I'm quite new to SwiftUI and I'm wondering how I should use a picker in an update view correctly.
At the moment I have a form and load the data in with .onAppear(). That works fine but when I try to pick something and go back to the update view the .onAppear() gets called again and I loose the picked value.
In the code it looks like this:
import SwiftUI
struct MaterialUpdateView: View {
// Bindings
#State var material: Material
// Form Values
#State var selectedUnit = ""
var body: some View {
VStack(){
List() {
Section(header: Text("MATERIAL")){
// Picker for the Unit
Picker(selection: $selectedUnit, label: Text("Einheit")) {
ForEach(API().units) { unit in
Text("\(unit.name)").tag(unit.name)
}
}
}
}
.listStyle(GroupedListStyle())
}
.onAppear(){
prepareToUpdate()
}
}
func prepareToUpdate() {
self.selectedUnit = self.material.unit
}
}
Does anyone has experience with that problem or am I doing something terribly wrong?
You need to create a custom binding which we will implement in another subview. This subview will be initialised with the binding vars selectedUnit and material
First, make your MaterialUpdateView:
struct MaterialUpdateView: View {
// Bindings
#State var material : Material
// Form Values
#State var selectedUnit = ""
var body: some View {
NavigationView {
VStack(){
List() {
Section(header: Text("MATERIAL")) {
MaterialPickerView(selectedUnit: $selectedUnit, material: $material)
}
.listStyle(GroupedListStyle())
}
.onAppear(){
prepareToUpdate()
}
}
}
}
func prepareToUpdate() {
self.selectedUnit = self.material.unit
}
}
Then, below, add your MaterialPickerView, as shown:
Disclaimer: You need to be able to access your API() from here, so move it or add it in this view. As I have seen that you are re-instanciating it everytime, maybe it is better that you store its instance with let api = API() and then refer to it with api, and even pass it to this view as such!
struct MaterialPickerView: View {
#Binding var selectedUnit: String
#Binding var material : Material
#State var idx: Int = 0
var body: some View {
let binding = Binding<Int>(
get: { self.idx },
set: {
self.idx = $0
self.selectedUnit = API().units[self.idx].name
self.material.unit = self.selectedUnit
})
return Picker(selection: binding, label: Text("Einheit")) {
ForEach(API().units.indices) { i in
Text(API().units[i].name).tag(API().units[i].name)
}
}
}
}
That should do,let me know if it works!
Related
I'm new to swiftui and doing an experiment with pushing and popping views with a stack. When I pop a view off the stack, the #State variable of the prior view has been reset and I don't understand why.
This demo code was tested on macos.
import SwiftUI
typealias Push = (AnyView) -> ()
typealias Pop = () -> ()
struct PushKey: EnvironmentKey {
static let defaultValue: Push = { _ in }
}
struct PopKey: EnvironmentKey {
static let defaultValue: Pop = {() in }
}
extension EnvironmentValues {
var push: Push {
get { self[PushKey.self] }
set { self[PushKey.self] = newValue }
}
var pop: Pop {
get { self[PopKey.self] }
set { self[PopKey.self] = newValue }
}
}
struct ContentView: View {
#State private var stack: [AnyView]
var body: some View {
currentView()
.environment(\.push, push)
.environment(\.pop, pop)
.frame(width: 600.0, height: 400.0)
}
public init() {
_stack = State(initialValue: [AnyView(AAA())])
}
private func currentView() -> AnyView {
if stack.count == 0 {
return AnyView(Text("stack empty"))
}
return stack.last!
}
public func push(_ content: AnyView) {
stack.append(content)
}
public func pop() {
stack.removeLast()
}
}
struct AAA : View {
#State private var data = "default text"
#Environment(\.push) var push
var body: some View {
VStack {
TextEditor(text: $data)
Button("Push") {
self.push(AnyView(BBB()))
}
}
}
}
struct BBB : View {
#Environment(\.pop) var pop
var body: some View {
VStack {
Button("Pop") {
self.pop()
}
}
}
}
If I type some text into the editor then hit Push, then Pop out of that view, I was expecting the text editor to maintain my changes but it reverts to the default text.
What am I missing?
