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//! Dispatches trace events to [`Subscriber`]s. //! //! The _dispatcher_ is the component of the tracing system which is responsible //! for forwarding trace data from the instrumentation points that generate it //! to the subscriber that collects it. //! //! # Using the Trace Dispatcher //! //! Every thread in a program using `tracing` has a _default subscriber_. When //! events occur, or spans are created, they are dispatched to the thread's //! current subscriber. //! //! ## Setting the Default Subscriber //! //! By default, the current subscriber is an empty implementation that does //! nothing. To use a subscriber implementation, it must be set as the default. //! There are two methods for doing so: [`with_default`] and //! [`set_global_default`]. `with_default` sets the default subscriber for the //! duration of a scope, while `set_global_default` sets a default subscriber //! for the entire process. //! //! To use either of these functions, we must first wrap our subscriber in a //! [`Dispatch`], a cloneable, type-erased reference to a subscriber. For //! example: //! ```rust //! # pub struct FooSubscriber; //! # use tracing_core::{ //! # dispatcher, Event, Metadata, //! # span::{Attributes, Id, Record} //! # }; //! # impl tracing_core::Subscriber for FooSubscriber { //! # fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) } //! # fn record(&self, _: &Id, _: &Record) {} //! # fn event(&self, _: &Event) {} //! # fn record_follows_from(&self, _: &Id, _: &Id) {} //! # fn enabled(&self, _: &Metadata) -> bool { false } //! # fn enter(&self, _: &Id) {} //! # fn exit(&self, _: &Id) {} //! # } //! # impl FooSubscriber { fn new() -> Self { FooSubscriber } } //! use dispatcher::Dispatch; //! //! let my_subscriber = FooSubscriber::new(); //! let my_dispatch = Dispatch::new(my_subscriber); //! ``` //! Then, we can use [`with_default`] to set our `Dispatch` as the default for //! the duration of a block: //! ```rust //! # pub struct FooSubscriber; //! # use tracing_core::{ //! # dispatcher, Event, Metadata, //! # span::{Attributes, Id, Record} //! # }; //! # impl tracing_core::Subscriber for FooSubscriber { //! # fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) } //! # fn record(&self, _: &Id, _: &Record) {} //! # fn event(&self, _: &Event) {} //! # fn record_follows_from(&self, _: &Id, _: &Id) {} //! # fn enabled(&self, _: &Metadata) -> bool { false } //! # fn enter(&self, _: &Id) {} //! # fn exit(&self, _: &Id) {} //! # } //! # impl FooSubscriber { fn new() -> Self { FooSubscriber } } //! # let my_subscriber = FooSubscriber::new(); //! # let my_dispatch = dispatcher::Dispatch::new(my_subscriber); //! // no default subscriber //! //! # #[cfg(feature = "std")] //! dispatcher::with_default(&my_dispatch, || { //! // my_subscriber is the default //! }); //! //! // no default subscriber again //! ``` //! It's important to note that `with_default` will not propagate the current //! thread's default subscriber to any threads spawned within the `with_default` //! block. To propagate the default subscriber to new threads, either use //! `with_default` from the new thread, or use `set_global_default`. //! //! As an alternative to `with_default`, we can use [`set_global_default`] to //! set a `Dispatch` as the default for all threads, for the lifetime of the //! program. For example: //! ```rust //! # pub struct FooSubscriber; //! # use tracing_core::{ //! # dispatcher, Event, Metadata, //! # span::{Attributes, Id, Record} //! # }; //! # impl tracing_core::Subscriber for FooSubscriber { //! # fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) } //! # fn record(&self, _: &Id, _: &Record) {} //! # fn event(&self, _: &Event) {} //! # fn record_follows_from(&self, _: &Id, _: &Id) {} //! # fn enabled(&self, _: &Metadata) -> bool { false } //! # fn enter(&self, _: &Id) {} //! # fn exit(&self, _: &Id) {} //! # } //! # impl FooSubscriber { fn new() -> Self { FooSubscriber } } //! # let my_subscriber = FooSubscriber::new(); //! # let my_dispatch = dispatcher::Dispatch::new(my_subscriber); //! // no default subscriber //! //! dispatcher::set_global_default(my_dispatch) //! // `set_global_default` will return an error if the global default //! // subscriber has already been set. //! .expect("global default was already set!"); //! //! // `my_subscriber` is now the default //! ``` //! <div class="information"> //! <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div> //! </div> //! <div class="example-wrap" style="display:inline-block"> //! <pre class="ignore" style="white-space:normal;font:inherit;"> //! <strong>Note</strong>: The thread-local scoped dispatcher (<code>with_default</code>) //! requires the Rust standard library. <code>no_std</code> users should //! use <a href="fn.set_global_default.html"><code>set_global_default</code></a> //! instead. //! </pre></div> //! //! ## Accessing the Default Subscriber //! //! A thread's current default subscriber can be accessed using the //! [`get_default`] function, which executes a closure with a reference to the //! currently default `Dispatch`. This is used primarily by `tracing` //! instrumentation. //! //! [`Subscriber`]: struct.Subscriber.html //! [`with_default`]: fn.with_default.html //! [`set_global_default`]: fn.set_global_default.html //! [`get_default`]: fn.get_default.html //! [`Dispatch`]: struct.Dispatch.html #[cfg(feature = "std")] #[cfg_attr(docsrs, doc(cfg(feature = "std")))] pub use tracing_core::dispatcher::set_default; #[cfg(feature = "std")] #[cfg_attr(docsrs, doc(cfg(feature = "std")))] pub use tracing_core::dispatcher::with_default; #[cfg(feature = "std")] #[cfg_attr(docsrs, doc(cfg(feature = "std")))] pub use tracing_core::dispatcher::DefaultGuard; pub use tracing_core::dispatcher::{ get_default, set_global_default, Dispatch, SetGlobalDefaultError, }; /// Private API for internal use by tracing's macros. /// /// This function is *not* considered part of `tracing`'s public API, and has no /// stability guarantees. If you use it, and it breaks or disappears entirely, /// don't say we didn;'t warn you. #[doc(hidden)] pub use tracing_core::dispatcher::has_been_set;