API.md

Controlling the Form Builder from an independent web page using JavaScript

Important: For this to work, make sure that the browser settings are not blocking the display of popup windows.

The form builder web application can be invoked using JavaScript from another web page. It can be opened in a new page/tab (sometimes referred as child window). The choice of new page or a new tab depends on the browser settings or preferences. The newly opened form builder can be loaded with an initial questionnaire. The parent page (sometimes referred as parent window) can listen to real time changes in the form builder as the user makes the changes to the questionnaire. When the user closes the form builder window, the parent window will receive a closed message with the final modifications to the questionnaire.

Set up event listener

Before opening the form builder window, add an event listener to the parent window. The form builder sends three types of messages, namely initialized, updateQuestionnaire, and closed. It can also receive a message with type initialQuestionnaire. The data object exchanged with the form builder consists of two fields, type and questionnaire. The type is one of the above defined message types. The questionnaire is the associated questionnaire for that message type. For the initialized message this field is undefined. Here is a code snippet to add an event listener.

window.addEventListener('message', handleFormBuilderMessages, true);

Handle form builder events

A window event listener is a callback function attached to the parent window listening to message events. The callback receives an event object, where event.data is the data object sent by the message emitter, in this case the form builder. The form builder defines its own messages with 'initialized', 'updateQuestionnaire', and 'closed' types. The message type is in event.data.type. Here is a typical handler to receive the messages from the form builder.

const fbUrl = 'https://formbuilder.nlm.nih.gov';

/**
Event handler to handle messages from form builder window.

@param: event - MessageEvent object. Read event.data.event to identify the form
  builder's message type, and expect event.data.questionnaire to contain the
  latest questionnaire from the form builder. For the 'initialized' event, the
  event.data.questionnaire is undefined.
*/
function handleFormBuilderMessages(event) {
  if(event.origin === fbUrl) {
    // Handling only form builder events.
    const eventType = event.data.type;
    
    // Receiving message: 'initialized' || 'updateQuestionnaire' || 'closed'
    switch (eventType) {
      case 'initialized':
        /*
         This is the first message from the child window.
         It indicates that the form builder is initialized and ready to
         receive the initial questionnaire. Use 'initialQuestionnaire' message to
         send the initial questionnaire.
        */

        // fbWin is the object reference returned by window.open().
        fbWin.postMessage({type: 'initialQuestionnaire', questionnaire: initialQ}, fbUrl);
        break;

      case 'updateQuestionnaire':
        // Use this to get continuous updates. The message is triggered by every
        // change in the form builder with about 0.5 second debounce.

        // Prints updated questionnaire.
        console.log(`${JSON.stringify(formBuilderMessage.questionnaire, null, 2)}`);
            
        // ...
        // Do something with formBuilderMessage.questionnaire.
        // ...

        break;

      case 'closed':
        // Triggered when the form builder window is closed.
        // Use this to get final updated questionnaire
        // ...
        // Do something with formBuilderMessage.questionnaire or any cleanup
        // ...

        break;

    }
  }
} 

Open form builder window

After setting up the message event handler, open the form builder in a new window. Use the window.open() method of the DOM Window interface. The form builder uses the parent window's location url to send the messages. Cross origin (CORS) restrictions prevent accessing the information, so the caller needs to provide the window.location.href as areferrer parameter in the url to establish the communication. The pathname to provide the url parameter is /window-open.

Here is an example:

const fbWin = window.open(fbUrl+'/window-open?referrer='+encodeURIComponent(window.location.href), 'formBuilderWindow');

Use fbWin.postMessage to send the initialQuestionnaire message to the child window, as described above.

Loading with an initial questionnaire

You can load a form builder with an existing FHIR questionnaire resource. You want to wait for the initialized message after opening the form builder window, before sending the message to the form builder.

// fbWin is the object reference returned by window.open().
// Use 'initialQuestionnaire' message.
fbWin.postMessage({type: 'initialQuestionnaire', questionnaire: initialQ}, fbUrl);

There is a working example in /tests/window-open-test.html on how to invoke the application and listen to the updates.