Understanding Brackets in Dart (Flutter)

问题

I have many confusion about Brackets in Dart(Flutter). Which bracket "(), {}, []" is used for what?

回答1:

() can group expressions:

var x = (1 + 2) * 3;

or can designate parameter lists for functions:

var getAnswer = () => 42;

int square(int x) => x * x;

or can designate function calls:

var answer = getAnswer();
var squared = square(4);

or is part of the syntax to some keywords. This includes (but is not limited to) if, assert, for, while, switch, catch:

if (condition) {
  ...
}

assert(condition);

for (var item in collection) {
  ...
}


while (condition) {
  ...
}

switch (value) {
  ...
}

try {
  ...
} on Exception catch (e) {
  ...
}

[] by itself creates List literals:

var list = [1, 2, 3];
var emptyList = []; // Creates a List<dynamic>.

or when used on an object, calls operator [], which usually accesses an element of a collection:

var element = list[index];
var value = map[key];

or in a parameter list, specifies optional positional parameters:

int function(int x, [int y, int z]) {
  return x + y ?? 0 + z ?? 0;
}

function(1, 2);

or in dartdoc comments, creates linked references to other symbols:

/// Creates a [Bar] from a [Foo].
Bar fooToBar(Foo foo) {
  // ...
}

{} can create a block of code, grouping lines together and limiting variable scope. This includes (but is not limited to) function bodies, class declarations, if-else blocks, try-catch blocks, for blocks, while blocks, switch blocks, etc.:

class Class {
  int member;
}

void doNothing() {}

void function(bool condition) {
  {
    int x = 'Hello world!';
    print(x);
  }

  if (condition) {
    int x = 42; // No name collision due to separate scopes.
    print(x);
  }
}

or by itself can create Set or Map literals:

var set = {1, 2, 3};
var emptySet = <int>{};

var map = {'one': 1, 'two': 2, 'three': 3};
var emptyMap = {}; // Creates a Map<dynamic, dynamic>

or in a parameter list, specifies optional named parameters:

int function(int x, {int y, int z}) {
  return x + y ?? 0 + z ?? 0;
}

function(1, y: 2);

or creates enumerations:

enum SomeEnumeration {
  foo,
  bar,
  baz,
}

or in Strings is used to disambiguate interpolated expressions:

var foo = 'foo';
var foobar = '${foo}bar';

var s = '${function(1, 2)}';

<> when used as a pair in function or class declarations creates generics:
```
class GenericClass<T> {
  T member;
}

T function<T>(T x) {
  // ...
}
```
or specifies explicit types when using generics:
```
var map = <String, int>{};
```

来源：https://stackoverflow.com/questions/61947930/understanding-brackets-in-dart-flutter

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