I'm trying to create a Fyne vertical box with a series of buttons but can't figure out the basic mechanism. I think this is a Go question, not a Fyne question, something I don't understand about go.
Here is a minimal program to show what I mean:
package main
import (
"fmt"
"fyne.io/fyne/v2/app"
"fyne.io/fyne/v2/container"
"fyne.io/fyne/v2/widget"
)
func main() {
a := app.New()
w := a.NewWindow("Button List")
btn0 := widget.NewButton("button 0", func() {
fmt.Println("Pressed 0")
})
btn1 := widget.NewButton("button 1", func() {
fmt.Println("Pressed 1")
})
btns := []*widget.Button{btn0, btn1}
vbox := container.NewVBox(
// does work
btns[0],
btns[1],
// doesn't work
// btns...,
)
w.SetContent(
vbox,
)
w.ShowAndRun()
}
My understanding is that the argument btns... should produce the same effect as the list of arguments btn[0], btn[1], but it apparently doesn't. If I comment out the lines
btn[0],
btn[1],
and uncomment the line
btns...
I get the error message
cannot use btns (type []*"fyne.io/fyne/v2/widget".Button) as type
[]fyne.CanvasObject in argument to container.NewVBox
So, my newbie questions:
whats going on here, i.e., why doesn't btns... work?
what should I be using as the argument to NewVBox instead?
To do what you're wanting to do here you need to modify the slice of *widget.Button to be a slice of fyne.CanvasObject.
When spreading into a variadic parameter like this, the types have to match exactly to what the variadic parameter is expecting. This means the type needs to be the interface itself and not a type that implements the interface.
In your case, the following will work:
btns := []fyne.CanvasObject{btn0, btn1}
vbox := container.NewVBox(btns...)
Related
I've been struggling with the reflect package. This code below does what I expect:
package main
import (
"reflect"
"log"
)
type Car struct {
Model string
}
type Person struct {
Name string
Cars []Car
}
func ModifyIt(parent interface{},fieldName string, val interface{}) {
slice := reflect.ValueOf(parent).Elem()
nth := slice.Index(0)
//row := nth.Interface() // this line causes errors
row := nth.Interface().(Person)
elem := reflect.ValueOf(&row).Elem()
field := elem.FieldByName(fieldName)
log.Println(field.CanSet())
}
func main() {
p := []Person{Person{Name:"john"}}
c := []Car{Car{"corolla"},Car{"jetta"}}
ModifyIt(&p,"Cars",&c)
}
However, if I replace the line row := nth.Interface().(Person) with row := nth.Interface(), that is I remove the type assertion, then I get the error:
panic: reflect: call of reflect.Value.FieldByName on interface Value
on line "field := elem.FieldByName(fieldName)
I've tried a bunch of other things the last few hours like trying to do reflect.TypeOf(), reflect.Indirect() etc... on some of the other variables but with no success.
I've read some other questions like these:
reflect: call of reflect.Value.FieldByName on ptr Value
Set a struct field with field type of a interface
Golang reflection: Can't set fields of interface wrapping a struct
They seem to suggest that I don't have a good understanding of how pointers or interfaces work.
So my question is, how do I go about setting the field of a struct when the struct is typed as an interface?
UPDATE
I posted a solution as an answer, but I have no confidence in whether it is the proper or safe way of doing things. I hope someone can explain, or post a better solution.
Try this:
func ModifyIt(slice interface{}, fieldName string, newVal interface{}) {
// Create a value for the slice.
v := reflect.ValueOf(slice)
// Get the first element of the slice.
e := v.Index(0)
// Get the field of the slice element that we want to set.
f := e.FieldByName(fieldName)
// Set the value!
f.Set(reflect.ValueOf(newVal))
}
Call it like this:
p := []Person{Person{Name: "john"}}
c := []Car{Car{"corolla"}, Car{"jetta"}}
ModifyIt(p, "Cars", c)
Note that the call passes the slices directly instead of using pointers to slices. The pointers are not needed and add extra complexity.
Run it on the Playground.
Out of sheer luck, I finally got something to work.
