How to create null safe block in flutter? - flutter

Question

How do I null check or create a null safe block in Flutter?
Here is an example:
class Dog {
final List<String>? breeds;
Dog(this.breeds);
}
void handleDog(Dog dog) {
printBreeds(dog.breeds); //Error: The argument type 'List<String>?' can't be assigned to the parameter type 'List<String>'.
}
void printBreeds(List<String> breeds) {
breeds.forEach((breed) {
print(breed);
});
}
If you try to surround it with an if case you get the same error:
void handleDog(Dog dog){
if(dog.breeds != null) {
printBreeds(dog.breeds); //Error: The argument type 'List<String>?' can't be assigned to the parameter type 'List<String>'.
}
}
If you create a new property and then null check it it works, but it becomes bothersome to create new properties each time you want to null check:
void handleDog(Dog dog) {
final List<String>? breeds = dog.breeds;
if (breeds != null) {
printBreeds(breeds); // OK!
}
}
Is there a better way to do this?
Like the ?.let{} syntax in kotlin?

Answer 1

To get something similar to Kotlins .let{} i created the following generic extension :
extension NullSafeBlock<T> on T? {
void let(Function(T it) runnable) {
final instance = this;
if (instance != null) {
runnable(instance);
}
}
}
And it can be used like this:
void handleDog(Dog dog) {
dog.breeds?.let((it) => printBreeds(it));
}
"it" inside the let function will never be null at runtime.
Thanks to all the suggestions, but they were all some variation of moving the null check further down the code execution cain, which was not what i was looking for.

Answer 2

Yes, you'll have create a local variable just like you did to handle those things because if you don't create a local variable then if there is a class which is extending the Dog class can override breeds which will then become nullable even after you had checked it in the first place.
The other solution you can try is changing the List<String> to nullable in printBreeds method.
void handleDog(Dog dog) {
printBreeds(dog.breeds);
}
void printBreeds(List<String>? breeds) {
breeds?.forEach((breed) {
print(breed);
});
}

Answer 3

This error is right //Error: The argument type 'List<String>?' can't be assigned to the parameter type 'List<String>'.
as null type list is passing to function which says it accepts a non-null list
By below way breeds can be accessible
void printBreeds(List<String>? breeds) {
breeds?.forEach((breed) {
print(breed);
});
}
Also, if we don't want nullable operation every time, we can handle it while calling
Example:
class Dog {
final List<String>? breeds;
Dog(this.breeds);
}
void handleDog(Dog dog) {
print("handleDog");
printBreeds(dog.breeds!);
}
// This method only called if breeds not null
void printBreeds(List<String> breeds) {
print("printBreeds");
breeds.forEach((breed) {
print(breed);
});
}
void main() {
var dog = Dog(null);
handleDog(dog);
}
Output:
printBreeds