I see this source from google_fonts of dart pub. It seems we have a function map? and asMap() has a arrow function? Don't understand this.
static Map<
String,
TextStyle Function({
TextStyle? textStyle,
Color? color,
Color? backgroundColor,
double? fontSize,
FontWeight? fontWeight,
FontStyle? fontStyle,
double? letterSpacing,
double? wordSpacing,
TextBaseline? textBaseline,
double? height,
Locale? locale,
Paint? foreground,
Paint? background,
List<ui.Shadow>? shadows,
List<ui.FontFeature>? fontFeatures,
TextDecoration? decoration,
Color? decorationColor,
TextDecorationStyle? decorationStyle,
double? decorationThickness,
})> asMap() => const {
'ABeeZee': GoogleFonts.aBeeZee,
'Abel': GoogleFonts.abel}
You're seeing a generated code, in the very beginning of the file:
// GENERATED CODE - DO NOT EDIT
// Copyright 2019 The Flutter team. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
This function is defined as:
Name: asMap.
Scope: top-level (Since it is a static method).
Return type: Map<String, TextStyle Function({ ...args })>.
It seems we have a function map?
We have a function that returns a map of Strings and Closures. In Dart this is perfectly fine:
final Map<String, int Function(int, int)> operations = {
'sum': (int a, int b) => a + b,
'subtract': (int a, int b) => a - b,
'multiply': (int a, int b) => a * b,
'divide': (int a, int b) => a * b
};
operations['sum']!(30, 30); // 60
operations['subtract']!(30, 30); // 0
operations['multiply']!(30, 30); // 900
operations['divide']!(30, 30); // 1
In general this is not something we do by raw hands due to type implications (hard to maintain, you can see that it's pretty confusing + it's pretty easy to lost track of type definitions and, consequently, all intellisense features). So that's why this is being done through code generation.
And in this particular case, we are generating a map, like this example below:
class Arithmetic {
static int sum(int a, int b) {
return a + b;
}
static int subtract(int a, int b) {
return a - b;
}
static int multiply(int a, int b) {
return a * b;
}
static int divide(int a, int b) {
return a * b;
}
static Map<String, int Function(int, int)> asMap() {
const Map<String, int Function(int, int)> operationsAsMap = {
'sum': sum,
'subtract': subtract,
'multiply': multiply,
'divide': divide
};
return operationsAsMap;
}
}
asMap() has a arrow function?
No, the asMap() implementation is using an arrow function, which is an alias for:
// This is a shorthand...
static Map<...> asMap() => const { ... };
// For this:
static Map<...> asMap() {
return const { ... };
}
Related
I was trying to use the compute function in Flutter.
void _blockPressHandler(int row, int col) async {
// Called when user clicks any block on the sudoku board . row and col are the corresponding row and col values ;
setState(() {
widget.selCol = col;
}
});
bool boardSolvable;
boardSolvable = await compute(SudokuAlgorithm.isBoardInSudoku , widget.board , widget.size) ;
}
isBoardInSudoku is a static method of class SudokuAlgorithm. Its present in another file. Writing the above code , tells me that
error: The argument type '(List<List<int>>, int) → bool' can't be assigned to the parameter type '(List<List<int>>) → bool'. (argument_type_not_assignable at [just_sudoku] lib/sudoku/SudokuPage.dart:161)
How do i fix this? Can it be done without bringing the SudokuAlgorithm class's methods out of its file ? How to send multiple arguments to the compute function ?
static bool isBoardInSudoku(List<List<int>>board , int size ){ } is my isBoardInSudoku function.
Just put the arguments in a Map and pass that instead.
There is no way to pass more than one argument to compute because it is a convenience function to start isolates which also don't allow anything but a single argument.
Use a map. Here is an example:
Map map = Map();
map['val1'] = val1;
map['val2'] = val2;
Future future1 = compute(longOp, map);
Future<double> longOp(map) async {
var val1 = map['val1'];
var val2 = map['val2'];
...
}
In OOP and in general, it is more elegant to create a class for that with fields you need, that gives you more flexibility and less hassle with hardcoded strings or constants for key names.
For example:
boardSolvable = await compute(SudokuAlgorithm.isBoardInSudoku , widget.board , widget.size) ;
replace with
class BoardSize{
final int board;
final int size;
BoardSize(this.board, this.size);
}
...
boardSolvable = await compute(SudokuAlgorithm.isBoardInSudoku, BoardSize(widget.board, widget.size)) ;
Use a Tuple
Here is some example code from my app:
#override
Future logChange(
String recordId, AttributeValue newValue, DateTime dateTime) async {
await compute(
logChangeNoCompute, Tuple2<String, AttributeValue>(recordId, newValue));
}
Future<void> logChangeNoCompute(Tuple2<String, AttributeValue> tuple) async {
_recordsById[tuple.item1]!.setAttributeValue(tuple.item2);
await storage.setItem(AssetsFileName, toJson());
}
You can have a function whose only argument is a Map so that you can pass multiple parameters by passing a Map with properties and values. However, the problem that I'm encountering now is that I cannot pass functions. If the value of a Map's property is a function I get an error when I run the compute function.
