2021-01-23flutter-isEmpty

1、判断空，标准写法--d如过是null或者没有isEmpty方法，就会抛异常
if (d?.isEmpty ?? true) {}

3、如果为null为解析成null字符串用$字面量
'订单编号: ${str}'

3、如果为null，会抛异常
Text(str);
data != null "A non-null String must be provided to a Text widget."

测试一段代码

void main() {
  String str = ''; //isEmpty可以判断长度为0的字符串
  String str2 = null; //需要用?
  List d1 = [];
  List d2 = null;//给对象传null，正常操作
  Map d3 = {};
  Map d4 = null;
  if (d1?.isEmpty) {
    print('空');
  }
  if (d2?.isEmpty ?? true) {
    print('空');
  }
  try {
    if (d2?.isEmpty) {//Assertion failed: "boolean expression must not be null"
      print('空');
    }
  } catch (e) {
    print(e);
  }
  if (d3?.isEmpty) {
    print('空');
  }
  if (d4?.isEmpty ?? true) {
    print('空');
  }
  if (str?.isEmpty ?? true) {
    print('空');
  }
  if (str2?.isEmpty ?? true) {
    print('空');
  }
  try {
    str.isEmpty;
  } catch (e) {
    print("1:$e");
  }
  try {
    str2.isEmpty; //NoSuchMethodError: method not found
  } catch (e) {
    print("2: $e");
  }
  print('订单编号: ${str}');
  print('订单编号: ${str2}');
}

运行结果

空
空
Assertion failed: "boolean expression must not be null"
空
空
空
空
2: NoSuchMethodError: method not found: 'get$isEmpty' on null
订单编号: 
订单编号: null

再测试：

  @override
  Widget build(BuildContext context) {
    String str;
    try {
      str = null;
    } catch (e) {
      print("e1: $e");
    }
    try {
      Text txt = Text(str, style: Theme.of(context).textTheme.headline4);

      return txt;
    } catch (e) {
      print("e2: $e");
    }
    return SizedBox();
  }

运行结果

e2: Assertion failed: file:///Users/brettmorgan/Documents/GitHub/dart-services/flutter/packages/flutter/lib/src/widgets/text.dart:379:10
data != null
"A non-null String must be provided to a Text widget."