frame-messenger.md

Frame Messenger

Axe frameMessenger can be used to configure how axe-core communicates information between frames. By default, axe-core uses window.postMessage(). Since other scripts on the page may also use window.postMessage, axe-core's use of it can sometimes disrupt page functionality. This can be avoided by providing axe.frameMessenger() a way to communicate to frames that does not use window.postMessage. If reliable and secure frame communication is not possible, use axe.runPartial and axe.finishRun instead.

Tools like browser extensions and testing environments often have different channels through which information can be communicated. axe.frameMessenger must be set up in every frame axe-core is included.

axe.frameMessenger({
  // Called to initialize message handling
  open(topicHandler) {
    // Map data from the bridge to topicHandler
    function subscriber(frameWin, data, response) {
      // Data deserializations / validation / etc. here
      topicHandler(data, response);
    }
    // Start listening for "axe-core" events
    const unsubscribe = bridge.subscribe('axe-core', subscriber);
    // Tell axe how to close the connection if it needs to
    return unsubscribe;
  },

  // Called when axe needs to send a message to another frame
  async post(frameWindow, data, replyHandler) {
    // Send a message to another frame for "axe-core"
    const replies = bridge.send(frameWindow, 'axe-core', data);
    // Async handling replies as they come back
    for await (let data of replies) {
      replyHandler(data);
    }
  }
});

axe.frameMessenger({ open })

open is a function that should setup the communication channel with iframes. It is passed a topicHandler function, which must be called when a message is received from another frame.

The topicHandler function takes two arguments: the data object and a callback function that is called when the subscribed listener completes. The data object is exclusively passed data that can be serialized with JSON.stringify(), which depending on the system may need to be used.

The open function can return an optional close function. Axe-core will only ever have one frameMessenger open at a time. The close function is called when another frameMessenger is registered.

axe.frameMessenger({ post })

post is a function that dictates how axe-core communicates with frames. It is passed three arguments: frameWindow, which is the frame's contentWindow, the data object, and a replyHandler that must be called when responses are received. To inform axe-core that no message was sent, return false. This informs axe-core not to await for the ping to time out.

Currently, axe-core will only require replyHandler to be called once, so promises can also be used here. This may change in the future, so it is preferable to make it possible for replyHandler to be called multiple times. Some axe-core plugins may rely on this feature.

A second frameMessenger feature available to plugins, but not used in axe-core by default is to reply to a reply. This works by passing replyHandler a responder callback as a second argument. This requires a different setup, in which callbacks are stored based on their channelId property.

// store handlers based on channelId
const channels = {};

axe.frameMessenger({
  post(frameWindow, data, replyHandler) {
    // Store the handler so it can be called later
    channels[data.channelId] = replyHandler;
    // Send a message to the frame
    bridge.send(frameWindow, data);
  },

  open(topicHandler) {
    function subscriber(frameWin, data) {
      const { channelId, message, keepalive } = data;
      // Create a callback to invoke on a reply.
      const responder = createResponder(frameWin, channelId);

      // If there is a topic, pass it to the axe supplied topic-handler
      if (data.topic) {
        topicHandler(data, responder);

        // If there is a replyHandler stored, invoke it
      } else if (channels[channelId]) {
        const replyHandler = channels[channelId];
        replyHandler(message, keepalive, responder);

        // Clean up replyHandler, as no further messages are expected
        if (!keepalive) delete channels[channelId];
      }
    }

    // Start listening for "axe-core" events
    const unsubscribe = bridge.subscribe('axe-core', subscriber);
    // Tell axe how to close the connection if it needs to
    return unsubscribe;
  }
});

// Return a function to be called when a reply is received
function createResponder(frameWin, channelId) {
  return function responder(message, keepalive, replyHandler) {
    // Store the new reply handler, possibly replacing a previous one
    //   to avoid receiving a message twice.
    channels[channelId] = replyHandler;
    // Send a message to the frame
    bridge.send(frameWin, { channelId, message, keepalive });
  };
}

Error handling & Timeouts

If for some reason the frameMessenger fails to open, post, or close you should not throw an error. Axe-core will handle missing results by reporting on them in the frame-tested rule. It should not be possible for the topicHandler and replyHandler callbacks to throw an error. If this happens, please file an issue.

Axe-core has a timeout mechanism built in, which pings frames to see if they respond before instructing them to run. There is no retry behavior in axe-core, which assumes that whatever channel is used is stable. If this isn't the case, this will need to be built into frameMessenger.

The message passed to responder may be an Error. If axe-core passes an Error, this should be propagated "as is". If this is not possible because the message needs to be serialized, a new Error object must be constructed as part of deserialization.