Dart 语言入门扫盲篇之4：【控制语句】

【量】的修饰符

关键词：条件、循环、控制、异常处理、assert语句

·条件语句

if、if-else、switch-case

·循环语句

for、for-in、while、do-while

·控制语句

break、continue

·assert 语句

开发期间用来检查参数的，assert(condition，optionalMessage), 如果 condition 为 false， 那么将会抛出 [AssertionError]异常，停止执行程序。（通常只用于开发阶段，在运行时被忽略）

void main() {
  var x = 5 / 2;
  var x1 = 5 ~/ 2;

  assert(x == 2, '测试x');
  assert(x1 == 2, '测试x1');
}
//结果：
 Failed assertion: line 48 pos 10: 'x == 2': 测试x
#0      _AssertionError._doThrowNew (dart:
core-patch/errors_patch.dart:42:39)
#1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:38:5)
#2      main (package:dartdemo/5_exception_main.dart:48:10)
#3      _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:307:19)
#4      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:174:12)

·异常处理语句

try-catch-finally、throw、rethrow
throw： 可以抛出任意对象
try-catch：捕获异常
finally： 确保某些代码一定能运行
rethrow： 抛给上一级处理

  执行顺序为： 内层 exception - 内层 finally -> rethrow -> 外层 exception - 外层 finally

void main() {
   try {
    try {
      List<String> numList = ['1', '2'];
      print(numList[6]);
    } on Exception catch (e) {
      //通过on 关键词来指定异常类型
      print('Exception details:\n $e');
    } catch (e, s) {
      //捕获异常对象
      print('Exception details:\n $e');
      //堆栈跟踪
      print('Stack trace:\n $s');
      rethrow; //抛给上一级处理
    } finally {
      print('Stack trace:finally');
    }
  } catch (e, s) {
    //捕获异常对象
    print('外层 Exception details:\n $e');
    //堆栈跟踪
    print('外层 Stack trace:\n $s');
  } finally {
    print('Stack trace:finally2222');
  }
}