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eventsource.js
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eventsource.js
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const EventEmitter = require('events').EventEmitter;
const undici = require('undici');
const backoff = require('backoff');
const {Writable} = require('stream');
const ReadyStates = {
CONNECTING: 0,
CONNECTED: 1,
DISCONNECTED: 2,
};
const COLON_CHARACTER = 58;
const SPACE_CHARACTER = 32;
const LINE_FEED_CHARACTER = 10;
const CARRIAGE_RETURN_CHARACTER = 13;
const DefaultOptions = {
initialReconnectDelay: 2 * 1000,
maximumReconnectDelay: 30 * 1000,
heartbeatTimeout: 30 * 1000,
};
class EventSource extends EventEmitter {
constructor(url, options = {}) {
super();
this.options = {...DefaultOptions, ...options};
this.url = url;
this.readyState = ReadyStates.DISCONNECTED;
this.lastEventId = null;
this.heartbeatTimeout = null;
this.backoff = backoff.exponential({
initialDelay: this.options.initialReconnectDelay,
maxDelay: this.options.maximumReconnectDelay,
});
this.backoff.on('ready', () => this.connect());
this.connect();
}
async connect() {
if ([ReadyStates.CONNECTING, ReadyStates.CONNECTED].includes(this.readyState)) return;
this.readyState = ReadyStates.CONNECTING;
this.emit('connecting');
const headers = {};
if (this.lastEventId) {
headers['Last-Event-ID'] = this.lastEventId;
}
try {
await undici.stream(this.url, {headers}, (response) => this.handleResponse(response));
} catch (err) {
this.emit('error', err);
}
this.emit('disconnected');
this.readyState = ReadyStates.DISCONNECTED;
try {
this.backoff.backoff();
} catch (_) {}
}
resetHeartbeatTimeout(callback) {
if (this.heartbeatInterval != null) return;
this.heartbeatInterval = setInterval(() => {
if (this.lastHeartbeat > Date.now() - this.options.heartbeatTimeout) {
return;
}
clearInterval(this.heartbeatInterval);
this.heartbeatInterval = null;
callback(new Error(`no heartbeat from server in ${this.options.heartbeatTimeout}ms`));
}, this.options.heartbeatTimeout);
return () => {
this.lastHeartbeat = Date.now();
};
}
handleResponse({statusCode, headers}) {
if (statusCode !== 200) {
throw new Error(`response not OK. status code: ${statusCode}`);
}
const contentType = headers['content-type'];
if (!contentType.includes('text/event-stream')) {
throw new Error(`response content type not text/event-stream: ${contentType}`);
}
let eventId = null;
let eventName = '';
let eventData = '';
const parseEventStreamLine = (buffer, position, fieldLength, lineLength) => {
if (lineLength === 0) {
if (eventData.length > 0) {
this.emit(eventName || 'message', {
id: eventId,
name: eventName,
data: eventData.slice(0, -1), // trim trailing new line
});
eventData = '';
}
if (eventId != null) {
this.lastEventId = eventId;
eventId = null;
}
eventName = null;
} else if (fieldLength > 0) {
const emptyValue = fieldLength < 0;
let step = 0;
const field = buffer.slice(position, position + (emptyValue ? lineLength : fieldLength)).toString();
if (emptyValue) {
step = lineLength;
} else if (buffer[position + fieldLength + 1] !== SPACE_CHARACTER) {
step = fieldLength + 1;
} else {
step = fieldLength + 2;
}
position += step;
const valueLength = lineLength - step;
const value = buffer.slice(position, position + valueLength).toString();
if (field === 'data') {
eventData += value + '\n';
} else if (field === 'event') {
eventName = value;
} else if (field === 'id') {
eventId = value;
}
}
}
let heartbeat;
let buffer;
let discardTrailingNewline = false;
const writable = new Writable({
write(chunk, encoding, callback) {
buffer = buffer != null ? Buffer.concat([buffer, chunk]) : chunk;
let position = 0;
let bufferLength = buffer.length;
while (position < bufferLength) {
if (discardTrailingNewline) {
if (buffer[position] === LINE_FEED_CHARACTER) {
++position;
}
discardTrailingNewline = false;
}
let lineLength = -1;
let fieldLength = -1;
for (let bufferIndex = position; lineLength < 0 && bufferIndex < bufferLength; ++bufferIndex) {
const character = buffer[bufferIndex];
if (character === COLON_CHARACTER) {
if (fieldLength < 0) {
fieldLength = bufferIndex - position;
}
} else if (character === CARRIAGE_RETURN_CHARACTER) {
discardTrailingNewline = true;
lineLength = bufferIndex - position;
} else if (character === LINE_FEED_CHARACTER) {
lineLength = bufferIndex - position;
}
}
if (lineLength < 0) {
break;
}
parseEventStreamLine(buffer, position, fieldLength, lineLength);
position += lineLength + 1;
}
if (position === bufferLength) {
buffer = null;
} else if (position > 0) {
buffer = buffer.slice(position);
}
heartbeat?.();
callback();
},
});
this.backoff.reset();
heartbeat = this.resetHeartbeatTimeout((err) => writable.destroy(err));
this.readyState = ReadyStates.CONNECTED;
this.emit('connected');
return writable;
}
}
module.exports = EventSource;