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85 lines
2.8 KiB
Markdown
85 lines
2.8 KiB
Markdown
---
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title: "E02. Use Named Events in SSE"
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order: 48
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status: "draft"
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---
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SSE lets you send multiple kinds of events over the same stream. Give each one a name with the `event:` field, and the client can dispatch to a different handler per type. Great for things like "new message", "user joined", "user left" in a chat app.
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## Send events with names
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```cpp
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auto send_event = [](httplib::DataSink &sink,
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const std::string &event,
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const std::string &data) {
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std::string msg = "event: " + event + "\n"
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+ "data: " + data + "\n\n";
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sink.write(msg.data(), msg.size());
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};
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svr.Get("/chat/stream", [&](const httplib::Request &req, httplib::Response &res) {
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res.set_chunked_content_provider(
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"text/event-stream",
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[&, send_event](size_t offset, httplib::DataSink &sink) {
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send_event(sink, "message", "Hello!");
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std::this_thread::sleep_for(std::chrono::seconds(2));
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send_event(sink, "join", "alice");
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std::this_thread::sleep_for(std::chrono::seconds(2));
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send_event(sink, "leave", "bob");
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std::this_thread::sleep_for(std::chrono::seconds(2));
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return true;
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});
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});
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```
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A message is `event:` → `data:` → blank line. If you omit `event:`, the client treats it as a default `"message"` event.
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## Attach IDs for reconnect
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When you include an `id:` field, the client automatically sends it back as `Last-Event-ID` on reconnect, telling the server "here's how far I got."
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```cpp
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auto send_event = [](httplib::DataSink &sink,
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const std::string &event,
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const std::string &data,
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const std::string &id) {
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std::string msg = "id: " + id + "\n"
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+ "event: " + event + "\n"
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+ "data: " + data + "\n\n";
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sink.write(msg.data(), msg.size());
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};
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send_event(sink, "message", "Hello!", "42");
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```
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The ID format is up to you. Monotonic counters or UUIDs both work — just pick something unique and orderable on the server side. See E03. Handle SSE Reconnection for details.
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## JSON payloads in data
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For structured data, the usual move is to put JSON in `data:`.
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```cpp
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nlohmann::json payload = {
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{"user", "alice"},
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{"text", "Hello!"},
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};
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send_event(sink, "message", payload.dump(), "42");
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```
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On the client, parse the incoming `data` as JSON to get the original object back.
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## Data with newlines
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If the data value contains newlines, split it across multiple `data:` lines.
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```cpp
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std::string msg = "data: line1\n"
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"data: line2\n"
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"data: line3\n\n";
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sink.write(msg.data(), msg.size());
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```
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On the client side, these come back as a single `data` string with newlines.
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> **Note:** Using `event:` makes client-side dispatch cleaner, but it also helps in the browser DevTools — events are easier to filter by type. That matters more than you'd expect while debugging.
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