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| </pre><pre class="rust"><code><span class="doccomment">//! 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 |
| //! ``` |
| //! |
| //! <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> |
| //! |
| //! ## 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`]: crate::Subscriber |
| </span><span class="attribute">#[cfg(feature = <span class="string">"std"</span>)] |
| #[cfg_attr(docsrs, doc(cfg(feature = <span class="string">"std"</span>)))] |
| </span><span class="kw">pub use </span>tracing_core::dispatcher::set_default; |
| <span class="attribute">#[cfg(feature = <span class="string">"std"</span>)] |
| #[cfg_attr(docsrs, doc(cfg(feature = <span class="string">"std"</span>)))] |
| </span><span class="kw">pub use </span>tracing_core::dispatcher::with_default; |
| <span class="attribute">#[cfg(feature = <span class="string">"std"</span>)] |
| #[cfg_attr(docsrs, doc(cfg(feature = <span class="string">"std"</span>)))] |
| </span><span class="kw">pub use </span>tracing_core::dispatcher::DefaultGuard; |
| <span class="kw">pub use </span>tracing_core::dispatcher::{ |
| get_default, set_global_default, Dispatch, SetGlobalDefaultError, WeakDispatch, |
| }; |
| |
| <span class="doccomment">/// 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. |
| </span><span class="attribute">#[doc(hidden)] |
| </span><span class="kw">pub use </span>tracing_core::dispatcher::has_been_set; |
| </code></pre></div> |
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