I want to feed Two to the function. When function receives Two as the data, it should print success to the console. But seems like it's not the correct way.
Error: Conditions must have a static type of 'bool'. Try changing the condition.
List
enum ButtonList {One, Two, Three}
Calling function
testFunc(ButtonList.Two)),
Function
testFunc( ButtonList type) {
if (type = ButtonList.Two ){print('sucess ')};
}
It should be:
testFunc(ButtonList type) {
if (type == ButtonList.Two) {
print('sucess ')
};
}
There's a big difference between = (assigning a value to a variable) and == (equality comparison). if expects a condition (==), not the assigning operation (=).
Formatting is important to read and understand the code. Please read Dart best code style practices: https://dart.dev/guides/language/effective-dart/style
You are trying to assign with =, use == instead
Related
error: The argument type 'Future<List<GalleryPictureInfo>>' can't be assigned to the parameter type 'Future<List<GalleryPictureInfo>>?'.
Is this Dart Analysis or me? The project still compiles.
Upd. Added code example
FutureBuilder<List<GalleryPictureInfo>>(
future: derpiService.getListOfImages(),
//other code
);
#override
Future<List<GalleryPictureInfo>> getListOfImages(arguments) async {
List<GalleryPictureInfo> listOfImages = [];
var searchImages = await getSearchImages(tags: tags, page: page);
//adding images to List
return listOfImages;
}
It's something with FutureBuilder actually. I should've mention this.
Upd. "Fixed" with // ignore: argument_type_not_assignable
Looks like a problem with Dart Analysis for now
Upd. Error
It actually is an error which is pretty self explanatory.
The acutal error comes because of null safety in dart.
For ex:
void main(){
var number = getNumber(true);
int parsedNumber = int.parse(number);
print(parsedNumber);
}
String? getNumber(boolean value) {
if (value){
return null;
} else return "1";
}
So here, getNumber function either returns null or "1" depending upon the value of value variable. So, number variable's type is String?.
But the error shall arise in the next line when you try to call int.parse(). int.parse function takes an argument which should be a String but the value passed in the function is of type String?. So if we pass null in int.parse it shall throw an error.
That's why Dart analysis makes it easier to identify such cases by telling us that the value can be null and it might throw.
However the code depends upon your actual code of your project. It says that you are passing Future<List<GalleryPictureInfo>>? which is of nullable type to a function which requires Future<List<GalleryPictureInfo>>. So, before passing the value you might want to check if the value you are passing is not null.
If you are sure that the value can never be null then if for ex: if you are passing a variable called value, you might wanna try someFunctionWhereYouPassValue(value!)
That ! means that you are sure that the value will never be null.
For more details about null safety you can see:
https://dart.dev/null-safety/understanding-null-safety
Is there a construct that communicates to the type checker a function's post-condition?
For example, in typescript it is possible to say
function assertIsNumber(value: any): asserts value is number {
if (typeof value !== 'number') {
throw new TypeError();
}
}
I would like to be able to do something like the following in dart:
class SomeClass {
int? value;
_checkPreconditions() {
if(value == null) {
throw MyPreconditionError()
}
// ...
}
somefunc() {
_checkPreconditions();
// here we know `value` is a non-null int.
final sum = value + 5;
}
}
I understand I could coerce the value to non-null sum = value! + 5, but I would prefer to allow the function to inform the type checker if possible.
It looks like the type system of Dart is not so powerful. The only thing that looks (from first glance) possible is to create a custom code analyzer package (or search for one that already exists).
Dart annotations don't actually do anything. They provide hints to tools such as the Dart analyzer (usually so that it can generate additional warnings), but they cannot change program behavior. Even if you could convince the analyzer to treat some variables as different types, you still wouldn't be able to compile and run your code.
Annotations can be used by code generation tools, so one possibility might be to generate a statement such as final value = this.value!; automatically. However, that would be a lot of trouble to go through (and would mean that code then would need to use this.value = 42; for assignments and would prevent your code from being analyzed directly).
How would I go about using NDepend to not only identify JustMyCode.Fields that are exactly a given type, but also indirectly, i.e. fields like IList<MyType>, IDictionary<int, MyType>, Lazy<T> and all those "nice" generic variants/usages?
Is there any helper method similar to .UsedBy(...) available by any chance that provides such a functionality?
Here is a query to get field typed with String or Int32:
let types = Types.WithFullNameIn(
"System.String",
"System.Int32").ToArray()
from f in Application.Fields
where !f.ParentType.IsEnumeration &&
f.FieldType != null &&
types.Contains(f.FieldType)
select new { f, type =f.FieldType }
For now you cannot detect when a type is used in a generic parameter.