I pieced together a bunch of random things I read with very little rhyme or reason. I even tried reading the Laws of Reflection on the Golang site, but I don't think I have a good grasp of how it relates to why I couldn't set variables typed as interface{}. In general, I still don't understand what I did.
My solution below is littered with comments to indicate my confusion, and lack of confidence in whether I did things properly or safely.
package main
import (
"reflect"
"log"
)
type Car struct {
Model string
}
type Person struct {
Name string
Cars []Car
}
func ModifyIt(parent interface{},fieldName string, val interface{}) {
log.Println(parent)
slice := reflect.ValueOf(parent).Elem()
nth := slice.Index(0)
row := nth.Interface()
log.Println(nth.CanSet()) // I can set this nth item
// I think I have a to make a copy, don't fully understand why this is necessary
newitem := reflect.New(reflect.ValueOf(row).Type())
newelem := newitem.Elem()
field := newelem.FieldByName(fieldName)
// I need to copy the values over from the old nth row to this new item
for c:=0; c<nth.NumField(); c++ {
newelem.Field(c).Set(reflect.Indirect(nth.Field(c)))
}
// now I can finally set the field for some reason I don't understand
field.Set(reflect.ValueOf(val).Elem())
// now that newitem has new contents in the field object, I need to overwrite the nth item with new item
// I don't know why I'm doing it, but I'll do it
// I also don't fully understand why I have to use Indirect sometimes, and not other times...it seems interchangeable with ValueOf(something).Elem(), I'm confused....
nth.Set(reflect.Indirect(newitem))
}
func main() {
p := []Person{Person{Name:"john"}}
c := []Car{Car{"corolla"},Car{"jetta"}}
ModifyIt(&p,"Cars",&c)
// now parent is up to date, although I have no idea how I got here.
log.Println(p)
}
If anyone can post a better answer that clears up my confusion, that will be great. I've been having a really hard time learning golang.
I am a beginner in Golang.
I have a problem with variable type assigning from user input.
When the user enters data like "2012BV352" I need to be able to ignore the BV and pass 2012352 to my next function.
There has a package name gopkg.in/validator.v2 in doc
But what it returns is whether or not the variable is safe or not.
I need to cut off the unusual things.
Any idea on how to achieve this?
You could write your own sanitizing methods and if it becomes something you'll be using more often, I'd package it out and add other methods to cover more use cases.
I provide two different ways to achieve the same result. One is commented out.
I haven't run any benchmarks so i couldn't tell you for certain which is more performant, but you could write your own tests if you wanted to figure it out. It would also expose another important aspect of Go and in my opinion one of it's more powerful tools... testing.
package main
import (
"fmt"
"log"
"regexp"
"strconv"
"strings"
)
// using a regex here which simply targets all digits and ignores everything else. I make it a global var and use MustCompile because the
// regex doesn't need to be created every time.
var extractInts = regexp.MustCompile(`\d+`)
func SanitizeStringToInt(input string) (int, error) {
m := extractInts.FindAllString(input, -1)
s := strings.Join(m, "")
return strconv.Atoi(s)
}
/*
// if you didn't want to use regex you could use a for loop
func SanitizeStringToInt(input string) (int, error) {
var s string
for _, r := range input {
if !unicode.IsLetter(r) {
s += string(r)
}
}
return strconv.Atoi(s)
}
*/
func main() {
a := "2012BV352"
n, err := SanitizeStringToInt(a)
if err != nil {
log.Fatal(err)
}
fmt.Println(n)
}
I have this summarized code to display tabs in a notebook in Gtk. Basically, I have created a custom struct with a embedded label, that is added to the notebook. After that, I want to get back that custom widget, but I get a Invalid type assertion. I have read a lot about structs and interfaces, but I can't get it to work.