This example works(keep in mind that I've imported libraries and that's the reason why some functions and classes definitions aren't in this example)
Future<List<int>> getPotentialKeys({
#required int p,
#required int q,
})async{
return await compute(allKeys,{
"p" : p,
"q" : q,
});
}
List<int> allKeys(Map<String,dynamic> parameters){
AdvancedCipherGen key = AdvancedCipherGen();
List<int> possibleE = key.step1(p: parameters["p"], q: parameters["q"]);
return possibleE;
}
This does not work(same thing with a function as the value of a property thows an error)
Future<List<int>> getPotentialKeys({
#required int p,
#required int q,
#required Function(AdvancedCipherGen key) updateKey,
})async{
return await compute(allKeys,{
"p" : p,
"q" : q,
"updateKey" : updateKey,
});
}
List<int> allKeys(Map<String,dynamic> parameters){
AdvancedCipherGen key = AdvancedCipherGen();
List<int> possibleE = key.step1(p: parameters["p"], q: parameters["q"]);
//TODO: Update the key value through callback
parameters["updateKey"](key);
return possibleE;
}
easily use a Class, you can Also Use Map or List But using class is Better and Cleaner
class MyFunctionInput{
final int first;
final int second;
MyFunctionInput({required this.first,required this.second});
}
change your function like this
doSomething(MyFunctionInput input){
}
and use it like below
compute(doSomething,MyFunctionInput(first: 1, second: 4));
Note: I initially posted an over-simplified version of my problem. A more
accurate description follows:
I have the following struct:
struct Thing(T) {
T[3] values;
int opApply(scope int delegate(size_t, ref T) dg) {
int res = 0;
foreach(idx, ref val; values) {
res = dg(idx, val);
if (res) break;
}
return res;
}
}
Foreach can be used like so:
unittest {
Thing!(size_t[]) thing;
foreach(i, ref val ; thing) val ~= i;
}
However, it is not #nogc friendly:
#nogc unittest {
Thing!size_t thing;
foreach(i, ref val ; thing) val = i;
}
If I change the signature to
int opApply(scope int delegate(size_t, ref T) #nogc dg) { ... }
It works for the #nogc case, but fails to compile for non-#nogc cases.
The solutions I have tried are:
Cast the delegate
int opApply(scope int delegate(size_t, ref T) dg) {
auto callme = cast(int delegate(size_t, ref T) #nogc) dg;
// use callme instead of dg to support nogc
This seems wrong as I am willfully casting a #nogc attribute even onto
functions that do may not support it.
Use opSlice instead of opApply:
I'm not sure how to return an (index, ref value) tuple from my range. Even if
I could, I think it would have to contain a pointer to my static array, which
could have a shorter lifetime than the returned range.
Use a templated opApply:
All attempts to work with this have failed to automatically determine the
foreach argument types. For example, I needed to specify:
foreach(size_t idx, ref int value ; thing)
Which I see as a significant hindrance to the API.
Sorry for underspecifying my problem before. For total transparency,
Enumap is the "real-world" example. It
currently uses opSlice, which does not support ref access to values. My
attempts to support 'foreach with ref' while maintaining #nogc support is what
prompted this question.
Instead of overloading the opApplyoperator you can implement an input range for your type. Input ranges work automatically as the agregate argument in foreach statements:
struct Thing(K,V) {
import std.typecons;
#nogc bool empty(){return true;}
#nogc auto front(){return tuple(K.init, V.init);}
#nogc void popFront(){}
}
unittest {
Thing!(int, int) w;
foreach(val ; w) {
int[] i = [1,2,3]; // spurious allocation
}
}
#nogc unittest {
Thing!(int, int) w;
foreach(idx, val ; w) { assert(idx == val); }
}
This solves the problem caused by the allocation of the delegate used in foreach.
Note that the example is shitty (the range doesn't work at all, and usually ranges are provided via opSlice, etc) but you should get the idea.
I have a class that has fields and I want call a method of this class and get the reference to one of the fields (not the value!!). Something like this:
class Test : Object{
uint8 x;
uint8 y;
uint8 z;
uint8 method(){
if (x == 1){
return y;
}else if (x == 2){
return z;
}
}
public static void main(string[] args){
uint8 i = method(); // get reference to y or z
i++; //this must update y or z
}
}
In C would be:
int& method()
{
if (x == 1){
return y;
}else if (x == 2){
return z;
}
}
How can I achieve this in vala?
