Dart: null safety variable and function

null safety variable

In dart, we can check whether a property is null or not, and inside the if check we can get the value directly, for example:void printValue(int? value) {
 if (value != null) {
   if (kDebugMode) {
     print(1 + value);
   }
 }
}

But if we define the value in the Widget or on the State object, then we can’t check the null like below:if (widget.value != null) {
 if (kDebugMode) {
   print(1 + widget.value);
 }
}

But we can do like below:int? value = widget.value;
if (value != null) {
 if (kDebugMode) {
   print(1 + value);
 }
}

From the code above, the first example that the we can use the value inside the if check because the value is a parameter, and it can’t be changed, but for the Widget or State, it is a member field, and it can be changed, so we can’t get the value inside the if check.

null safety function

If I define a function on the widget, then how can I call the method? We can use ?.call to call the function, for example, I define an onDelete function like below:final Function(int)? onDelete;

We can call the onDelete function like below:widget.onDelete?.call(1);

Resources

• Understanding null safety