Struct state::Container [−][src]
pub struct Container { /* fields omitted */ }Expand description
A container for global type-based state.
A container stores at most one global instance of given type as well as n thread-local instances of a given type.
Global State
Global state is set via the set method and retrieved via the
get method. The type of the value being set must be
thread-safe and transferable across thread boundaries. In other words, it
must satisfy Sync + Send + 'static.
Example
Set and later retrieve a value of type T:
static CONTAINER: state::Container = state::Container::new();
CONTAINER.set(T::new());
CONTAINER.get::<T>();Freezing
By default, all get, set, get_local, and set_local calls result in
synchronization overhead for safety. However, if calling set or
set_local is no longer required, the overhead can be eliminated by
freezing the Container. A frozen container can only be read and never
written to. Attempts to write to a frozen container will fail.
To freeze a Container, call freeze(). A frozen
container can never be thawed. To check if a container is frozen, call
is_frozen().
Thread-Local State
Thread-local state is set via the set_local method and
retrieved via the get_local method. The type of the
value being set must be transferable across thread boundaries but need not
be thread-safe. In other words, it must satisfy Send + 'static but not
necessarily Sync. Values retrieved from thread-local state are exactly
that: local to the current thread. As such, you cannot use thread-local
state to synchronize across multiple threads.
Thread-local state is initialized on an as-needed basis. The function used
to initialize the thread-local state is passed in as an argument to
set_local. When the state is retrieved from a given thread for the first
time, the function is executed to generate the initial value. The function
is executed at most once per thread. The same function is used for
initialization across all threads.
Note: Rust reuses thread IDs across multiple threads. This means that is possible to set thread-local state in thread A, have that thread die, start a new thread B, and access the state set in A in B.
Example
Set and later retrieve a value of type T:
static CONTAINER: state::Container = state::Container::new();
CONTAINER.set_local(|| T::new());
CONTAINER.get_local::<T>();Implementations
Creates a new container with no stored values.
Example
Create a globally available state container:
static CONTAINER: state::Container = state::Container::new();Freezes the container. A frozen container disallows writes allowing for synchronization-free reads.
Example
use state::Container;
// A new container starts unfrozen and can be written to.
let mut container = Container::new();
assert_eq!(container.set(1usize), true);
// While unfrozen, `get`s require synchronization.
assert_eq!(container.get::<usize>(), &1);
// After freezing, calls to `set` or `set_local `will fail.
container.freeze();
assert_eq!(container.set(1u8), false);
assert_eq!(container.set("hello"), false);
// Calls to `get` or `get_local` are synchronization-free when frozen.
assert_eq!(container.try_get::<u8>(), None);
assert_eq!(container.get::<usize>(), &1);Returns true if the container is frozen and false otherwise.
Example
use state::Container;
// A new container starts unfrozen and is frozen using `freeze`.
let mut container = Container::new();
assert_eq!(container.is_frozen(), false);
container.freeze();
assert_eq!(container.is_frozen(), true);Sets the global state for type T if it has not been set before and
self is not frozen.
If the state for T has previously been set or self is frozen, the
state is unchanged and false is returned. Otherwise true is
returned.
Example
Set the state for AtomicUsize. The first set is succesful while the
second fails.
static CONTAINER: state::Container = state::Container::new();
assert_eq!(CONTAINER.set(AtomicUsize::new(0)), true);
assert_eq!(CONTAINER.set(AtomicUsize::new(1)), false);Attempts to retrieve the global state for type T.
Returns Some if the state has previously been set.
Otherwise returns None.
Example
struct MyState(AtomicUsize);
static CONTAINER: state::Container = state::Container::new();
// State for `T` is initially unset.
assert!(CONTAINER.try_get::<MyState>().is_none());
CONTAINER.set(MyState(AtomicUsize::new(0)));
let my_state = CONTAINER.try_get::<MyState>().expect("MyState");
assert_eq!(my_state.0.load(Ordering::Relaxed), 0);Retrieves the global state for type T.
Panics
Panics if the state for type T has not previously been
set. Use try_get for a non-panicking
version.
Example
struct MyState(AtomicUsize);
static CONTAINER: state::Container = state::Container::new();
CONTAINER.set(MyState(AtomicUsize::new(0)));
let my_state = CONTAINER.get::<MyState>();
assert_eq!(my_state.0.load(Ordering::Relaxed), 0);