Edit:
I guess this is really a question of how are NavigationView and NavigationLink implemented. This simple code does the what I'm trying to do:
import SwiftUI
struct MyView: View {
#State var text = "default text"
var body: some View {
VStack {
TextEditor(text: $text)
NavigationLink(destination: MyView()) {
Text("Push")
}
}
}
}
struct ContentView: View {
var body: some View {
NavigationView {
MyView()
}
}
}
run that on iOS so you get a nav stack. edit the text, then push. Edit again if you want, then go back and see state is retained.
My code is trying to do the same thing in principle.
I'll share this attempt maybe it will help you create your version of this.
This all started with an attempt to create something like NavigationView and NavigationLink but being able to back track to a random View in the stack
I have a protocol where an object returns a View. Usually it is an enum. The view() references a View with a switch that provides the correct child View. The ContentView/MainView works almost like a storyboard and just presents whatever is designated in the current or path variables.
//To make the View options generic
protocol ViewOptionsProtocol: Equatable {
associatedtype V = View
#ViewBuilder func view() -> V
}
This is the basic navigation router that keep track of the main view and the NavigationLink/path. Which looks similar to what you want to do.
//A generic Navigation Router
class ViewNavigationRouter<T: ViewOptionsProtocol>: ObservableObject{
//MARK: Variables
var home: T
//Keep track of your current screen
#Published private (set) var current: T
//Keep track of the path
#Published private (set) var path: [T] = []
//MARK: init
init(home: T, current: T){
self.home = home
self.current = current
}
//MARK: Functions
//Control how you get to the screen
///Navigates to the nextScreen adding to the path/cookie crumb
func push(nextScreen: T){
//This is a basic setup just going forward
path.append(nextScreen)
}
///Goes back one step in the path/cookie crumb
func pop(){
//Use the stored path to go back
_ = path.popLast()
}
///clears the path/cookie crumb and goes to the home screen
func goHome(){
path.removeAll()
current = home
}
///Clears the path/cookie crumb array
///sets the current View to the desired screen
func show(nextScreen: T){
goHome()
current = nextScreen
}
///Searches in the path/cookie crumb for the desired View in the latest position
///Removes the later Views
///sets the nextScreen
func dismissTo(nextScreen: T){
while !path.isEmpty && path.last != nextScreen{
pop()
}
if path.isEmpty{
show(nextScreen: nextScreen)
}
}
}
It isn't an #Environment but it can easily be an #EnvrionmentObject and all the views have to be in the enum so the views are not completely unknown but it is the only way I have been able to circumvent AnyView and keep views in an #ViewBuilder.
I use something like this as the main portion in the main view body
router.path.last?.view() ?? router.current.view()
Here is a simple implementation of your sample
import SwiftUI
class MyViewModel: ViewNavigationRouter<MyViewModel.ViewOptions> {
//In some view router concepts the data that is /preserved/shared among the views is preserved in the router itself.
#Published var preservedData: String = "preserved"
init(){
super.init(home: .aaa ,current: .aaa)
}
enum ViewOptions: String, ViewOptionsProtocol, CaseIterable{
case aaa
case bbb
#ViewBuilder func view() -> some View{
ViewOptionsView(option: self)
}
}
struct ViewOptionsView: View{
let option: ViewOptions
var body: some View{
switch option {
case .aaa:
AAA()
case .bbb:
BBB()
}
}
}
}
struct MyView: View {
#StateObject var router: MyViewModel = .init()
var body: some View {
NavigationView{
ScrollView {
router.path.last?.view() ?? router.current.view()
}
.toolbar(content: {
//Custom back button
ToolbarItem(placement: .navigationBarLeading, content: {
if !router.path.isEmpty {
Button(action: {
router.pop()
}, label: {
HStack(alignment: .center, spacing: 2, content: {
Image(systemName: "chevron.backward")
if router.path.count >= 2{
Text(router.path[router.path.count - 2].rawValue)
}else{
Text(router.current.rawValue)
}
})
})
}
})
})
.navigationTitle(router.path.last?.rawValue ?? router.current.rawValue)
}.environmentObject(router)
}
}
struct MyView_Previews: PreviewProvider {
static var previews: some View {
MyView()
}
}
struct AAA : View {
//This will reset because the view is cosmetic. the data needs to be preserved somehow via either persistence or in the router for sharing with other views.
