Flutter開發之Dart語法基礎

var name = 'name';
dynamic name1 = 'name1';
String name2 = 'name2';

// 變數的賦值
name = 'a';
name1 = 'a1';
name2 = 'a2';

var name;
dynamic name1;
String name2;

String name2 = 'name2';

final age = 10;
final int age1 = 20;

// final修飾的變數不能重新賦值, 會報錯
age = 20;
age1 = 30;

// const
const m1 = 12;
const double m2 = 23;
const m3 = m1 + m2;

// final修飾的變數不能重新賦值, 會報錯
m1 = 10;
m2 = 1.02;

assert(2 + 3 == 5);
assert(2 - 3 == -1);
assert(2 * 3 == 6);
assert(5 / 2 == 2.5);// 結果是double型別
assert(5 ~/ 2 == 2);// 結果是integer型別 
assert(5 % 2 == 1);// 餘數

var a = 0, b = 0;

b = a++;
print('a = $a, b = $b'); //a = 1, b = 0
b = ++a;
print('a = $a, b = $b'); //a = 2, b = 2


b = a--;
print('a = $a, b = $b'); //a = 1, b = 2
b = --a;
print('a = $a, b = $b'); //a = 0, b = 0

external bool identical(Object a, Object b);

if (emp is Person) { // Type check
emp.firstName = 'Bob';
}

// 上面程式碼可簡化為
(emp as Person).firstName = 'Bob';

// 給 a 變數賦值
a = value;

// 複合賦值操作符
a += b;// 等價於a = a + b;

// 如果 b 是 null，則賦值給 b；
// 如果不是 null，則 b 的值保持不變
b ??= value; 

// 如下所示:
var s;
print(s);// null
print(s ?? 'str');// str
s ??= 'string';
print(s);// string

condition ? expr1 : expr2
// 如果 condition 是 true，執行 expr1 (並返回執行的結果)； 否則執行 expr2 並返回其結果

expr1 ?? expr2
// 如果 expr1 是 non-null，返回其值； 否則執行 expr2 並返回其結果

String toString() => msg ?? super.toString();

// 上面的程式碼等價於
String toString() => msg == null ? super.toString() : msg;

// 等價於
String toString() {
if (msg == null) {
return super.toString();
} else {
return msg;
}
}

querySelector('#button') // Get an object.
..text = 'Confirm'// Use its members.
..classes.add('important')
..onClick.listen((e) => window.alert('Confirmed!'));

// 上面程式碼等價於
var button = querySelector('#button');
button.text = 'Confirm';
button.classes.add('important');
button.onClick.listen((e) => window.alert('Confirmed!'));

final addressBook = (new AddressBookBuilder()
..name = 'jenny'
..email = '[email protected]'
..phone = (new PhoneNumberBuilder()
..number = '415-555-0100'
..label = 'home')
.build())
.build();

if (a == 0) {
print('a = 0');
} else if (a == 1) {
print('a = 1');
} else {
print('a = 2');
}

var arr = [0, 1, 2];

// for迴圈
for (var i = 0; i < arr.length; i++) {
print(arr[i]);
}

// for-in迴圈
for (var x in arr) {
print(x);
}

// while 迴圈在執行迴圈之前先判斷條件是否滿足：
while (c == 0) {
print('c = $c');
}

// 而do-while迴圈是先執行迴圈程式碼再判斷條件：
do {
print('c = $c');
} while (c == 0);

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();
}

var command = 'OPEN';
switch (command) {
case 'CLOSED':
print('CLOSED');
break;
case 'APPROVED':
print('APPROVED');
// break;
// 這裡非空的case, 沒有break會報錯
case 'DENIED':
// 這裡空的case, 可以不要break
case 'OPEN':
print('OPEN');
continue nowClosed;
//如果你需要實現這種繼續到下一個 case 語句中繼續執行，則可以 使用 continue 語句跳轉到對應的標籤（label）處繼續執行：
nowClosed:
case 'PENDING':
print('PENDING');
break;
default:
print('default');
}

// Make sure the variable has a non-null value
assert(text != null);

// Make sure the value is less than 100
assert(number < 100);

// Make sure this is an https URL
assert(urlString.startsWith('https'));