In my Flutter app I want to build this function:
bool isValidType(element, Type cls) {
return element is cls;
}
The issue with this is: The name 'cls' isn't a type and can't be used in an 'is' expression. Try correcting the name to match an existing type. dart(type_test_with_non_type)
element is dynamic, but I expect it to be an Object of a class that extends the class Element (like class Node extends Element). The way I want to be able to use this function is:
isValidType(node, Node) -> true
isValidType(node, Element) -> true
I first thought I could do this:
bool isValidType(element, Type cls) {
return element.runtimeType == cls;
}
But the problem with that is that for the examples provided above it returns the following:
isValidType(node, Node) -> true
isValidType(node, Element) -> false
The reason is of course that the runtimeType of node is Node and not Element.
What do I have to change in the function provided first to work for my requirements?
There currently isn't much you can do with a Type object. The general recommendation is to avoid using them.
In your case, if you want to check against types known at compilation-time, you could make isValidType a generic function instead:
bool isValidType<T>(dynamic element) => element is T;
and then use isValidType<Node>(node) or isValidType<Element>(node). However, I don't think such a function adds anything, so you might as well replace isValidType<Node>(Node) with node is Node at the callsite.
Related
I want to have a method that takes a parameter of type enum as a parameter and then operates on it to get all possible values of the enum type, and do some work with each of those values.
I'd be hoping for something like:
Widget foo (EnumType enumType) {
for(var value in enumType.values) {
print(value.name);
}
}
I've looked for solutions but the only ones I can find are addressed by passing a list of the values as a parm, however this doesn't solve my problem as, even with the list of objects, I can't know that they are enum values so if I try do the .name operation on them, dart throws an error Error: The getter 'name' isn't defined for the class 'Object'.
Maybe my only problem is that I don't know how to specify a variable as an EnumType but I haven't been able to find a correct type for this.
EnumType.values is the equivalent of an automatically generated static method on EnumType and as such is not part of any object's interface. You therefore will not be able to directly call .values dynamically.
I've looked for solutions but the only ones I can find are addressed by passing a list of the values as a [parameter], however this doesn't solve my problem as, even with the list of objects, I can't know that they are enum values so if I try do the .name operation on them, dart throws an error
You can use a generic function that restricts its type parameter to be an Enum:
enum Direction {
north,
east,
south,
west,
}
List<String> getNames<T extends Enum>(List<T> enumValues) =>
[for (var e in enumValues) e.name];
void main() {
print(getNames(Direction.values)); // Prints: [north, east, south, west]
}
The problem with enums is that they can't be passed as a parameter, what you can do instead is that pass all the values and then extract the name from the toString method.
void printEnumValues<T>(List<T> values){
for(var value in values){
final name = value.toString().split('.')[1];
print(name);
}
}
Also I would recommend you look into freezed union classes as that might allow you an alternative approach towards the problem you're trying to solve.
I'd like to store instances of models in a common provider using their classes or interfaces as a keys and then pop them up by class references. I have written some code:
class Provider {
public function new() { }
public function set<T:Any>(instance:T, ?type:Class<T>) {
if (type == null)
type = Type.getClass(instance);
if (type != null && instance != null)
map.set(type, instance);
}
public function get<T:Any>(type:Class<T>):Null<T> {
return cast map.get(type);
}
var map = new Map<Class<Any>, Any>();
}
...alas, it's even doesn't compile.
Probably I have to use qualified class name as a key rather than class/interface reference? But I'd like to keep neat get function design that takes type as argument and returns object just of type taken, without additional type casting.
Is it possible or should I change my approach to this problem?
The issue of using Class<T> as a Map key come up every so often, here is a related discussion. The naive approach of Map<Class<T>, T> fails to compile with something like this:
Abstract haxe.ds.Map has no #:to function that accepts haxe.IMap<Class<Main.T>, Main.T>`
There's several different approaches to this problem:
One can use Type reflection to obtain the fully qualified name of a class instance, and then use that as a key in a Map<String, T>:
var map = new Map<String, Any>();
var name = Type.getClassName(Main);
map[name] = value;
For convenience, you would probably want to have a wrapper that does this for you, such as this ClassMap implementation.
A simpler solution is to simply "trick" Haxe into compiling it by using an empty structure type ({}) as the key type. This causes ObjectMap to be chosen as the underlying map implementation.
var map = new Map<{}, Any>();
map[Main] = value;
However, that allows you to use things as keys that are not of type Class<T>, such as:
map[{foo: "bar"}] = value;
The type safety issues of the previous approach can be remedied by using this ClassKey abstract:
#:coreType abstract ClassKey from Class<Dynamic> to {} {}
This still uses ObjectMap as the underlying map implementation due to the to {} implicit cast. However, using a structure as a key now fails at compile time:
var map = new Map<ClassKey, Any>();
map[{foo: "bar"}] = value; // No #:arrayAccess function accepts arguments [...]