#State private var data = "default text"
#EnvironmentObject var vm: MyViewModel
var body: some View {
VStack {
TextEditor(text: $data)
TextEditor(text: $vm.preservedData)
Button("Push") {
vm.push(nextScreen: .bbb)
}
}
}
}
struct BBB : View {
#EnvironmentObject var vm: MyViewModel
var body: some View {
VStack {
Button("Pop") {
vm.pop()
}
}
}
}
I know that State wrappers are for View and they designed for this goal, but I wanted to try build and test some code if it is possible, my goal is just for learning purpose,
I have 2 big issues with my code!
Xcode is unable to find T.
How can I initialize my state?
import SwiftUI
var state: State<T> where T: StringProtocol = State(get: { state }, set: { newValue in state = newValue })
struct ContentView: View {
var body: some View {
Text(state)
}
}
Update: I could do samething for Binding here, Now I want do it for State as well with up code
import SwiftUI
var state2: String = String() { didSet { print(state2) } }
var binding: Binding = Binding.init(get: { state2 }, set: { newValue in state2 = newValue })
struct ContentView: View {
var body: some View {
TextField("Enter your text", text: binding)
}
}
If I could find the answer of my issue then, i can define my State and Binding both outside of View, 50% of this work done and it need another 50% for State Wrapper.
New Update:
import SwiftUI
var state: State<String> = State.init(initialValue: "Hello") { didSet { print(state.wrappedValue) } }
var binding: Binding = Binding.init(get: { state.wrappedValue }, set: { newValue in state = State(wrappedValue: newValue) })
struct ContentView: View {
var body: some View {
Text(state) // <<: Here is the issue!
TextField("Enter your text", text: binding)
}
}
Even if you create a State wrapper outside a view, how will the view know when to refresh its body?
Without a way to notify the view, your code will do the same as:
struct ContentView: View {
var body: some View {
Text("Hello")
}
}
What you can do next depends on what you want to achieve.
If all you need is a way to replicate the State behaviour outside the view, I recommend you take a closer look at the Combine framework.
An interesting example is CurrentValueSubject:
var state = CurrentValueSubject<String, Never>("state1")
It stores the current value and also acts as a Publisher.
What will happen if we use it in a view that doesn't observe anything?
struct ContentView: View {
var body: some View {
Text(state.value)
.onAppear {
DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
state.value = "state2"
}
}
}
}
The answer is: nothing. The view is drawn once and, even if the state changes, the view won't be re-drawn.
You need a way to notify the view about the changes. In theory you could do something like:
var state = CurrentValueSubject<String, Never>("state1")
struct ContentView: View {
#State var internalState = ""
var body: some View {
Text(internalState)
.onAppear {
DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
state.value = "state2"
}
}
.onReceive(state) {
internalState = $0
}
}
}
But this is neither elegant nor clean. In these cases we should probably use #State:
struct ContentView: View {
#State var state = "state1"
var body: some View {
Text(state)
.onAppear {
DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
state = "state2"
}
}
}
}
To sum up, if you need a view to be refreshed, just use the native SwiftUI property wrappers (like #State). And if you need to declare state values outside the view, use ObservableObject + #Published.
Otherwise there is a huge Combine framework which does exactly what you want. I recommend you take a look at these links:
Combine: Getting Started
Using Combine
I am building a complex interface in SwiftUI that I need to break into multiple extensions in order to be able to compile the code, but I can't figure out how to pass data between the extension and the body structure.
I made a simple code to explain it :
class Search: ObservableObject {
#Published var angle: Int = 10
}
struct ContentView: View {
#ObservedObject static var search = Search()
var body: some View {
VStack {
Text("\(ContentView.self.search.angle)")
aTest()
}
}
}
extension ContentView {
struct aTest: View {
var body: some View {
ZStack {
Button(action: { ContentView.search.angle = 11}) { Text("Button")}
}
}
}
}
When I press the button the text does not update, which is my issue. I really appreciate any help you can provide.
