如何在 SwiftUI 中打印()到 Xcode 控制台？

You can't because you're in a computed property. You need for example a button and in the action you define the print. Or work with breakpoints

You can not print in body structure i.e. a structure which is some view type.For print you need to make function out of body structure and call it using button or something else.

Try right-clicking on the live preview play button and selecting 'Debug Preview from the popup

It is possible to use print() remembering that all SwiftUI View content are (a) implicit closures and (b) it is highly recommended to decompose views as much as possible to have simple structure, so it might look like the following...

struct Model: Identifiable {
let value: String
var id: String {
value
}
init (_ value: String) {
self.value = value
}
}


struct TestView: View {
@State var showFavoritesOnly = false
@State var listData: [Model] = [Model("one"), Model("two"), Model("three")]


var body: some View {
NavigationView {
List {
Toggle(isOn: $showFavoritesOnly) {
Text("Favorite only")
}
ForEach(listData) { data in
self.rowView(data: data)
}
}
}
}


private func rowView(data: Model) -> some View {
#if DEBUG
print(">> \(data.value)")
#endif
return NavigationLink(destination: Text("Details")) {
Text("Go next from \(data.value)")
}
}
}

... and right clicking in Preview to select run as Debug Preview we get:

2019-10-31 14:28:03.467635+0200 Test[65344:11155167] [Agent] Received connection, creating agent
2019-10-31 14:28:04.472314+0200 Test[65344:11155168] [Agent] Received display message
>> one
>> two
>> three

Here's a helper Print( ... ) View that acts like a print( ... ) function but within a View

Put this in any of your view files

extension View {
func Print(_ vars: Any...) -> some View {
for v in vars { print(v) }
return EmptyView()
}
}

and use inside of body like so

Print("Here I am", varOne, varTwo ...)

or inside a ForEach {} like so

self.Print("Inside ForEach", varOne, varTwo ...)

Note: you might need to put Print() into a Group {} when combining with existing views

You can print in the body structure but to do so you have to explcitly return the view you want to render. Normally in SwiftUI, the body property implicitly returns the view. For example, this will throw an error when you try to print:

struct SomeView: View {
@State var isOpen = false


var body: some View {
print(isOpen) // error thrown here


VStack {
// other view code
|
}
}

But if we explicitly return the view we want then it will work e.g.

struct SomeView: View {
@State var isOpen = false


var body: some View {
print(isOpen) // this ok because we explicitly returned the view below


// Notice the added 'return' below
return VStack {
// other view code
}
}
}

The above will work well if you're looking to view how state or environment objects are changing before returning your view, but if you want to print something deeper down within the view you are trying to return, then I would go with @Rok Krulec answer.

You can easily add a print statement anywhere in a function builder by simply storing its return value in a wildcard, effectively ignoring it:

let _ = print("hi!")

No setup or other verbosity needed!

Why does this work while a regular print() doesn't?

The way SwiftUI's @ViewBuilder (and result builders in general) is that they consume any values in a closure that aren't used otherwise (e.g. if you just have 42 on its own line). The print function returns Void (nothing), which the builder would have to build into a view, so it fails. By instead assigning it to a variable (in this case _, basically a variable that you can never access), the Void is never offered to the view builder in the first place.

You could argue the builder should simply accept and ignore Void values, but the idea is that your builder closures should not have side effects (I'd remove print statements after finishing debugging too)—you should not rely on these closures being called at certain times.

EDIT: Debug Preview is no longer supported in the latest versions of Xcode.

Very easy way to debug your Preview:

1. Open your Swift project in Xcode 11.
2. Right-click (or Control-click) on the Live Preview button in the bottom right corner of the preview.
3. Select Debug Preview.

How to debug your SwiftUI previews in Xcode

It can be generalized to:

extension View {
func Perform(_ block: () -> Void) -> some View {
block()
return EmptyView()
}
}

So in your example:

ForEach(landmarkData) { landmark in
Perform { print("landmark: \(landmark)") }
if !self.userData.showFavoritesOnly || landmark.isFavorite {
NavigationButton(destination: LandmarkDetail(landmark: landmark)) {
LandmarkRow(landmark: landmark)
}
}
}

Here you go. It will just work like simple print but inside a view.

      func printv( _ data : Any)-> EmptyView{
print(data)
return EmptyView()
}

and use it like that

struct ContentView: View {
var body: some View  {
VStack() {
        

Text("hello To SwiftUI")
        

printv("its easy to code in SwiftUI")
        

Text("And Good to have you here")
}
}
}

// Try this, add a 'return' on a view then the 'print' can stay alive in.

struct ContentView: View {
var num: Int = 1
    

var body: some View {
print(num)
        

return  Text("hello")
    

}
}

The following extension on View is as intuitive as print because it's made to replicate the default print(_:separator:terminator:) function signature & behavior.

extension View {
func printUI(_ args: Any..., separator: String = " ", terminator: String = "\n") -> EmptyView {
let output = args.map(String.init(describing:)).joined(separator: separator)
print(output, terminator: terminator)
return EmptyView()
}
}

Usage Example:

struct ContentView: View {
var body: some View {
VStack {
printUI("ContentView", "1")
printUI("ContentView", "2", separator: ", ", terminator: "\n.\n.\n")
printUI("ContentView", "3", separator: "; ")


Text("Hello, World!")
}
}
}

Console Output:

ContentView 1
ContentView, 2
.
.
ContentView; 3

This should work

if true {
print(aVar, "xx")
}


return ZStack {
...
}

The safest and easiest way to print while debugging in a SwiftUI View.

extension View {
func Print(_ item: Any) -> some View {
#if DEBUG
print(item)
#endif
return self
}
}

Usage Example:

struct ContentView: View {
var body: some View {
VStack {
ForEach((1...5), id: \.self) { number in
Text("\(number)")
.Print(number)
}
}
}
}

Console output:

1
2
3
4
5

You can declare a printing() method that includes print() and returns EmptyView struct.

struct ContentView: View {
    

@State private var offset = CGSize.zero
    

func printing(_ items: Any...) -> some View {
let _ = print(items)
return EmptyView()
}
        

var body: some View {
        

#if DEBUG
printing(offset)                // prints [(0.0, 0.0)]
#endif


ZStack {
Text("Hello")
}
}
}