bool.fromEnvironment constructor
Boolean value for name
in the compilation configuration environment.
The compilation configuration environment is provided by the
surrounding tools which are compiling or running the Dart program.
The environment is a mapping from a set of string keys to their associated
string value.
The string value, or lack of a value, associated with a name
must be consistent across all calls to String.fromEnvironment,
int.fromEnvironment, bool.fromEnvironment
and bool.hasEnvironment
in a single program.
The string values can be directly accessed using String.fromEnvironment.
This constructor parses the string value associated with name
as
a boolean, as if by bool.tryParse,
meaning that it accepts only the strings "true"
and "false"
.
If there is no value associated with name
in the compilation
configuration environment, or if the associated string value is not one
of "true"
or "false"
, the value of the constructor invocation
is the defaultValue
boolean, which defaults to the boolean value
false
.
The result is the same as that of:
(const String.fromEnvironment(name) == "true")
|| ((const String.fromEnvironment(name) != "false") && defaultValue)
Example:
const bool loggingEnabled = bool.fromEnvironment("logging");
In order to check whether a value is there at all, use bool.hasEnvironment. Example:
const bool? yesNoMaybe = bool.hasEnvironment("optionalFlag")
? bool.fromEnvironment("optionalFlag")
: null;
To accept other strings than "true"
or "false"
, use the
String.fromEnvironment constructor directly. Example:
const isLoggingOn = (const String.fromEnvironment("logging") == "on");
This constructor is only guaranteed to work when invoked as const
.
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.
Implementation
external const factory bool.fromEnvironment(String name,
{bool defaultValue = false});