WKPostMessenger

WKPostMessenger creates a synced channel via postMessage to facilitate communication between a Swift and JavaScript.

It's kind of like Postmate, except between webview and the iOS application where it live, not between two windows.

Messages passed back and forth contain an action (analogous to a function call) and data (an optional object with a bunch of data.). These messages also send an ID that is used as a token to keep track of multiple messages and their responses.

Why

Swift provides evaluateJavaScript and addScriptMessageHandler for communication to and from JS in a webview, but these commands do not inherently verify that messages are received by the other side.

This library defines a common format for messages sent back and forth and allows for asynchronous return values (ie. Promises).

Installation and Usage

iOS

There isn't a corresponding Swift library for this (...yet?), but here's a rough outline of how to get it working.

Setup

• Add a script message handler.

  class WKPostMessageScriptMessageHandler: NSObject, WKScriptMessageHandler {
    func userContentController(userContentController: WKUserContentController, didReceiveScriptMessage message: WKScriptMessage!) {
      if message.name == "wkPostMessage" {
        let payload = // decoded message.body
  
        switch action {
        case .__WK_HANDSHAKE__:
          // Handshake data contains global JS function for sending messages
          self.wkPostMessage = // payload.data to a string
          // Acknowledge the handshake
          let messageID = "\"\(payload.id)\"" // needs to be unique and escaped for JS
          webview?.evaluateJavaScript("\(payload.callback)(\(messageID))", completionHandler: nil)
        case .__WK_CALLBACK__:
          // payload.id contains the ID of the message originally sent from Swift
          // payload.data contains anything returned by the callback, which can be of any type
        case .yourAction:
          // payload.data contains anything returned by the callback, which can be of any type
          let messageID = "\"\(payload.id)\"" // needs to be unique and escaped for JS
          let jsonStringResult = // whatever you want, encoded for JS
          let callbackJs = "\(payload.callback)(\(messageID), \(jsonStringResult));"
          webview?.evaluateJavaScript(callbackJs, completionHandler: nil)
        }
      }
    }
  }
  
  let contentController = WKUserContentController()
  let handler = WKPostMessageScriptMessageHandler()
  contentController.addScriptMessageHandler(handler, name: "wkPostMessage")

Sending messages

if let wkPostMessage = self.wkPostMessage {
  // needs to be unique and escaped for JS
  let messageID = "\"\(NSUUID().UUIDString)\""
  let messageAction = "\"resetForm\""
  let messageDataJsonString = // whatever you want, encoded for JS

  let messageJs = "\(wkPostMessage)(\(messageID), \(messageAction), \(messageDataJsonString));"
  webview?.evaluateJavaScript(messageJs, completionHandler: nil)

  // the __WK_CALLBACK__ you receive with messageID returned as the message ID
  // is the response from JavaScript!
}

Receiving messages

• Define actions as cases in the script message handler.
• Do whatever decoding is necessary on payload.data. This is the trickiest part, because JS is loosey-goosey with types. Ideally, you want to be able to handle anything gracefully on the decoding side, even if you're expecting data in a specific format.

JavaScript

Setup

• Install this library.

  npm install --save wk-postmessenger

  Depending on your use case, grabbing the minified build might be more convenient. Follow your heart.

• Build the page that you want to use in your webview and create a WKPostMessenger instance that defines message handling behavior.

  import WKPostMessenger from 'wk-postmessenger';
  const postMessenger = new WKPostMessenger({
    handleMessage(action, data) {
      // Your code to handle messages from the iOS app
    },
  });

• A handshake postMessage sent from the webview to the application establishes the channel. This message uses a special __WK_HANDSHAKE__ action and data containing a string indicating the global function in the webview that accept messages from the iOS side.

Sending messages

• Call sendMessage with an action name and optional data.

  postMessenger.sendMessage('doSomethingInApp', {
    quickly: 'Sure, I guess.',
  })
    .then((results) => {
      // Resolves when the app sends a response acknowledging that the message
      // was received and processed. Note that it doesn't necessarily have to
      // return any data, but it can!
      console.log('It did something!', results);
    })
    .catch(() => {
      // If iOS doesn't acknowledge the message within the timeout, the Promise
      // automatically rejects.
    });

• sendMessage also takes a third parameter for a custom timeout, which might come in handy if the application has to do an asynchronous operation that takes longer than the default timeout.

Receiving messages

• The handleMessage option allows you to define the behavior for messages from the iOS app. The return value is sent back to iOS.

  const postMessenger = new WKPostMessenger({
    handleMessage(action, data) {
      switch (action) {
        case 'getDogProperties':
          if (data.name === 'Clifford') {
            return { color: 'red', size: 'big' };
          }
          return { color: 'furry', size: 'dog-sized' };
        default:
          // Unrecognized message?
          break;
      }
    },
  });

• The return value can also be a Promise, in which case the response will be sent to iOS when it resolves.

  const postMessenger = new WKPostMessenger({
    handleMessage(action, data) {
      switch (action) {
        case 'doAsyncThing':
          return new Promise((resolve) => {
            setTimeout(() => resolve('at long last!'), 9000);
          });
        default:
          break;
      }
    },
  });

License

MIT