HeaderValue abstract interface

abstract interface class HeaderValue

Representation of a header value in the form:

value; parameter1=value1; parameter2=value2

HeaderValue can be used to conveniently build and parse header values on this form.

Parameter values can be omitted, in which case the value is parsed as null. Values can be doubled quoted to allow characters outside of the RFC 7230 token characters and backslash sequences can be used to represent the double quote and backslash characters themselves.

To build an "accepts" header with the value

text/plain; q=0.3, text/html

use code like this:

HttpClientRequest request = ...;
var v = HeaderValue("text/plain", {"q": "0.3"});
request.headers.add(HttpHeaders.acceptHeader, v);
request.headers.add(HttpHeaders.acceptHeader, "text/html");

To parse the header values use the parse static method.

HttpRequest request = ...;
List<String> values = request.headers[HttpHeaders.acceptHeader];
values.forEach((value) {
  HeaderValue v = HeaderValue.parse(value);
  // Use v.value and v.parameters
});

An instance of HeaderValue is immutable.

Implementers

• ContentType

Constructors

HeaderValue() factory

factory HeaderValue([
  String value = "",
  Map<String, String?> parameters = const {},
])

Creates a new header value object setting the value and parameters.

Implementation

factory HeaderValue([
  String value = "",
  Map<String, String?> parameters = const {},
]) => _HeaderValue(value, parameters);

Properties

hashCode no setter inherited

int get hashCode

The hash code for this object.

A hash code is a single integer which represents the state of the object that affects operator == comparisons.

All objects have hash codes. The default hash code implemented by Object represents only the identity of the object, the same way as the default operator == implementation only considers objects equal if they are identical (see identityHashCode).

If operator == is overridden to use the object state instead, the hash code must also be changed to represent that state, otherwise the object cannot be used in hash based data structures like the default Set and Map implementations.

Hash codes must be the same for objects that are equal to each other according to operator ==. The hash code of an object should only change if the object changes in a way that affects equality. There are no further requirements for the hash codes. They need not be consistent between executions of the same program and there are no distribution guarantees.

Objects that are not equal are allowed to have the same hash code. It is even technically allowed that all instances have the same hash code, but if clashes happen too often, it may reduce the efficiency of hash-based data structures like HashSet or HashMap.

If a subclass overrides hashCode, it should override the operator == operator as well to maintain consistency.

Inherited from Object.

Implementation

external int get hashCode;

parameters no setter

Map<String, String?> get parameters

A map of parameters.

This map cannot be modified.

Implementation

Map<String, String?> get parameters;

runtimeType no setter inherited

Type get runtimeType

A representation of the runtime type of the object.

Inherited from Object.

Implementation

external Type get runtimeType;

value no setter

String get value

The value of the header.

Implementation

String get value;

Methods

noSuchMethod() inherited

dynamic noSuchMethod(Invocation invocation)

Invoked when a nonexistent method or property is accessed.

A dynamic member invocation can attempt to call a member which doesn't exist on the receiving object. Example:

dynamic object = 1;
object.add(42); // Statically allowed, run-time error

This invalid code will invoke the noSuchMethod method of the integer 1 with an Invocation representing the .add(42) call and arguments (which then throws).

Classes can override noSuchMethod to provide custom behavior for such invalid dynamic invocations.

A class with a non-default noSuchMethod invocation can also omit implementations for members of its interface. Example:

class MockList<T> implements List<T> {
  noSuchMethod(Invocation invocation) {
    log(invocation);
    super.noSuchMethod(invocation); // Will throw.
  }
}
void main() {
  MockList().add(42);
}

This code has no compile-time warnings or errors even though the MockList class has no concrete implementation of any of the List interface methods. Calls to List methods are forwarded to noSuchMethod, so this code will log an invocation similar to Invocation.method(#add, [42]) and then throw.

If a value is returned from noSuchMethod, it becomes the result of the original invocation. If the value is not of a type that can be returned by the original invocation, a type error occurs at the invocation.

The default behavior is to throw a NoSuchMethodError.

Inherited from Object.

Implementation

@pragma("vm:entry-point")
@pragma("wasm:entry-point")
external dynamic noSuchMethod(Invocation invocation);

toString() override

String toString()

Returns the formatted string representation in the form:

value; parameter1=value1; parameter2=value2

Implementation

String toString();

Operators

operator ==() inherited

bool operator ==(Object other)

The equality operator.

The default behavior for all Objects is to return true if and only if this object and other are the same object.

Override this method to specify a different equality relation on a class. The overriding method must still be an equivalence relation. That is, it must be:

• Total: It must return a boolean for all arguments. It should never throw.

• Reflexive: For all objects o, o == o must be true.

• Symmetric: For all objects o1 and o2, o1 == o2 and o2 == o1 must either both be true, or both be false.

• Transitive: For all objects o1, o2, and o3, if o1 == o2 and o2 == o3 are true, then o1 == o3 must be true.

The method should also be consistent over time, so whether two objects are equal should only change if at least one of the objects was modified.

If a subclass overrides the equality operator, it should override the hashCode method as well to maintain consistency.

Inherited from Object.

Implementation

external bool operator ==(Object other);

Static Methods

parse()

HeaderValue parse(
  String value, {
  String parameterSeparator = ";",
  String? valueSeparator,
  bool preserveBackslash = false,
})

Creates a new header value object from parsing a header value string with both value and optional parameters.

Implementation

static HeaderValue parse(
  String value, {
  String parameterSeparator = ";",
  String? valueSeparator,
  bool preserveBackslash = false,
}) {
  return _HeaderValue.parse(
    value,
    parameterSeparator: parameterSeparator.codeUnitAt(0),
    valueSeparator: valueSeparator?.codeUnitAt(0) ?? _CharCode.NONE,
    preserveBackslash: preserveBackslash,
  );
}