Flutter - Function is called OnTap: but not with Ontap () {...}

I'm pretty inexperienced with Flutter, I created a widget with this piece of code inside:

With code written like this: (the ToolModify function is not called)

final VoidCallback ToolModify;


onTap: () {
  // rest of the code
  widget.ToolModify;
},

Instead, it is called with code written like this:

onTap: widget.ToolModify,

Can anyone explain to me why this happens?

Since I need to write other lines of code besides widget.ToolModify; how can i call the ToolModify function inside OnTap: () {...} ??

Hope someone can help me. Thanks :)

Solution

It is perfectly legal and good style to code like this:

onTap: widget.ToolModify,

In this case, widget.ToolModify denotes a function and you assign the function to onTap to make onTap call the function later on.

If you call it like this, you make a new anonymous and parameterless function () {} that just calls the other function widget.ToolModify():

onTap: () {
  widget.ToolModify(); 
},

Note, that widget.ToolModify denotes the function itself. While widget.ToolModify() calls the function with no parameters.

Moreover, you may write this, since you return just one expression:

onTap: () => widget.ToolModify(),

Example

Have a look at this example that defines a function ad(), that returns an anonymous function and later on applies it to some arguments:

  // This function takes one argument x and returns a new function
  // of one parameter
  Function(int) add( y ) {
    return ( x ) => ( x + y );
  }
  
  // The above => notation is only applicable, if exactly one expression is returned
  // In case the structure of the inner function is more complex, write
  // it using non shorthand notation like so
  Function(int) divideBy( y ) {
    return ( x ) {
      assert( y > 0 );
      return ( x / y );
    };
  }
  
  // Use the above function to create two specialist functions
  Function(int) add100 = add( 100 );
  
  // Simmilar, but implicitly typed
  var add10 = add( 10 );
  
  var halfOf = divideBy( 2 );

  var doubleOf = divideBy( 0.2 );


  // Now apply two specialized functions
  print( add10( 1 ) );
  print( add100( 1) );
  
  print( halfOf( 10 ) );
  print( doubleOf( 10 ) );

Outputs this: