if else

  1. if (isRaining()) {
  2.   you.bringRainCoat();
  3. } else if (isSnowing()) {
  4.   you.wearJacket();
  5. } else {
  6.   car.putTopDown();
  7. }

for

标准 for:

  1. var message = StringBuffer('Dart is fun');
  2. for (var i = 0; i < 5; i++) {
  3.   message.write('!');
  4. }

这种利用闭包的是安全的, 输出 0, 1:

  1. var callbacks = [];
  2. for (var i = 0; i < 2; i++) {
  3.   callbacks.add(() => print(i));
  4. }
  5. callbacks.forEach((c) => c());

Iterable 的 forEach:

  1. candidates.forEach((candidate) => candidate.interview());

Iterable 的 for-in:

  1. var collection = [0, 1, 2];
  2. for (var x in collection) {
  3.   print(x); // 0 1 2
  4. }

while

先判断条件:

  1. while (!isDone()) {
  2.   doSomething();
  3. }

do-whild

先进行循环体:

  1. do {
  2.   printLine();
  3. } while (!atEndOfPage());

break

  1. while (true) {
  2.   if (shutDownRequested()) break;
  3.   processIncomingRequests();
  4. }

continue

  1. for (int i = 0; i < candidates.length; i++) {
  2.   var candidate = candidates[i];
  3.   if (candidate.yearsExperience < 5) {
  4.     continue;
  5.   }
  6.   candidate.interview();
  7. }

dart 风格

  1. candidates
  2.     .where((c) => c.yearsExperience >= 5)
  3.     .forEach((c) => c.interview());

switch-case

  • 比较的类型都是相同类
  • 不能有子类

可以使用:

  • break
  • continue
  • throw
  • return
  1. var command = 'OPEN';
  2. switch (command) {
  3.   case 'CLOSED':
  4.     executeClosed();
  5.     break;
  6.   case 'PENDING':
  7.     executePending();
  8.     break;
  9.   case 'APPROVED':
  10.     executeApproved();
  11.     break;
  12.   case 'DENIED':
  13.     executeDenied();
  14.     break;
  15.   case 'OPEN':
  16.     executeOpen();
  17.     break;
  18.   default:
  19.     executeUnknown();
  20. }

必须要有 clause:

  1. var command = 'OPEN';
  2. switch (command) {
  3.   case 'OPEN':
  4.     executeOpen();
  5.     // ERROR: Missing break
  7.   case 'CLOSED':
  8.     executeClosed();
  9.     break;
  10. }

fall-through 形式允许省略 clause:

  1. var command = 'CLOSED';
  2. switch (command) {
  3.   case 'CLOSED': // Empty case falls through.
  4.   case 'NOW_CLOSED':
  5.     // Runs for both CLOSED and NOW_CLOSED.
  6.     executeNowClosed();
  7.     break;
  8. }

continue 形式的 fall-through:

  • 注意下面代码中 nowClosed 的作用域
  1. var command = 'CLOSED';
  2. switch (command) {
  3.   case 'CLOSED':
  4.     executeClosed();
  5.     continue nowClosed;
  6.   // Continues executing at the nowClosed label.
  8.   nowClosed:
  9.   case 'NOW_CLOSED':
  10.     // Runs for both CLOSED and NOW_CLOSED.
  11.     executeNowClosed();
  12.     break;
  13. }

assert

断言失败会抛出 AssertionError 异常.

当是以下情况时, 断言功能启用:

  • Flutter 启用 debug mode
  • dartdev 开发工具
  • dar, dart2js 工具使用 --enable-asserts 命令标记
  1. // Make sure the variable has a non-null value.
  2. assert(text != null);
  4. // Make sure the value is less than 100.
  5. assert(number < 100);
  7. // Make sure this is an https URL.
  8. assert(urlString.startsWith('https'));

附加字符串:

  1. assert(urlString.startsWith('https'),
  2.     'URL ($urlString) should start with "https".');