4.Operators-操作符(Dart中文文档)

Posted 2021-01-28 lowezheng

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
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