You can try the following:
struct ContentView: View {
#ObservedObject var search = Search()
var body: some View {
VStack {
Text("\(ContentView.self.search.angle)")
aTest // call as a computed property
}
}
}
extension ContentView {
var aTest: some View { // not a separate `struct` anymore
ZStack {
Button(action: { self.search.angle = 11 }) { Text("Button")}
}
}
}
I'm trying to call a method of a child view which includes clearing some of its fields. When the method is called from a parent view, nothing happens. However, calling the method from the child view will clear its field. Here is some example code:
struct ChildView: View {
#State var response = ""
var body: some View {
TextField("", text: $response)
}
func clear() {
self.response = ""
}
}
struct ParentView: View {
private var child = ChildView()
var body: some View {
HStack {
self.child
Button(action: {
self.child.clear()
}) {
Text("Clear")
}
}
}
}
Can someone tell me why this happens and how to fix it/work around it? I can't directly access the child view's response because there are too many fields in my actual code and that would clutter it up too much.
SwiftUI view is not a reference-type, you cannot create it once, store in var, and then access it - SwiftUI view is a struct, value type, so storing it like did you work with copies it values, ie
struct ParentView: View {
private var child = ChildView() // << original value
var body: some View {
HStack {
self.child // created copy 1
Button(action: {
self.child.clear() // created copy 2
}) {
Here is a correct SwiftUI approach to construct parent/child view - everything about child view should be inside child view or injected in it via init arguments:
struct ChildView: View {
#State private var response = ""
var body: some View {
HStack {
TextField("", text: $response)
Button(action: {
self.clear()
}) {
Text("Clear")
}
}
}
func clear() {
self.response = ""
}
}
struct ParentView: View {
var body: some View {
ChildView()
}
}
Try using #Binding instead of #State. Bindings are a way of communicating state changes down to children.
Think of it this way: #State variables are used for View specific state. They are usually made private for this reason. If you need to communicate anything down, then #Binding is the way to do it.
struct ChildView: View {
#Binding var response: String
var body: some View {
TextField("", text: $response)
}
}
struct ParentView: View {
#State private var response = ""
var body: some View {
HStack {
ChildView(response: $response)
Button(action: {
self.clear()
}) {
Text("Clear")
}
}
}
private func clear() {
self.response = ""
}
}
I have created a View that provides a convinient save button and a save method. Both can then be used inside a parent view.
The idea is to provide these so that the navigation bar items can be customized, but keep the original implementation.
Inside the view there is one Textfield which is bound to a #State variable. If the save method is called from within the same view everthing works as expected. If the parent view calls the save method on the child view, the changes to the #State variable are not applied.
Is this a bug in SwiftUI, or am I am missing something? I've created a simple playbook implementation that demonstrates the issue.
Thank you for your help.
import SwiftUI
import PlaygroundSupport
struct ContentView: View {
// Create the child view to make the save button available inside this view
var child = Child()
var body: some View {
NavigationView {
NavigationLink(
destination: child.navigationBarItems(
// Set the trailing button to the one from the child view.
// This is required as this view might be inside a modal
// sheet, and we need to add the cancel button as a leading
// button:
// leading: self.cancelButton
trailing: child.saveButton
)
) {
Text("Open")
}
}
}
}
struct Child: View {
// Store the value from the textfield
#State private var value = "default"
// Make this button available inside this view, and inside the parent view.
// This makes sure the visibility of this button is always the same.
var saveButton: some View {
Button(action: save) {
Text("Save")
}
}
var body: some View {
VStack {
// Simple textfield to allow a string to change.
TextField("Value", text: $value)
// Just for the playground to change the value easily.
// Usually it would be chnaged through the keyboard input.
Button(action: {
self.value = "new value"
}) {
Text("Update")
}
}
}
func save() {
// This always displays the default value of the state variable.
// Even after the Update button was used and the value did change inside
// the textfield.
print("\(value)")
}
}
PlaygroundPage.current.setLiveView(ContentView())
I think a more SwiftUi way of doing it:
import SwiftUI
import PlaygroundSupport
struct ContentView: View {
var body: some View {
return NavigationView {
// tell the child view where to render it's navigation item
// Instead of configuring navigation items.
NavigationLink(destination: Child(navigationSide: .left)) {
Text("Open")
}
}
}
}
struct Child: View {
enum NavigationSide { case left, right }
// If you really want to encapsulate all state in this view then #State
// is a good choice.
