Pass method as parameter to a widget

Question

I have a custom button widget:

class Button extends StatelessWidget {
  final String text;

  Button(this.text);

  @override
  Widget build(BuildContext context) {
    return Container(
      height: 50,
      child: SizedBox(
        width: double.infinity,
        child: RaisedButton(
          onPressed: () => {}, // Use the function from parent Widget
          child: Padding(
              padding: EdgeInsets.symmetric(vertical: 13),
              child: Text(
                text,
                style: TextStyle(fontWeight: FontWeight.bold),
              )),
          color: COLOR_BLUE,
          textColor: Colors.white,
          shape:
              RoundedRectangleBorder(borderRadius: BorderRadius.circular(25)),
        ),
      ),
    );
  }
}

Then in parent Widget I want to pass a onPressed method to this button widget:

...
myMethod () => {
   // do some stuff
}
...
Padding(
    padding: EdgeInsets.only(bottom: 10),
    child: Button("Log in", myMethod),
),
...

How can I tell a button widget to use the myMethod for onPress?

Applejack

see how it is done for a RawMaterialButton – Alyworth

Answer 1

Use VoidCallback type

Check the line comments on the code as well for more information:

class Button extends StatelessWidget {
  final String text;
  final VoidCallback callback; // Notice the variable type

  Button(this.text, this.callback);

  @override
  Widget build(BuildContext context) {
    return Container(
      height: 50,
      child: SizedBox(
        width: double.infinity,
        child: RaisedButton(
          onPressed: callback, // Simply put the function name here, DON'T use ()
          child: Padding(
              padding: EdgeInsets.symmetric(vertical: 13),
              child: Text(
                text,
                style: TextStyle(fontWeight: FontWeight.bold),
              )),
          color: COLOR_BLUE,
          textColor: Colors.white,
          shape:
              RoundedRectangleBorder(borderRadius: BorderRadius.circular(25)),
        ),
      ),
    );
  }
}

• By omitting the parentheses, you are passing the function itself as a parameter. If you added the parentheses, the function would be executed and you would pass the return of the function to the widget.

Then, initialize your widget:

Button("Log in", myMethod)

or

Button("Log in", (){
    print('something');
})

Wassyngton

+1. You can also define your own typedefs for methods if you want one that takes parameters and/or returns a result: dartlang.org/guides/language/language-tour#typedefs – Takao

Any reason for not putting () ? I put onPressed:(){ callback() ; }, and it still worked fine!?! – Censorship

@Censorship when you add (), you are executing the function and getting the response when the code is run. When you skip the parentheses, you are passing the function as a parameter, as if it was a variable. So onPressed actually takes the function itself, not the result of the function. When you put () { callback(); } , you are passing a function with one line as a parameter, you are not executing it :) If you use callback(), onPressed will receive nothing (void) as value, because that what it would return. – Wassyngton

Ahh.. thanks! So correct me if Im wrong... Skipping the parenthesis allows us to accept a returning value, (which I think is an Advantage!)... BUT otherwise () { callback(); } will always have a void return. – Censorship

VoidCallback is the answer yes , thank you bro – Hominy

I get The values in a const list literal must be constants. Try removing the keyword 'const' from the list literal. when I do this. Any ideas? – Cerebrate

Answer 2

There are a few predefined types that already exist.

VoidCallback

If you want to create a parameter something like this:

onPressed: () { },

Then you can define it in your class like so:

class MyWidget extends StatelessWidget {

  MyWidget({Key key, this.onPressed}) : super(key: key);

  final VoidCallback onPressed;

  // ...
}

Notes

The typedef is defined in the source code like this:

typedef VoidCallback = void Function();

The asynchronous version is AsyncCallback.

typedef AsyncCallback = Future<void> Function();

ValueSetter

If you want to create a parameter something like this:

onPressed: (value) { },

Then you can define it in your class like so:

class MyWidget extends StatelessWidget {

  MyWidget({Key key, this.onPressed}) : super(key: key);

  final ValueSetter<String> onPressed;

  // ...
}

Notes

The typedef is defined in the source code like this:

typedef ValueSetter<T> = void Function(T value);

If you want to specify that the function only gets called when there is a change then use ValueChanged instead.

typedef ValueChanged<T> = void Function(T value);

The asynchronous version is AsyncValueSetter.

typedef AsyncValueSetter<T> = Future<void> Function(T value);

ValueGetter

If you want to create a parameter something like this:

onPressed: () => value,

Then you can define it in your class like so:

class MyWidget extends StatelessWidget {

  MyWidget({Key key, this.onPressed}) : super(key: key);

  final ValueGetter<String> onPressed;

  // ...
}

Notes

The typedef is defined in the source code like this:

typedef ValueGetter<T> = T Function();

The asynchronous version is AsyncValueGetter.

typedef AsyncValueGetter<T> = Future<T> Function();

Define your own type

As you can see from all of the examples above, everything is just a typedef for a Function. So it is easy enough to make your own.

Say you want to do something like this:

onEvent: (context, child) => value,

Then you would make the typedef like this:

typedef MyEventCallback = int Function(BuildContext context, Widget widget);

And use it like this:

class MyWidget extends StatelessWidget {

  MyWidget({Key key, this.onEvent}) : super(key: key);

  final MyEventCallback onEvent;

  // ...
}

See the documentation for more.

Anthropophagite

Say I create a CustomTextField that returns a TextField. How would you make a constructor for the onChanged property and pass it correctly? I'm trying to make a constructor for the CustomTextField that passes the function correctly to the TextField inside. Things would be like onChanged: onChanged. – Telemetry

@IvánYoed, You can check out how TextField does it and do the same thing. They have ValueChanged<String>? onChanged where ValueChanged is defined to be void Function(T value). – Anthropophagite