flutterdartwidgetontap

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:

    11
    101
    5
    50