// If the parent view needs to read it, too, #Binding would be your friend here
#State private var value: String = "default"
// no need for #State as it's never changed from here.
var navigationSide = NavigationSide.right
// wrap in AnyView here to make ternary in ui code easier readable.
var saveButton: AnyView {
AnyView(Button(action: save) {
Text("Save")
})
}
var emptyAnyView: AnyView { AnyView(EmptyView()) }
var body: some View {
VStack {
TextField("Value", text: $value)
Button(action: {
self.value = "new value"
}) {
Text("Update")
}
}
.navigationBarItems(leading: navigationSide == .left ? saveButton : emptyAnyView,
trailing: navigationSide == .right ? saveButton : emptyAnyView)
}
func save() {
print("\(value)")
}
}
TextField will only update your value binding when the return button is pressed. To get text changes that occur during editing, set up an observed object on Child with didSet. This was the playground I altered used from your example.
struct ContentView: View {
var child = Child()
var body: some View {
NavigationView {
NavigationLink(
destination: child.navigationBarItems(
trailing: child.saveButton
)
) {
Text("Open")
}
}
}
}
class TextChanges: ObservableObject {
var completion: (() -> ())?
#Published var text = "default" {
didSet {
print(text)
}
}
}
struct Child: View {
#ObservedObject var textChanges = TextChanges()
var saveButton: some View {
Button(action: save) {
Text("Save")
}
}
var body: some View {
VStack {
TextField("Value", text: $textChanges.text).multilineTextAlignment(.center)
Button(action: {
print(self.textChanges.text)
}) {
Text("Update")
}
}
}
func save() {
print("\(textChanges.text)")
}
}
PlaygroundPage.current.setLiveView(ContentView())
Inside Child: value is mutable because it's wrapped with #State.
Inside ContentView: child is immutable because it's not wrapped with #State.
Your issue can be fixed with this line: #State var child = Child()
Good luck.
Child view needs to keep its state as a #Binding. This works:
import SwiftUI
import PlaygroundSupport
struct ContentView: View {
#State var v = "default"
var body: some View {
let child = Child(value: $v)
return NavigationView {
NavigationLink(
destination: child.navigationBarItems(trailing: child.saveButton)
) {
Text("Open")
}
}
}
}
struct Child: View {
#Binding var value: String
var saveButton: some View {
Button(action: save) {
Text("Save")
}
}
var body: some View {
VStack {
TextField("Value", text: $value)
Button(action: {
self.value = "new value"
}) {
Text("Update")
}
}
}
func save() {
print("\(value)")
}
}
PlaygroundPage.current.setLiveView(ContentView())
Based on this commend from #nine-stones (thank you!) I implemented a more SwiftUI way so solve my problem. It does not allow the customization of the navigation items as I planned, but that was not the problem that needed to be solved. I wanted to use the Child view in a navigation link, as well as inside a modal sheet. The problem was how to perform custom cancel actions. This is why I removed the button implementation and replaced it with a cancelAction closure. Now I can display the child view wherever and however I want.
One thing I still do not know why SwiftUI is not applying the child context to the button inside the saveButton method.
Still, here is the code, maybe it helps someone in the future.
import SwiftUI
import PlaygroundSupport
struct ContentView: View {
#Environment(\.presentationMode) var presentationMode
var body: some View {
NavigationView {
NavigationLink(
destination: Child(
// Instead of defining the buttons here, I send an optional
// cancel action to the child. This will make it possible
// to use the child view on navigation links, as well as in
// modal dialogs.
cancelAction: {
self.presentationMode.wrappedValue.dismiss()
}
)
) {
Text("Open")
}
}
}
}
struct Child: View {
// Store the value from the textfield
#State private var value = "default"
#Environment(\.presentationMode) var presentationMode
var cancelAction: (() -> Void)?
// Make this button available inside this view, and inside the parent view.
// This makes sure the visibility of this button is always the same.
var saveButton: some View {
Button(action: save) {
Text("Save")
}
}
var body: some View {
VStack {
// Simple textfield to allow a string to change.
TextField("Value", text: $value)
// Just for the playground to change the value easily.
// Usually it would be chnaged through the keyboard input.
Button(action: {
self.value = "new value"
}) {
Text("Update")
}
}
.navigationBarItems(
leading: self.cancelAction != nil ? Button(action: self.cancelAction!, label: {
Text("Cancel")
}) : nil,
trailing: self.saveButton
)
}
func save() {
// This always displays the default value of the state variable.
// Even after the Update button was used and the value did change inside
// the textfield.
print("\(value)")
}
}
PlaygroundPage.current.setLiveView(ContentView())