如何有条件地将参数传递给 Flutter/Dart 中的 Widget？

Question

I want to conditionally pass an argument while using the widget. Is there any way to do so?

...

return ListTile(
   title: Text("${product.name}"),
   
   // want to conditionally pass subtitle, which is not the right syntax (???)
   if(condition) subtitle: Text("subtitle"), 

   // I know about this
   subtitle: condition? Text("subtitle"): null,
 );
},

...

I know we can conditionally pass the value as null using ternary operator here but what I am looking for is to skip the argument itself.

The above example is for ListTile widget. But my curiosity is to know the syntax for doing so for any widget.

Answer 1

Using optional named parameters , using parameter:value when a parameter is required pass its value else it can be skipped completely. Inside the called method null handling is required to be done by the developer(not handled internally).

This is a simplified sample:

void main() {
  doNothing();
  doNothing(index: 1);
  doNothing(description: 'Printing');
  doNothing(index: 1,description: 'Printing');
  
}

void doNothing({int index, String description}) {
  print('Received => $index : $description');
}

Output:

Received => null : null
Received => 1 : null
Received => null : Printing
Received => 1 : Printing

Note: Default values can be assigned in the Widget/methods implementation and may not necessarily be always null .

Example:

void doNothing({int index, String description, String otherDesc = 'Provided By Default'}) {
  print('Received => $index : $description : $otherDesc ');
}

Output:

Received => null : null : Provided By Default

Answer 2

Since not explicitly initialized variables in dart get automatically initialized to null (if they don't have a default value), for the widget there is no difference between not giving an argument to the optional parameter and giving the argument null .

Because of that setting it to null is actually skipping the argument. I would recommend just using the ternary operator in case you don't want to give an argument.