bool.hasEnvironment constructor Null safety
- String name
Whether there is an environment declaration
Returns true iff there is an environment declaration with the name
If there is then the value of that declaration can be accessed using
const String.fromEnvironment(name). Otherwise,
String.fromEnvironment(name, defaultValue: someString)
will evaluate to the given
This constructor can be used to handle an absent declaration
specifically, in ways that cannot be represented by providing
a default value to the
C.fromEnvironment constructor where
is one of String, int, or bool.
const loggingIsDeclared = bool.hasEnvironment("logging"); const String? logger = loggingIsDeclared ? String.fromEnvironment("logging") : null;
The string value, or lack of a value, associated with a
must be consistent across all calls to String.fromEnvironment,
int.fromEnvironment, bool.fromEnvironment and
in a single program.
This constructor is only guaranteed to work when invoked as
It may work as a non-constant invocation on some platforms which
have access to compiler options at run-time, but most ahead-of-time
compiled platforms will not have this information.
// The .hasEnvironment() constructor is special in that we do not want // users to call them using "new". We prohibit that by giving them bodies // that throw, even though const constructors are not allowed to have bodies. // Disable those static errors. //ignore: const_constructor_with_body //ignore: const_factory external const factory bool.hasEnvironment(String name);