How to use conditional statement within child attr

问题:

So far whenever I needed to use a conditional statement within a Widget I have done the following (Using Center and Containers as simplified dummy examples):

new Center(
  child: condition == true ? new Container() : new Container()
)

Though when I tried using an if/else statement it would lead to an Dead code warning:

new Center(
  child: 
    if(condition == true){
      new Container();
    }else{
      new Container();
    }
)

Interestingly enough I tried with a switch case statement and it gives me the same warning and thus I cannot run the code. Am I doing something wrong or is it so that one cannot use if/else or switch statements without flutter thinking there is dead code?

回答1:

In Dart, if/else and switch are statements not expressions. They don't return a value so you can't pass them to constructor params. If you have a lot of conditional logic in your build method, then it is a good practice to try and simplify it. For example, you can move self-contained logic to methods, and use if/else statements to initialize local variables which you can later use.

Using a method and if/else

Widget _buildChild() {
  if (condition) {
    return ...
  }
  return ...
}

Widget build(BuildContext context) {
  return new Container(child: _buildChild());
}

Using an if/else

Widget build(BuildContext context) {
  Widget child;
  if (condition) {
    child = ...
  } else {
    child = ...
  }
  return new Container(child: child);
}

回答2:

I found out that an easy way to use conditional logic to build Flutter UI is to keep the logic outside of the UI. Here is a function to return two different colors:

Color getColor(int selector) {
  if (selector % 2 == 0) {
    return Colors.blue;
  } else {
    return Colors.blueGrey;
  }
}

The function is used below to to set the background of the CircleAvatar.

new ListView.builder(
  itemCount: users.length,
  itemBuilder: (BuildContext context, int index) {
    return new Column(
      children: <Widget>[
        new ListTile(
          leading: new CircleAvatar(
            backgroundColor: getColor(index),
            child: new Text(users[index].name[0])
          ),
          title: new Text(users[index].login),
          subtitle: new Text(users[index].name),
        ),
        new Divider(height: 2.0),
      ],
    );
  },
);

Very neat as you can reuse your color selector function in several widgets.

回答3:

Here is the solution. I have fixed it. Here is the code

child: _status(data[index]["status"]),

Widget _status(status) {
  if (status == "3") {
    return Text('Process');
  } else if(status == "1") {
    return Text('Order');
  } else {
    return Text("Waiting");
  }
}