javascript - When is WebSocket.onmessage triggered? -

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(1) open websocket connection.

var ws = new websocket(url);

(2) when connection established, send message trigger blob sent in response.

ws.onopen = function () {     ws.send('1000000'); }

(3) onmessage triggered when response begins or complete?

ws.onmessage = function () {     // has entire blob arrived or has download begun? }

the w3c spec websocket api says message event should dispatched "when websocket message has been received":

when websocket message has been received type type , data data, user agent must queue task follow these steps:

...

  1. let event event uses messageevent interface, event type message, not bubble, not cancelable, , has no default action.

...

  1. dispatch event @ websocket object.

to understand meant "when websocket message has been received," must consult rfc 6455, "the websocket protocol". in websockets, participants send messages contained in 1 or more frames:

after successful handshake, clients , servers transfer data , forth in conceptual units referred in specification "messages". on wire, message composed of 1 or more frames.

once understand that, can understand section 6 of rfc 6455, defines phrase "a websocket message has been received":

if frame comprises unfragmented message (section 5.4), said _a websocket message has been received_ type /type/ , data /data/. if frame part of fragmented message, "application data" of subsequent data frames concatenated form /data/. when last fragment received indicated fin bit (frame-fin), said _a websocket message has been received_ data /data/...

assuming sever-side library's send method treats argument "message", specification requires client wait receipt of entire message before firing message event.

a server-side api allowed streaming (e.g., kept message incomplete , withheld fin bit until kind of finish method called) not cause client fire message event until message concluded.


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