Swiftui Binding Default Value (Argument Labels '(Wrappedvalue:)' Do Not Match Any Available Overloads)

SwiftUI Binding default value (Argument labels '(wrappedValue:)' do not match any available overloads)

Here it is

struct MyView: View {
@Binding var a: Bool
init(a: Binding<Bool> = .constant(true)) {
_a = a
}

var body: some View {
Text("MyView")
}
}

SwiftUI: Binding in initializer: default value

Use the constant factory method:

init(value: Binding<String> = .constant("")) {
self._value = value
}

Now you can do TestView().

How can gave a View as default value in a View Function in SwiftUI?

The most possible way is having 2 function's. Like down codes:

PS: We cannot have the function which carries the default value for a generic type.

struct ContentView: View {

var body: some View {

viewFunction(content: Text("Hello, world!"))

viewFunction()
}
}

func viewFunction<Content: View>(content: Content) -> some View {

return content

}

func viewFunction() -> some View {

return Text("Hello, world!")

}

Cannot convert value of type 'Binding Double ' to expected argument type 'Binding Double? '

Here is a solution that allows to disambiguate w/ and w/o value binding.

Tested with Xcode 11.4 / iOS 13.4

struct ChildView: View {
@Binding var prop: Double?

init() {
_prop = .constant(nil)
}

init(prop: Binding<Double>) {
_prop = Binding(prop)
}

var body: some View {
Group {
if prop != nil {
Text("Child View: \(prop!)")
} else {
Text("Child View")
}
}
}
}

SwiftUI: How to implement a custom init with @Binding variables

Argh! You were so close. This is how you do it. You missed a dollar sign (beta 3) or underscore (beta 4), and either self in front of your amount property, or .value after the amount parameter. All these options work:

You'll see that I removed the @State in includeDecimal, check the explanation at the end.

This is using the property (put self in front of it):

struct AmountView : View {
@Binding var amount: Double

private var includeDecimal = false

init(amount: Binding<Double>) {

// self.$amount = amount // beta 3
self._amount = amount // beta 4

self.includeDecimal = round(self.amount)-self.amount > 0
}
}

or using .value after (but without self, because you are using the passed parameter, not the struct's property):

struct AmountView : View {
@Binding var amount: Double

private var includeDecimal = false

init(amount: Binding<Double>) {
// self.$amount = amount // beta 3
self._amount = amount // beta 4

self.includeDecimal = round(amount.value)-amount.value > 0
}
}

This is the same, but we use different names for the parameter (withAmount) and the property (amount), so you clearly see when you are using each.

struct AmountView : View {
@Binding var amount: Double

private var includeDecimal = false

init(withAmount: Binding<Double>) {
// self.$amount = withAmount // beta 3
self._amount = withAmount // beta 4

self.includeDecimal = round(self.amount)-self.amount > 0
}
}
struct AmountView : View {
@Binding var amount: Double

private var includeDecimal = false

init(withAmount: Binding<Double>) {
// self.$amount = withAmount // beta 3
self._amount = withAmount // beta 4

self.includeDecimal = round(withAmount.value)-withAmount.value > 0
}
}

Note that .value is not necessary with the property, thanks to the property wrapper (@Binding), which creates the accessors that makes the .value unnecessary. However, with the parameter, there is not such thing and you have to do it explicitly. If you would like to learn more about property wrappers, check the WWDC session 415 - Modern Swift API Design and jump to 23:12.

As you discovered, modifying the @State variable from the initilizer will throw the following error: Thread 1: Fatal error: Accessing State outside View.body. To avoid it, you should either remove the @State. Which makes sense because includeDecimal is not a source of truth. Its value is derived from amount. By removing @State, however, includeDecimal will not update if amount changes. To achieve that, the best option, is to define your includeDecimal as a computed property, so that its value is derived from the source of truth (amount). This way, whenever the amount changes, your includeDecimal does too. If your view depends on includeDecimal, it should update when it changes:

struct AmountView : View {
@Binding var amount: Double

private var includeDecimal: Bool {
return round(amount)-amount > 0
}

init(withAmount: Binding<Double>) {
self.$amount = withAmount
}

var body: some View { ... }
}

As indicated by rob mayoff, you can also use $$varName (beta 3), or _varName (beta4) to initialise a State variable:

// Beta 3:
$$includeDecimal = State(initialValue: (round(amount.value) - amount.value) != 0)

// Beta 4:
_includeDecimal = State(initialValue: (round(amount.value) - amount.value) != 0)

Checking the value of an Optional Bool

With optional booleans it's needed to make the check explicit:

if boolean == true {
...
}

Otherwise you can unwrap the optional:

if boolean! {
...
}

But that generates a runtime exception if boolean is nil - to prevent that:

if boolean != nil && boolean! {
...
}

Before beta 5 it was possible, but it has been changed as reported in the release notes:

Optionals no longer implicitly evaluate to true when they have a value and false when they do not, to avoid confusion when working with optional Bool values. Instead, make an explicit check against nil with the == or != operators to find out if an optional contains a value.

Addendum: as suggested by @MartinR, a more compact variation to the 3rd option is using the coalescing operator:

if boolean ?? false {
// this code runs only if boolean == true
}

which means: if boolean is not nil, the expression evaluates to the boolean value (i.e. using the unwrapped boolean value), otherwise the expression evaluates to false

What's the cleanest way of applying map() to a dictionary in Swift?

Swift 4+

Good news! Swift 4 includes a mapValues(_:) method which constructs a copy of a dictionary with the same keys, but different values. It also includes a filter(_:) overload which returns a Dictionary, and init(uniqueKeysWithValues:) and init(_:uniquingKeysWith:) initializers to create a Dictionary from an arbitrary sequence of tuples. That means that, if you want to change both the keys and values, you can say something like:

let newDict = Dictionary(uniqueKeysWithValues:
oldDict.map { key, value in (key.uppercased(), value.lowercased()) })

There are also new APIs for merging dictionaries together, substituting a default value for missing elements, grouping values (converting a collection into a dictionary of arrays, keyed by the result of mapping the collection over some function), and more.

During discussion of the proposal, SE-0165, that introduced these features, I brought up this Stack Overflow answer several times, and I think the sheer number of upvotes helped demonstrate the demand. So thanks for your help making Swift better!

SwiftUI Optional TextField

Ultimately the API doesn't allow this - but there is a very simple and versatile workaround:

extension Optional where Wrapped == String {
var _bound: String? {
get {
return self
}
set {
self = newValue
}
}
public var bound: String {
get {
return _bound ?? ""
}
set {
_bound = newValue.isEmpty ? nil : newValue
}
}
}

This allows you to keep the optional while making it compatible with Bindings:

TextField($test.bound)


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