I know that in the latest version of dart we can use if else statements inside the build method. Does anyone know if we can use also if else statement when we setting class parameters? I know I can do inline statement there but inline is a bit hard to read when there are multiple conditions
const int i = 0;
class Person {
// NewClass n = NewClass(a: i == 0 ? 'a' : 'b'); //<- inline statement working
NewClass n = NewClass(a: if(i == 0) 'a' else 'b'); //<- if statement doesn't
}
class NewClass {
final String a;
const NewClass({this.a});
}
Edit:
Basically in my case I've got an TextField widget where I set its's type parameter from enum (Type.text, Type.numeric...) According to this parameter I want to set The textField parameters (textCapitalization, maxLength and so on)
As per your comment, you are already creating an enum for specifying the type of the fields.
enum Type {text, numeric}
Now for specifying the properties of that particular type, you can add an extension on this enum, as shown below:
extension TextFieldProperties on Type {
int get maxLength {
if (this == Type.text) {
return 10;
}
return 12;
}
}
So in your field class you already have a type defined, you can use that type variable to get the properties of that particular type of field.
Type type = Type.text;
print(type.maxLength); // Will print 10
type = Type.numeric;
print(type.maxLength); // Will print 12
Note: It will work only in Dart 2.7 and above
You want the conditional expression (?:), not the conditional statement or literal entry (if), as you have already discovered.
The reason if doesn't work is that if only works as a statement or as a collection literal entry. It doesn't work in arbitrary expressions.
The reason for the distinction is that the if syntax allows you to omit the else branch. That only makes sense in places where "nothing" is a valid alternative. For a statement, "doing nothing" is fine. For a collection, "adding nothing" is also fine.
In an expression context, you must evaluate to a value or throw. There is no reasonable default that we can use instead of "nothing", so an if is not allowed instead of an expression.
Doesn't work because this syntax doesn't exist in Dart. The only way to do what you would like to do is to use the ternary operator.
If you try it in the DartPad you will get an error.
I suggest you to use a function to return the right value.
I'm trying to replace a function call like (simplified) Utility.GetString(MyEntity.SomePropertyWithRelatedEntity)=="abc" with an expression visitor into something like p => p.SubRelatedEntities.FirstOrDefault(sre => sre.SomeFlag==true).SomePropertyWithRelatedEntity.
It means, the datamodel goes like:
MyEntity -> RelatedEntity -> SubRelatedEntity
I'm trying to return a string value from the SubRelatedEntity, based on some rules in the RelatedEntity, so I don't have to re-write / copy/paste the whole filtering rules in every usage; that's why I put inside a "call-signature", so my expression visitor can identify it and replace the fake-call to Utility.GetString to some complicated lambda expressions.
My expression visitor contains something like:
public override Expression Visit(Expression node)
{
if (node == null)
return null;
Expression result = null;
if (node.NodeType == ExpressionType.Call)
{
MethodCallExpression mce = node as MethodCallExpression;
if (mce.Method.DeclaringType == typeof(Utility) && mce.Method.Name == "GetString")
{
Expression<Func<RelatedEntity, string>> exp = re => re.SubRelatedEntities.FirstOrDefault(sre => sre.SomeFlag == true).SomeStringValue;
result = exp.Body;
}
else
result = base.Visit(node);
}
else
result = base.Visit(node);
return result;
}
Now, the problem is, the "sre" parameter is not bound when called the injected lambda expression. After much research, I see the lambda parameters should be replaced with another expression visitor, specifically searching for the new parameters and replacing them with the old ones. In my situation, however, I don't have an "old parameter" - I have the expression MyEntity.SomePropertyWithRelatedEntity (e.g. an property filled with the related entities) which I need to insert somehow in the generated lambda.
I hope my problem is understandable. Thank you for any insights!
After getting no answers for long time and trying hard to find a solution, I've solved it at the end :o)! It goes like this:
The newly injected lambda expression gets an ParameterExpression - well, this is a 'helper', used when directly calling the lambda, what I don't want (hence, 'parameter not bound' exception when ToEnumerable is called). So, the clue is to make a specialized ExpressionVisitor, which replaces this helper with the original expression, which is of course available in the Arguments[] for the method call, which I try to replace.
Works like a charm, like this you can reuse the same LINQ expressions, something like reusable sub-queries, instead of writing all the same LINQ stuff all time. Notice as well, that expression calling a method is not allowed in EF, in Linq2Sql it worked. Also, all the proposed web articles only replace the parameter instances, when constructing/merging more LINQ expressions together - here, I needed to replace a parameter with an faked-method-call argument, e.g. the method should not be called, it only stands for a code-marker, where I need to put my LINQ sub-query.
Hope this helps somebody, at the end it's pretty simple and logical, when one knows how the expression trees are constructed ;-).
Bye,
Andrej