package main
import "github.com/gotk3/gotk3/gtk"
type NotebookPage struct {
gtk.Label
}
func main() {
gtk.Init(nil)
win, _ := gtk.WindowNew(gtk.WINDOW_TOPLEVEL)
notebook, _ := gtk.NotebookNew()
win.Add(notebook)
content1, _ := gtk.LabelNew("Content 1")
page1 := NotebookPage{Label: *content1}
label1, _ := gtk.LabelNew("Label 1")
notebook.AppendPage(&page1, label1)
content2, _ := gtk.LabelNew("Content 2")
page2 := NotebookPage{Label: *content2}
label2, _ := gtk.LabelNew("Label 2")
notebook.AppendPage(&page2, label2)
win.ShowAll()
win.Connect("destroy", func() {
gtk.MainQuit()
})
backwidget1, _ := notebook.GetNthPage(0)
backpage1, _ := backwidget1.(*NotebookPage)
gtk.Main()
}
Looks like your problem is here on line 35 as the message says:
backpage1, _ := backwidget1.(*NotebookPage)
notebook.GetNthPage returns a *Widget, *NotebookPage is not a *Widget, so you're not allowed to cast to it. If the function didn't return a concrete type (if it returned the same IWidget interface), and if it didn't roundtrip through gtk C libraries, you could do this.
As it is if you want to get your custom widget back you probably need to get at the underlying gtk widget or serialisation (which presumably stores your custom label), extract the label and build a new NotebookPage.
So you need something like :
func NewNotebookPage(widget *Widget) {
return &NotebookPage{Label: widget.GetLabelSomehow()}
}
It looks like Label is also a widget :) This is painful because it's trying to work with C++ inheritance in Go. You'll have to find out how to unfreeze your label from the widget.C.GtkWidget
package main
import (
"fmt"
"encoding/json"
"reflect"
)
type GeneralConfig map[string]interface{}
var data string = `
{
"key":"value",
"important_key":
{"foo":"bar"}
}`
func main() {
jsonData := &GeneralConfig{}
json.Unmarshal([]byte(data), jsonData)
fmt.Println(reflect.TypeOf(jsonData)) //main.GeneralConfig
jsonTemp := (*jsonData)["important_key"]
fmt.Println(reflect.TypeOf(jsonTemp)) //map[string]interface {}
//newGeneralConfig := GeneralConfig(jsonTemp)
//cannot convert jsonTemp (type interface {}) to type GeneralConfig:
//need type assertion
newGeneralConfig := jsonTemp.(GeneralConfig)
//fmt.Println(reflect.TypeOf(newGeneralConfig))
//panic: interface conversion: interface {} is map[string]interface {},
//not main.GeneralConfig
}
Available at the playground
I understand that I can use a nested struct in lieu of GeneralConfig, but that would require me knowing the exact structure of the payload, ie it wouldn't work for different keys (I would be locked into "important_key").
Is there a golang workaround for when I don't know what the value of "important_key" is? I say golang, because if possible, one could require all "important_keys" to have a constant parent key, which could resolve this issue.
To summarize, given an arbitrary json object, there must be a way that I can traverse its keys, and if a value is a custom type, convert the value to that type. Right now it seems that if I use type conversion, it tells me that the type is interface{} and I need to use type assertion; however, if I use type assertion, it tells me that interface{} is map[string]interface{} not main.GeneralConfig.
I agree the comments about trying to utilise the expected structure of the incoming JSON in order to write well-defined Structs, but I'll attempt to answer the question anyway.
The thing to take away from what you're seeing printed versus the error messages that you're seeing is that the compiler knows less about the type than the runtime because the runtime can look at the actual value. To bring the compiler up-to-speed we must (i) assert (*jsonData)["important_key"] is a map[string]interface{} -- the compiler only knows it to be an interface{} -- and then (ii) type-cast that to a GeneralConfig type. See:
package main
import (
"fmt"
"encoding/json"
)
type GeneralConfig map[string]interface{}
func main() {
jsonStruct := new(GeneralConfig)
json.Unmarshal([]byte(`{"parent_key": {"foo": "bar"}}`), jsonStruct)
fmt.Printf("%#v\n", jsonStruct)
// => &main.GeneralConfig{"parent_key":map[string]interface {}{"foo":"bar"}}
nestedStruct := (*jsonStruct)["parent_key"]
fmt.Printf("%#v\n", nestedStruct)
// => map[string]interface {}{"foo":"bar"}
// Whilst this shows the runtime knows its actual type is
// map[string]interface, the compiler only knows it to be an interface{}.
