条件

conditioned

A package that contains a set of Widgets to remove ugly if, if else and switch statements from your layout code and keep it more consistent.

Usage

The Conditioned Widget is just like any other and can be used in your Widget tree. Conditioned will have one child. What that child will be is up to you. You will provide the cases and Conditioned will figure out which builder to call.

Note: You will always provide a builder method that results in a Widget. Providing builders instead of Widgets is better for performance and allows us to use null objects.

Basics

When you want to add a specific child based on a condition you can avoid the if, else if and else drama by using the Conditioned Widget. Conditioned will use the builder of the first Case whos expression validates as true.

Without Conditioned

We cannot use if .. else if .. else directly in our layout code. We have to propagate it to a function (not recommended) or to a new Widget.

Widget build(BuildContext context){
  return Container(
    child: _getIcon(),
  );
}

Widget _getIcon() {
  if (myNumber < 25)
    return Icon(Icons.ac_unit);
  else if (myNumber < 50)
    return Icon(Icons.home);
  else if (myNumber < 75)
    return Icon(Icons.wb_cloudy);
  else
    return Icon(Icons.wb_sunny);
}

With Conditioned

With the use of Conditioned you don't have to break up the tree at the point of the if .. else if .. else. You can break up the tree where it makes sense, which is often right after the condition.

Widget build(BuildContext context) {
  return Container(
    child: Conditioned(
      cases: [
        Case(myNumber < 25, builder: () => Icon(Icons.ac_unit)),
        Case(myNumber < 50, builder: () => Icon(Icons.home)),
        Case(myNumber < 75, builder: () => Icon(Icons.wb_cloudy)),
      ],
      defaultBuilder: () => Icon(Icons.wb_sunny),
    ),
  );
}

Simple boolean checks

For simple true/false checks you can use the Conditioned.boolean constructor. It will not reduce your lines of code, it does however make it more consistent with the rest of your layout code.

Without Conditioned.boolean

Widget build(BuildContext context) {
    return Container(
        child: myNumber > 50
          ? Text(
              'The condition is true!',
              style: TextStyle(color: Colors.green[200]),
            )
          : Text(
              'The condition is false!',
              style: TextStyle(color: Colors.green[800]),
        ),
    );
}

With Conditioned.boolean

Widget build(BuildContext context) {
    return Container(
        child: Conditioned.boolean( myNumber > 50,
            trueBuilder: () => Text(
              'The condition is true!',
              style: TextStyle(color: Colors.orange[200]),
            ),
            falseBuilder: () => Text(
              'The condition is false!',
              style: TextStyle(color: Colors.orange[800]),
            ),
        ),
    );
}

Switch cases

If instead of checking for boolean conditions but want to check by equality where you would normally want to use a switch, you can use the Conditioned.equality<T> static method.

Let's say we need to build a screen based on the following information:

enum State {empty, loading, full, error}
final state = State.idle;

Without Conditioned.equality<T>

We cannot use a switch directly in our layout code. We have to propagate it to a function (not recommended) or to a new Widget.

Widget build(BuildContext context) {
  return Container(
    child: _getStateText(),
  );
}

Widget _getStateText() {
  switch(state) {
    case State.loading:
      return Text('The device is loading');
    case State.full:
      return Text('The device is fully loaded');
    case State.error:
      return Text('An error occured');
    default:
      return Text('The device is operating as expected');
    }
}

With Conditioned.equality<T>

With the use of Conditioned.equality<T> you don't have to break up the tree at the point of the switch or introduce a Builder Widget. You can break up the tree where it makes more sense, right after the switch for example.

Widget build(BuildContext context) {
  return Container(
    child: Conditioned.equality<State>( state,
      values: [
        Value(State.loading, builder: () => Text('The device is loading')),
        Value(State.full, builder: () => Text('The device is fully loaded')),
        Value(State.error, builder: () => Text('An error occured')),
      ],
      defaultBuilder: () => Text('The device is operating as expected')
    ),
  );
}