4.Operators-操作符(Dart中文文档)

Dart有如下操作符：

Description	Operator
unary postfix	expr++ expr-- () [] . ?.
unary prefix	-expr !expr ~expr ++expr --expr
multiplicative	* / % ~/
additive	+ -
shift	<< >>
bitwise AND	&
bitwise XOR	^
bitwise OR	\
relational and type test	>= > <= < as is is!
equality	== !=
logical AND	&&
logical OR	|\
if null	??
conditional	expr1 ? expr2 : expr3
cascade	..
assignment	= *= /= ~/= %= += -= <<= >>= &= ^= |= ??=

在操作符表中，优先级是从上到下递减。例如，即便&&在之前，优先级是求余（%）>判断相等（）> 且(&&)。下面代码中，计算顺序是一致的

// Parentheses improve readability.
if ((n % i == 0) && (d % i == 0)) ...

// Harder to read, but equivalent.
if (n % i == 0 && d % i == 0) ...

注意：操作符左右各有一个对象，操作符具体行为是由左边的对象决定的，这个涉及到操作符重载，比如Vector和Point对象重载了+号操作符的行为，Vector+Point的+号具体行为由Vector决定。

Arithmetic operators 算术运算符

Dart supports the usual arithmetic operators, as shown in the following table.

Dart支持常见的几种算术运算符

Operator	Meaning
+	Add
–	Subtract
-expr	Unary minus, also known as negation (reverse the sign of the expression)
*	Multiply
/	Divide
~/	Divide, returning an integer result
%	Get the remainder of an integer division (modulo)

assert(2 + 3 == 5);
assert(2 - 3 == -1);
assert(2 * 3 == 6);
assert(5 / 2 == 2.5); // Result is a double
assert(5 ~/ 2 == 2); // Result is an int
assert(5 % 2 == 1); // Remainder

assert('5/2 = ${5 ~/ 2} r ${5 % 2}' == '5/2 = 2 r 1');

Dart也支持自增自减运算符

Operator	Meaning
++var	var = var + 1 (expression value is var + 1)
var++	var = var + 1 (expression value is var)
--var	var = var – 1 (expression value is var – 1)
var--	var = var – 1 (expression value is var)

var a, b;

a = 0;
b = ++a; // Increment a before b gets its value.
assert(a == b); // 1 == 1

Equality and relational operators 关系运算符

Operator	Meaning
==	Equal; see discussion below
!=	Not equal
>	Greater than
<	Less than
>=	Greater than or equal to
<=	Less than or equal to

验证两个对象是否相同，用操作符,（在极少数情况下，你要判断两个对象是否完全相同，需要用identical()函数）

下面是的判断逻辑：

步骤1.如果x,y都是null,返回true ,x,y只有一个是null,返回false

步骤2.返回x.(y)的结果，这种写法类似a.method(params),可以将看做x的方法。

assert(2 == 2);
assert(2 != 3);
assert(3 > 2);
assert(2 < 3);
assert(3 >= 3);
assert(2 <= 3);
a = 0;
b = a++; // Increment a AFTER b gets its value.
assert(a != b); // 1 != 0
a = 0;
b = --a; // Decrement a before b gets its value.
assert(a == b); // -1 == -1

a = 0;
b = a--; // Decrement a AFTER b gets its value.
assert(a != b); // -1 != 0

Type test operators 类操作符

Operator	Meaning
as	Typecast (also used to specify library prefixes)
is	True if the object has the specified type
is!	False if the object has the specified type

如果对象A是实现T这个类的，那么 A is T == true,is!正好相反。as的用法是将对象A强制转换为指定类型。

下面的例子是判断对象类型，如果是true，可执行对应类型的方法

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

这是上面的简化写法，如果emp是null或者不是Person类型，后续代码不会执行

(emp as Person).firstName = 'Bob';

Assignment operators 赋值运算符

As you’ve already seen, you can assign values using the = operator. To assign only if the assigned-to variable is null, use the ??= operator.

如你所知，可以用=进行赋值，如果只对空对象赋值，可以用??=。

// Assign value to a
a = value;
// Assign value to b if b is null; otherwise, b stays the same
b ??= value;

组合赋值操作符：

= |–= |/= |%= |>>= |^=

+= |*= |~/= |<<= |&=| ||=

等效说明：

	Compound assignment	Equivalent expression
For an operator op:	a op= b	a = a op b
Example:	a += b	a = a + b

The following example uses assignment and compound assignment operators:

var a = 2; // Assign using =
a *= 3; // Assign and multiply: a = a * 3
assert(a == 6);
```

### Logical operators 逻辑运算符

|Operator|	Meaning|
|--|--|
!expr	|inverts the following expression (changes false to true, and vice versa)
||	|logical OR
&&	|logical AND

下面是逻辑运算符示例：
···

if (!done && (col == 0 || col == 3)) {
  // ...Do something...
}

···

### Bitwise and shift operators 位和移位操作符

你可以对数值进行移位操作

Operator|	Meaning
--|--
|&|	AND
\||	OR
^|	XOR
~expr|	Unary bitwise complement (0s become 1s; 1s become 0s)
<<|	Shift left
>>|	Shift right

下面是移位操作示例：
```
final value = 0x22;
final bitmask = 0x0f;

assert((value & bitmask) == 0x02); // AND
assert((value & ~bitmask) == 0x20); // AND NOT
assert((value | bitmask) == 0x2f); // OR
assert((value ^ bitmask) == 0x2d); // XOR
assert((value << 4) == 0x220); // Shift left
assert((value >> 4) == 0x02); // Shift right
```
### Conditional expressions 条件表达式

Dart有两种条件表达式写法，来取代if逻辑。

```
condition ? expr1 : expr2
```
如果condition是true,返回expr1,否则返回expr2
```
expr1 ?? expr2
```
如果expr1不是null,返回expr1,否则返回expr2.
```
var visibility = isPublic ? 'public' : 'private';

String playerName(String name) => name ?? 'Guest';
```

```
// Slightly longer version uses ?: operator.
String playerName(String name) => name != null ? name : 'Guest';
// Very long version uses if-else statement.
String playerName(String name) {
  if (name != null) {
    return name;
  } else {
    return 'Guest';
  }
}
```

### Cascade notation (..) 链式调用操作符
链式操作符允许你对相同对象进行多次调用而不用多次写对象名。

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

上面的示例等效于：
```
var button = querySelector('#confirm');
button.text = 'Confirm';
button.classes.add('important');
button.onClick.listen((e) => window.alert('Confirmed!'));
```
You can also nest your cascades. For example:
```
final addressBook = (AddressBookBuilder()
      ..name = 'jenny'
      ..email = 'jenny@example.com'
      ..phone = (PhoneNumberBuilder()
            ..number = '415-555-0100'
            ..label = 'home')
          .build())
    .build();
```
注意，链式调用必须由对象发起，比如下面的write返回值为void,所以无法使用链式调用。
```
var sb = StringBuffer();
sb.write('foo')
  ..write('bar'); // Error: method 'write' isn't defined for 'void'.
```

### Other operators 其它操作符

Operator|	Name|	Meaning
--|--|--
()	|Function application|	Represents a function call
[]	|List access	|Refers to the value at the specified index in the list
.	|Member access	|Refers to a property of an expression; example: foo.bar selects property bar from expression foo
?.	|Conditional |member access	Like ., but the leftmost operand can be null; example: foo?.bar selects property bar from expression foo unless foo is null (in which case the value of foo?.bar is null)

第四篇准备翻译 Control flow statements 流程控制