I created an extension method that lets me treat a List as DbSet for testing purposes (actually, I found this idea in another question here on stack overflow, and it's been fairly useful). Coded as follows:
public static DbSet<T> AsDbSet<T>(this List<T> sourceList) where T : class
{
var queryable = sourceList.AsQueryable();
var mockDbSet = new Mock<DbSet<T>>();
mockDbSet.As<IQueryable<T>>().Setup(m => m.Provider).Returns(queryable.Provider);
mockDbSet.As<IQueryable<T>>().Setup(m => m.Expression).Returns(queryable.Expression);
mockDbSet.As<IQueryable<T>>().Setup(m => m.ElementType).Returns(queryable.ElementType);
mockDbSet.As<IQueryable<T>>().Setup(m => m.GetEnumerator()).Returns(queryable.GetEnumerator());
mockDbSet.Setup(d => d.Add(It.IsAny<T>())).Callback<T>(sourceList.Add);
mockDbSet.Setup(d => d.Find(It.IsAny<object[]>())).Callback(sourceList.Find);
return mockDbSet.Object;
}
I had been using Add for awhile, and that works perfectly. However, when I try to add the callback for Find, I get a compiler error saying that it can't convert a method group to an action. Why is sourceList.Add an Action, but sourceList.Find is a method group?
I'll admit I'm not particularly familiar with C# delegates, so it's likely I'm missing something very obvious. Thanks in advance.
The reason Add works is because the List<T>.Add method group contains a single method which takes a single argument of type T and returns void. This method has the same signature as an Action<T> which is one of the overloads of the Callback method (the one with a single generic type parameter, Callback<T>), therefore the List<T>.Add method group can be converted to an Action<T>.
With Find, you are trying to call the Callback method (as opposed to Callback<T>) which expects an Action parameter (as opposed to Action<T>). The difference here is that an Action does not take any parameters, but an Action<T> takes a single parameter of type T. The List<T>.Find method group cannot be converted to an Action because all the Find methods (there is only one anyway) take input parameters.
The following will compile:
public static DbSet<T> AsDbSet<T>(this List<T> sourceList) where T : class
{
var mockDbSet = new Mock<DbSet<T>>();
mockDbSet.Setup(d => d.Find(It.IsAny<object[]>())).Callback<Predicate<T>>(t => sourceList.Find(t));
return mockDbSet.Object;
}
Note that I have called .Callback<Predicate<T>> because the List<T>.Find method expects and argument of type Predicate. Also note I have had to write t => sourceList.Find(t) instead of sourceList.Find because Find returns a value (which means it doesn't match the signature of Action<Predicate<T>>). By writing it as a lambda expression the return value will be thrown away.
Note that although this compiles it will not actually work because the DbSet.Find method actually takes an object[] for it's parameter, not a Predicate<T>, so you will likely have to do something like this:
public static DbSet<T> AsDbSet<T>(this List<T> sourceList) where T : class
{
var mockDbSet = new Mock<DbSet<T>>();
mockDbSet.Setup(d => d.Find(It.IsAny<object[]>())).Callback<object[]>(keyValues => sourceList.Find(keyValues.Contains));
return mockDbSet.Object;
}
This last point has more to do with how to use the Moq library that how to use method groups, delegates and lambdas - there is all sorts of syntactic sugar going on with this line which is hiding what is actually relevant to the compiler and what isn't.
I have the following classes & interfaces:
export interface IBody {
body : ListBody;
}
export class Element {
// ...
}
export class Paragraph extends Element implements IBody {
// ...
}
export class Character extends Element {
// ...
}
I have code where I will get an array of Element derived objects (there are more than just Paragraph & Character). In the case of those that implement IBody, I need to take action on the elements in the body.
What is the best way to see if it implements IBody? Is it "if (element.body !== undefined)"?
And then how do I access it? "var bodyElement = <IBody> element;" gives me an error.
C:/src/jenova/Dev/Merge/AutoTagWeb/client/layout/document/elements/factory.ts(34,27): error TS2012: Cannot convert 'Element' to 'IBody':
Type 'Element' is missing property 'body' from type 'IBody'.
