1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160
//! Dispatches trace events to a [`Collect`].
//!
//! 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 collector that collects it.
//!
//! # Using the Trace Dispatcher
//!
//! Every thread in a program using `tracing` has a _default collector_. When
//! events occur, or spans are created, they are dispatched to the thread's
//! current collector.
//!
//! ## Setting the Default Collector
//!
//! By default, the current collector is an empty implementation that does
//! nothing. Trace data provided to this "do nothing" implementation is
//! immediately discarded, and is not available for any purpose.
//!
//! To use another collector 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 collector for the
//! duration of a scope, while `set_global_default` sets a default collector
//! for the entire process.
//!
//! To use either of these functions, we must first wrap our collector in a
//! [`Dispatch`], a cloneable, type-erased reference to a collector. For
//! example:
//! ```rust
//! # pub struct FooCollector;
//! # use tracing_core::{
//! # dispatch, Event, Metadata,
//! # span::{Attributes, Current, Id, Record}
//! # };
//! # impl tracing_core::Collect for FooCollector {
//! # 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) {}
//! # fn current_span(&self) -> Current { Current::unknown() }
//! # }
//! # impl FooCollector { fn new() -> Self { FooCollector } }
//! # #[cfg(feature = "alloc")]
//! use dispatch::Dispatch;
//!
//! # #[cfg(feature = "alloc")]
//! let my_collector = FooCollector::new();
//! # #[cfg(feature = "alloc")]
//! let my_dispatch = Dispatch::new(my_collector);
//! ```
//! Then, we can use [`with_default`] to set our `Dispatch` as the default for
//! the duration of a block:
//! ```rust
//! # pub struct FooCollector;
//! # use tracing_core::{
//! # dispatch, Event, Metadata,
//! # span::{Attributes, Current, Id, Record}
//! # };
//! # impl tracing_core::Collect for FooCollector {
//! # 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) {}
//! # fn current_span(&self) -> Current { Current::unknown() }
//! # }
//! # impl FooCollector { fn new() -> Self { FooCollector } }
//! # #[cfg(feature = "alloc")]
//! # let my_collector = FooCollector::new();
//! # #[cfg(feature = "alloc")]
//! # let my_dispatch = dispatch::Dispatch::new(my_collector);
//! // no default collector
//!
//! # #[cfg(feature = "std")]
//! dispatch::with_default(&my_dispatch, || {
//! // my_collector is the default
//! });
//!
//! // no default collector again
//! ```
//! It's important to note that `with_default` will not propagate the current
//! thread's default collector to any threads spawned within the `with_default`
//! block. To propagate the default collector 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 FooCollector;
//! # use tracing_core::{
//! # dispatch, Event, Metadata,
//! # span::{Attributes, Current, Id, Record}
//! # };
//! # impl tracing_core::Collect for FooCollector {
//! # 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) {}
//! # fn current_span(&self) -> Current { Current::unknown() }
//! # }
//! # impl FooCollector { fn new() -> Self { FooCollector } }
//! # #[cfg(feature = "alloc")]
//! # let my_collector = FooCollector::new();
//! # #[cfg(feature = "alloc")]
//! # let my_dispatch = dispatch::Dispatch::new(my_collector);
//! // no default collector
//!
//! # #[cfg(feature = "alloc")]
//! dispatch::set_global_default(my_dispatch)
//! // `set_global_default` will return an error if the global default
//! // collector has already been set.
//! .expect("global default was already set!");
//!
//! // `my_collector` is now the default
//! ```
//! <div class="example-wrap" style="display:inline-block">
//! <pre class="ignore" style="white-space:normal;font:inherit;">
//!
//! **Note**: The thread-local scoped dispatcher (`with_default`)
//! requires the Rust standard library. `no_std` users should
//! use [`set_global_default()`] instead.
//!
//! </pre></div>
//!
//! ## Accessing the Default Collector
//!
//! A thread's current default collector 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.
//!
//! [`Collect`]: tracing_core::Collect
#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
pub use tracing_core::dispatch::set_default;
#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
pub use tracing_core::dispatch::with_default;
#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
pub use tracing_core::dispatch::DefaultGuard;
pub use tracing_core::dispatch::{
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::dispatch::has_been_set;