Handle empty and contains

[appsup-dart]

Handle empty and contains

[oodavid]

@MatteoAntolini take a peek at the example on pub.dev

You'll see that the expression contains functions cos and sin, which are made available to the evaluator through context. The code below shows how you might implement contains:

var expression = Expression.parse('contains(a,b)');
var context = {
  'a': 'Hello World',
  'b': 'Hello',
  'contains': (String a, String b) {
    return a.contains(b);
  }
};
final evaluator = const ExpressionEvaluator();
print(evaluator.eval(expression, context));

If we change the context for different values of a and b, we'll see false instead of true:

var expression = Expression.parse('contains(a,b)');
var context = {
  'a': 'Hello World',
  'b': 'Hi',
  'contains': (String a, String b) {
    return a.contains(b);
  }
};
final evaluator = const ExpressionEvaluator();
print(evaluator.eval(expression, context));

I hope this helps

[rbellens]

If you want to be able to call a member method of some objects, you need to implement a custom evaluator. See the example directory for an example of this.

[MatteoAntolini]

Ok, but I have it in the expression.

+a empty or +a contains "b"

How can I do it?

[rbellens]

For this the parser should be changed. This is currently not supported. You can however copy the code of the default parser to your own code and alter it as you wish. If you find a solution that is generic enough, you can also do a pull request.