int.fromEnvironment constructor

Integer 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 looks up the string value for name, then attempts to parse it as an integer, using the same syntax rules as int.parse/int.tryParse. That is, it accepts decimal numerals and hexadecimal numerals with a 0x prefix, and it accepts a leading minus sign. If there is no value associated with name in the compilation configuration environment, or if the associated string value cannot be parsed as an integer, the value of the constructor invocation is the defaultValue integer, which defaults to the integer zero.

The result is effectively the same as that of:

int.tryParse(const String.fromEnvironment(name, defaultValue: ""))
    ?? defaultValue

except that the constructor invocation can be a constant value.

Example:

const defaultPort = int.fromEnvironment("defaultPort", defaultValue: 80);

In order to check whether a value is there at all, use bool.hasEnvironment. Example:

const int? maybeDeclared = bool.hasEnvironment("defaultPort")
    ? int.fromEnvironment("defaultPort")
    : null;

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.

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.