Ignoring cast fail from JSArray to List

问题:

I receive json from api with string array. Then I set it to local variable with type List.

if (json['x'] is List) {
  List<String> x = json['x'];
  print(x);
}

When I run application, Chrome show me a warning: "Ignoring cast fail from JSArray to List"

What should I do with that?

---

• Angular 5.0.0-alpha+6
• Chrome 63.0.3239.132
• Dart VM version: 2.0.0-dev.32.0

回答1:

JSON encodes all arrays as List<dynamic>, since that is the specification.

If you want to cast something to List<String>, you can't rely on just an implicit cast like you did in Dart 2. You must either use the real type:

List x = json['x'];

Or use the .cast function:

var x = (json['x'] as List).cast<String>();

I realize that is more writing than before. You might want to look at JSON serialization package such as json_serializable or built_value if you don't like the boiler-plate around this.