Edit: I'm trying use pointers, I have the following
public class Test : Object {
private Struct1 stru;
struct Struct1{
uint8 _a;
public uint8 a{
get{ return _a; }
set{ _a = value; }
}
public Struct1(Struct1? copy = null){
if (copy != null){
this._a = copy.a;
}else{
this._a = 0;
}
}
public uint8* get_aa(){
return (uint8*)a;
}
}
public void get_pointer(){
uint8* dst = stru.get_aa();
}
public static int main (string[] args){
Test t = new Test();
return 0;
}
}
but when I compile I get
/home/angelluis/Documentos/vala/test.vala.c: In function ‘test_struct1_get_aa’:
/home/angelluis/Documentos/vala/test.vala.c:130:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
result = (guint8*) _tmp1_;
^
Compilation succeeded - 2 warning(s)
Why? I am returning an uint8* type and I attempt to store it in an uint8* pointer.
C doesn't have references (C++ does). Keep in mind that Vala compiles to C as an intermediate language.
I think that there are only two ways to do this in Vala:
Use a box type to encapsulate your uint8 values and return a reference to that box type.
Use a pointer. (Which opens the obvious pointer can of worms)
Edit: Answer to your updated example code problem:
You must be very careful with casting something to some pointer type. In this case the C compiler caught your spurious cast and emited a warning.
uint8 _a;
// This property will get and set the *value* of _a
public uint8 a{
get{ return _a; }
set{ _a = value; }
}
public uint8* get_aa(){
// Here you are casting a *value* of type uint8 to a pointer
// Which doesn't make any sense, hence the compiler warning
return (uint8*)a;
}
Note that you can't get a pointer or a reference to a property, because properties have no memory location on their own.
You can however get a pointer to the field _a in this case:
public uint8* get_aa(){
return &_a;
}
If you insist to go through the property, you have to make your property operate on the pointer as well:
uint8 _a;
public uint8* a{
get{ return &_a; }
}
Notice that in this version I have removed the get_aa () method which is now equivalent to the getter for a.
Also since in this code the property is returning a pointer there is no need for a setter, you can just dereference the pointer to set the value.
I know you can specify function types in formal arg list, but how would I do this for instance variables? I would like to do this:
class A<T> {
int compare(T a, T b);
}
where compare is a function variable with the appropriate type. I would like to be able to write:
A a = new A();
a.compare = ...
You can use typedef :
typedef Comparison<T> = int Function(T a, T b);
class A<T> {
Comparison<T> compare;
}
main() {
A a = new A<int>();
a.compare = (int a, int b) => a.compareTo(b);
print(a.compare(1, 2));
}
In addition to the Alexandre Ardhuin's answer, direct declaration, without typedef:
class A<T> {
late int Function(T a, T b) compare;
}
main() {
A<int> a = new A<int>();
a.compare = (int a, int b) => a.compareTo(b);
print(a.compare(1, 2));
}
Does Dart support the concept of variable functions/methods? So to call a method by its name stored in a variable.
For example in PHP this can be done not only for methods:
// With functions...
function foo()
{
echo 'Running foo...';
}
$function = 'foo';
$function();
// With classes...
public static function factory($view)
{
$class = 'View_' . ucfirst($view);
return new $class();
}
I did not found it in the language tour or API. Are others ways to do something like this?
To store the name of a function in variable and call it later you will have to wait until reflection arrives in Dart (or get creative with noSuchMethod). You can however store functions directly in variables like in JavaScript
main() {
var f = (String s) => print(s);
f("hello world");
}
and even inline them, which come in handy if you are doing recusion:
main() {
g(int i) {
if(i > 0) {
print("$i is larger than zero");
g(i-1);
} else {
print("zero or negative");
}
}
g(10);
}
The functions stored can then be passed around to other functions
main() {
var function;
function = (String s) => print(s);
doWork(function);
}
doWork(f(String s)) {
f("hello world");
}
I may not be the best explainer but you may consider this example to have a wider scope of the assigning functions to a variable and also using a closure function as a parameter of a function.
void main() {
// a closure function assigned to a variable.
var fun = (int) => (int * 2);
// a variable which is assigned with the function which is written below
var newFuncResult = newFunc(9, fun);
print(x); // Output: 27
}
//Below is a function with two parameter (1st one as int) (2nd as a closure function)
int newFunc(int a, fun) {
int x = a;
int y = fun(x);
return x + y;
}