class Channel
Usage in Deno
```typescript import { Channel } from "node:node__diagnostics_channel.d.ts"; ```The class `Channel` represents an individual named channel within the data
pipeline. It is used to track subscribers and to publish messages when there
are subscribers present. It exists as a separate object to avoid channel
lookups at publish time, enabling very fast publish speeds and allowing
for heavy use while incurring very minimal cost. Channels are created with [channel](../.././node__diagnostics_channel.d.ts/~/channel), constructing a channel directly
with `new Channel(name)` is not supported.
ContextType = StoreType
readonly
hasSubscribers: boolean
Check if there are active subscribers to this channel. This is helpful if
the message you want to send might be expensive to prepare.
This API is optional but helpful when trying to publish messages from very
performance-sensitive code.
```js
import diagnostics_channel from 'node:diagnostics_channel';
const channel = diagnostics_channel.channel('my-channel');
if (channel.hasSubscribers) {
// There are subscribers, prepare and publish message
}
```
readonly
name: string | symbol
bindStore(store: AsyncLocalStorage<StoreType>,transform?: (context: ContextType) => StoreType,): void
When `channel.runStores(context, ...)` is called, the given context data
will be applied to any store bound to the channel. If the store has already been
bound the previous `transform` function will be replaced with the new one.
The `transform` function may be omitted to set the given context data as the
context directly.
```js
import diagnostics_channel from 'node:diagnostics_channel';
import { AsyncLocalStorage } from 'node:async_hooks';
const store = new AsyncLocalStorage();
const channel = diagnostics_channel.channel('my-channel');
channel.bindStore(store, (data) => {
return { data };
});
```
publish(message: unknown): void
Publish a message to any subscribers to the channel. This will trigger
message handlers synchronously so they will execute within the same context.
```js
import diagnostics_channel from 'node:diagnostics_channel';
const channel = diagnostics_channel.channel('my-channel');
channel.publish({
some: 'message',
});
```
runStores(): void
Applies the given data to any AsyncLocalStorage instances bound to the channel
for the duration of the given function, then publishes to the channel within
the scope of that data is applied to the stores.
If a transform function was given to `channel.bindStore(store)` it will be
applied to transform the message data before it becomes the context value for
the store. The prior storage context is accessible from within the transform
function in cases where context linking is required.
The context applied to the store should be accessible in any async code which
continues from execution which began during the given function, however
there are some situations in which `context loss` may occur.
```js
import diagnostics_channel from 'node:diagnostics_channel';
import { AsyncLocalStorage } from 'node:async_hooks';
const store = new AsyncLocalStorage();
const channel = diagnostics_channel.channel('my-channel');
channel.bindStore(store, (message) => {
const parent = store.getStore();
return new Span(message, parent);
});
channel.runStores({ some: 'message' }, () => {
store.getStore(); // Span({ some: 'message' })
});
```
deprecated
subscribe(onMessage: ChannelListener): void
Register a message handler to subscribe to this channel. This message handler
will be run synchronously whenever a message is published to the channel. Any
errors thrown in the message handler will trigger an `'uncaughtException'`.
```js
import diagnostics_channel from 'node:diagnostics_channel';
const channel = diagnostics_channel.channel('my-channel');
channel.subscribe((message, name) => {
// Received data
});
```
unbindStore(store: any): void
Remove a message handler previously registered to this channel with `channel.bindStore(store)`.
```js
import diagnostics_channel from 'node:diagnostics_channel';
import { AsyncLocalStorage } from 'node:async_hooks';
const store = new AsyncLocalStorage();
const channel = diagnostics_channel.channel('my-channel');
channel.bindStore(store);
channel.unbindStore(store);
```
deprecated
unsubscribe(onMessage: ChannelListener): void
Remove a message handler previously registered to this channel with `channel.subscribe(onMessage)`.
```js
import diagnostics_channel from 'node:diagnostics_channel';
const channel = diagnostics_channel.channel('my-channel');
function onMessage(message, name) {
// Received data
}
channel.subscribe(onMessage);
channel.unsubscribe(onMessage);
```