Type 'IBody' is missing property 'type' from type 'Element'.
thanks - dave
An interface in TypeScript is a compile-time only construct, with no run-time representation. You might find section 7 of the TypeScript specification interesting to read as it has the complete details.
So, you can't "test" for an interface specifically. Done correctly and completely, you generally shouldn't need to test for it as the compiler should have caught the cases where an object didn't implement the necessary interface. If you were to try using a type assertion:
// // where e has been typed as any, not an Element
var body = <IBody> e;
The compiler will allow it without warning as you've asserted that the type is an IBody. If however, e were an Element in scope, the compiler as you've shown will check the signature of the Element and confirm that it has the properties/methods declared by IBody. It's important to note that it's checking the signature -- it doesn't matter that it may not implement IBody as long as the signature matches up.
Assuming that Element has a signature that matches IBody, it will work. If it does not, you'll get the compiler error you're receiving. But, again, if it's declared as any, the assertion will pass and at run-time, unless the type has the methods defined on IBody, the script will fail.
As your Element is the base class, you cannot check for IBody. You could declare an argument as any:
function someFeature(e: any) {
}
And then assert that the IBody is present:
function someFeature(e: any) {
var body :IBody = <IBody> e;
// do something
}
However, if you do need a run-time check, you'd need to look for the function on the prototype or as a property before using it. While that could be misleading in some cases, the interface in TypeScript also may not have caught the mismatch either. Here's an example of how you could check for the existence of a specific function.
It might look like this:
function someFeature(e: any) {
var body = <IBody> e;
if (typeof (body.someFunctionOnBodyInterface) === "undefined") {
// not safe to use the function
throw new Error("Yikes!");
}
body.someFunctionOnBodyInterface();
}
I've got an object, which I'll call MyObject. It's a class that controls a particular data row.
I've then got a collection class, called MyObjectCollection:
public class MyObjectCollection : List<MyObject> {}
Why can I not do the following:
List<MyObject> list = this.DoSomethingHere();
MyObjectCollection collection = (MyObjectCollection)list;
Thanks in advance.
Edit: The error is InvalidCastException
My guess is that DoSomethingHere doesn't return an instance of MyObjectCollection.
Let's get rid of all the generics etc here, as they're not relevant. Here's what I suspect you're trying to do:
public static object CreateAnObject()
{
return new object();
}
object o = CreateAnObject();
string s = (string) o;
That will fail (at execution time) and quite rightly so.
To bring it back to your code, unless DoSomethingHere actually returns a MyObjectCollection at execution time, the cast will fail.
Because a List<MyObject> is not a MyObjectCollection. The reverse is true: you could cast a MyObjectCollection to a List because MyObjectCollection inherits from List<MyObject> and thus, for all intents and purposes, IS A List<MyObject>.
The only thing you can do is to define a constructor on MyObjectCollection that takes an Ienumerable as a parameter and initalizes itself with the data in the other one, but that will make a new object containing the same data:
public class MyObjectCollection : List<MyObject>
{
public MyObjectCollection(IEnumerable<MyObject> items)
{
Addrange(items);
}
}
UPDATE:
As noted in the comment, you COULD have the cast succeed at runtime, provided that DoSomething actually returns an instance of MyObjectCollection. If it does, the object effectively is a MyObjectCollection, and the cast is completely legal.
I'd have to say, it is bad practice in my view to upcast something like that. If the function returns a List, you should not rely on a specific implementation of List. Either modify the return type of DoSomething, if you own that function, and return a MyObjectCollection, or deal with it as a list.
Without knowing what exactly is created inside DoSomething() we have to assume either:
You have a misunderstanding about the inheritence in .Net.
you have
A : B
B DoSomething()
{
return new B();
}
// then this is
B b = new B();
A a = (A)b;
Clearly b is a B but not an A. B might look much like A but it is not (if you traverse the parentage of b you won't find A anywhere)
This is true irrespective of the Generics involved (though that sometimes can cause situations where something that could work doesn't see the co-contra variance in c# 4.0)
or
A : B
B DoSomething()
{
return new A();
}
// then this is
B b = new A();
A a = (A)b;
Which in the absence of Generics will work.
You can't do it because (I guessing) the list instance returned from DoSomethingHere isn't derived from MyObjectCollection
You could create an implicit operator that would allow you to convert between your object and the list. You would need an constructor that takes a list and to property that returns the underlaying list.
public static implicit operator List<MyObject>(MyObjectCollection oCollection)
{
//Convert here
return MyObjectCollection.BaseList;
}
public static implicit operator MyObjectCollection(List<MyObject> oList)
{
//Convert here
return new MyObjectCollection(oList);
}