How to check if a function parameter is null

Question

I want to know if a function passed by the constructor is null or not, but when I check it, it doesn't recognize the function as null, even though I didn't pass anything as a parameter.

This is my widget:

const CropImageWidget({
    this.navBack,
    super.key,
  });

final Function? navBack;

Here is where I check if navBack is null or not:

if (widget.navBack == null) {
   Navigator.pop(context);
}
widget.navBack!();

The problem is that widget.navBack is never null.

I need to check if a function is or isn't passed in the constructor to execute function or to just pop the screen.

Solution: I thought the Navigator.pop(context) would return and not execute the code after it, however it executes the pop and then executes the function, which gives me the error. So, to make the code work I just did an If/else like this:

if (widget.navBack == null) {
    Navigator.pop(context);
} else {
    widget.navBack!();
}

actually you can answer to your own question. do not put the answer in the question section. to help other people find the answer who has same issue later — pmatatias
– pmatatias, Commented Oct 26, 2022 at 15:27

WilliamKose · Accepted Answer · 2022-10-26 15:55:24Z

2

I thought the Navigator.pop(context) would return and not execute the code after it, however it executes the pop and then executes the function, which gives me the error. So, to make the code work I just did an If/else like this:

if (widget.navBack == null) {
    Navigator.pop(context);
} else {
    widget.navBack!();
}

answered Oct 26, 2022 at 15:55

WilliamKose

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Comments

john · Accepted Answer · 2022-10-26 15:15:17Z

0

try this:

           final VoidCallback? navBack;

            onPressed: (){
              if(navBack == null){
             Navigator.pop(context);
              }
            }

answered Oct 26, 2022 at 15:15

john

2,01417 silver badges23 bronze badges

2 Comments

WilliamKose Over a year ago

This didn't work. I think I have found the solution. I thought the Navigator.pop(context); would return and not execute the code after it, however it executes the pop and then executes the function, which gives me the error.

john Over a year ago

it works for me. I run the code before posting. it might be a problem with your linter.

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