// First we assert for the compiler that it is indeed a
// map[string]interface{} we are working with. You can imagine the issues
// that might arrise if we has passed in `{"parent_key": 123}`.
mapConfig, ok := nestedStruct.(map[string]interface{})
if !ok {
// TODO: Error-handling.
}
// Now that the compiler can be sure mapConfig is a map[string]interface{}
// we can type-cast it to GeneralConfig:
config := GeneralConfig(mapConfig)
fmt.Printf("%#v\n", config)
// => main.GeneralConfig{"foo":"bar"}
}
You are looking for json.RawMessage.
You can delay unmarshalling based upon some other value and then force it to unmarshal to a specific type.
This is not a good idea, but might be closer to what you are looking for.
http://play.golang.org/p/PWwAUDySE0
This is a standard "workaround" if get what you're after. When handling unknown data you can implement this pattern (modified from your example) of switching on the type recursively to get to the concrete values in an unknown body of json data.
package main
import (
"encoding/json"
"fmt"
"reflect"
)
var data = `
{
"key":"value",
"important_key":
{"foo":"bar"}
}`
func main() {
var jsonData interface{}
json.Unmarshal([]byte(data), &jsonData)
fmt.Println(reflect.TypeOf(jsonData))
parseArbitraryJSON(jsonData.(map[string]interface{}))
}
func parseArbitraryJSON(data map[string]interface{}) {
for k, v := range data {
switch a := v.(type) {
case string:
fmt.Printf("%v:%v\n", k, a)
case map[string]interface{}:
fmt.Printf("%v:%v\n", k, a)
parseArbitraryJSON(a)
}
}
}
The resulting output is:
map[string]interface {}
key:value
important_key:map[foo:bar]
foo:bar
This example only accounts for the base data being a string type but you can switch on any type that you expect to receive, and like any switch you can group your cases, so you can treat all numbers similarly for example.
I am looking for an elegant way to unzip a list of arguments in Go. I do not want to use a variadic function for that purpose because in my usecase when writing a function I already know the number of arguments and I want to keep that part simple. However in my usecase the parameters arrive as []interface{}.
I could not find a solution but hey maybe someone out there already knows how to do that?
package main
import (
"fmt"
)
// NON-VARIADIC greater
func greet(n1, n2 string) {
fmt.Printf("%s %s\n", n1, n2)
}
func main() {
l := []interface{}{"hello", "world"}
// works
greet(l[0].(string), l[1].(string))
// does not work: "./args.go:20: not enough arguments in call to greet"
//greet(l...)
// is there something more elegant to unzip the list?
}
You could create a "generic" caller using reflect package, although this comes with overhead and lacks type safety. Unless you have some special case situation and don't know what you want to call in the code, it would be wiser to use the snippet from your question which works, but you consider not elegant.
Example usage of reflect which could be your starting point:
package main
import (
"fmt"
"reflect"
)
func call(f interface{}, args []interface{}) {
// Convert arguments to reflect.Value
vs := make([]reflect.Value, len(args))
for n := range args {
vs[n] = reflect.ValueOf(args[n])
}
// Call it. Note it panics if f is not callable or arguments don't match
reflect.ValueOf(f).Call(vs)
}
func greet(n1, n2 string) {
fmt.Printf("%s %s\n", n1, n2)
}
func main() {
l := []interface{}{"hello", "world"}
call(greet, l)
}
// Output: hello world
https://play.golang.org/p/vbi3CChCdV
I'm not quite sure what you're trying to do. If you want a way to easily pass a slice of two empty interfaces to a function that accepts two strings, you can create a little helper:
func twoStrings(vs []interface{}) (string, string) {
return vs[0].(string), vs[1].(string)
}
Use it as
greet(twoStrings(l))
Playground: http://play.golang.org/p/R8KFwMUT_V.
But honestly, it seems like you're doing something wrong, trying to make the Go type system to do something it cannot do.