前端学习-Dart官方文档学习-005-控制流

官方文档链接

Loops循环

• for loops
• while and do while loops
• break and continue
• Branching, like if and switch
• Exceptions, like try, catch, and throw

for、for-in、forEach

var callbacks = [];
for (var i = 0; i < 2; i++) {
  callbacks.add(() => print(i));
}
// The output is 0 and then 1
for (final c in callbacks) {
  c();
}

// 使用pattern
for (final Candidate(:name, :yearsExperience) in candidates) {
  print('$name has $yearsExperience of experience.');
}

// forEach
var collection = [1, 2, 3];
collection.forEach(print); // 1 2 3

while and do while

while (!isDone()) {
  doSomething();
}

do {
  printLine();
} while (!atEndOfPage());

Break and continue

while (true) {
  if (shutDownRequested()) break;
  processIncomingRequests();
}

for (int i = 0; i < candidates.length; i++) {
  var candidate = candidates[i];
  if (candidate.yearsExperience < 5) {
    continue;
  }
  candidate.interview();
}

// If you're using an Iterable such as a list or set, how you write the previous example might differ:
candidates
    .where((c) => c.yearsExperience >= 5)
    .forEach((c) => c.interview());

Branches 分支

• if statements and elements
• if-case statements and elements
• switch statements and expressions
• Loops, like for and while
• Exceptions, like try, catch, and throw

if、if-case

if (isRaining()) {
  you.bringRainCoat();
} else if (isSnowing()) {
  you.wearJacket();
} else {
  car.putTopDown();
}

if (pair case [int x, int y]) {
  print('Was coordinate array $x,$y');
} else {
  throw FormatException('Invalid coordinates.');
}

Switch

switch语句

var command = 'OPEN';
switch (command) {
  case 'CLOSED':
    executeClosed();
  case 'PENDING':
    executePending();
  case 'APPROVED':
    executeApproved();
  case 'DENIED':
    executeDenied();
  case 'OPEN':
    executeOpen();
  default:
    executeUnknown();
}

switch (command) {
  case 'OPEN':
    executeOpen();
    continue newCase; // Continues executing at the newCase label.

  case 'DENIED': // Empty case falls through.
  case 'CLOSED':
    executeClosed(); // Runs for both DENIED and CLOSED,

  newCase:
  case 'PENDING':
    executeNowClosed(); // Runs for both OPEN and PENDING.
}

switch 表达式

var x = switch (y) { ... };

print(switch (x) { ... });

return switch (x) { ... };

如何将switch语句转为表达式

// switch语句
// Where slash, star, comma, semicolon, etc., are constant variables...
switch (charCode) {
  case slash || star || plus || minus: // Logical-or pattern
    token = operator(charCode);
  case comma || semicolon: // Logical-or pattern
    token = punctuation(charCode);
  case >= digit0 && <= digit9: // Relational and logical-and patterns
    token = number();
  default:
    throw FormatException('Invalid');
}

// switch表达式
token = switch (charCode) {
  slash || star || plus || minus => operator(charCode),
  comma || semicolon => punctuation(charCode),
  >= digit0 && <= digit9 => number(),
  _ => throw FormatException('Invalid')
};

Exhaustiveness checking（穷尽检查）

穷尽检查是一种在编程中用于确保所有可能情况都被处理的技术。在 Dart 中，通常用于处理枚举类型或联合类型。

// Non-exhaustive switch on bool?, missing case to match null possibility:
switch (nullableBool) {
  case true:
    print('yes');
  case false:
    print('no');
}

例如，在处理枚举类型时，可以使用switch语句来进行穷尽检查。确保在switch语句中处理了枚举的所有可能值，否则会产生编译错误或运行时异常。如果没有处理完所有的枚举值，Dart 编译器会给出相应的错误提示，强制要求进行完整的处理，从而实现穷尽检查。

enum Day { monday, tuesday, wednesday, thursday, friday, saturday, sunday }

void handleDay(Day day) {
  switch (day) {
    case Day.monday:
      // 处理周一的逻辑
      break;
    case Day.tuesday:
      // 处理周二的逻辑
      break;
    case Day.wednesday:
      // 处理周三的逻辑
      break;
    case Day.thursday:
      // 处理周四的逻辑
      break;
    case Day.friday:
      // 处理周五的逻辑
      break;
    case Day.saturday:
      // 处理周六的逻辑
      break;
    case Day.sunday:
      // 处理周日的逻辑
      break;
  }
}

sealed class Shape {}

class Square implements Shape {
  final double length;
  Square(this.length);
}

class Circle implements Shape {
  final double radius;
  Circle(this.radius);
}

double calculateArea(Shape shape) => switch (shape) {
      Square(length: var l) => l * l,
      Circle(radius: var r) => math.pi * r * r
    };

Guard clause

在 Dart 中，Guard clause（卫语句）指的是在函数开头用来判断一些明显非法情况并立即退出的语句。例如常见的指针判空操作，函数参数的校验等。它的主要作用是增强代码的健壮性，减少嵌套的条件表达式，使代码更易于理解和维护。
当guard子句的计算结果为false时，执行将继续执行下一个case，而不是退出整个switch。

// Switch statement:
switch (something) {
  case somePattern when some || boolean || expression:
    //             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Guard clause.
    body;
}

// Switch expression:
var value = switch (something) {
  somePattern when some || boolean || expression => body,
  //               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Guard clause.
}

// If-case statement:
if (something case somePattern when some || boolean || expression) {
  //                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Guard clause.
  body;
}