for JustKey and increments the value that we pass to .id(i). When the singular argument to id(_:) changes, the id modifier tells SwiftUI that this is a different view, so when SwiftUI runs it's diffing algorithm it throws away the old one and creates a new one (with new #State variables). Read more about id here.
The explanation above provides a workaround, but it's not a good solution, IMO. A much better solution is to use an ObservableObject. It looks like this:
#ObservedObject condition = KeyPathObserver(\.JustKey, on: UserDefaults.standard)
You can find the code that implements the KeyPathObserver and a full working Playground example at this SO answer