A utility function to detect window focusing without false positives from iframe focus events

onWindowFocus.ts

type State = {
  added: boolean;
  interval: false | ReturnType<typeof setInterval>;
  inFrame: boolean;
  callbacks: Array<SetFocusedCallback>;
};

type EnrichedHTMLIFrameElement = HTMLIFrameElement & { ___onWindowFocusHandled: boolean };

type SetFocusedCallback = (focused?: boolean) => void;

const state: State = {
  added: false,
  interval: false,
  inFrame: false,
  callbacks: [],
};

export const onWindowFocus = (newCallback: SetFocusedCallback) => {
  state.callbacks.push(newCallback);
  start();
  return () => {
    state.callbacks = state.callbacks.filter(
      (registeredCallback) => registeredCallback !== newCallback
    );
    stop();
  };
};

const runIFrameCheck = () => {
  const iframes = Array.from(document.getElementsByTagName('iframe'));
  console.debug('Polling iframes... found: ', iframes.length);

  (iframes as EnrichedHTMLIFrameElement[]).forEach((iframe) => {
    if (iframe.___onWindowFocusHandled) {
      return;
    }
    iframe.___onWindowFocusHandled = true;
    iframe.addEventListener('touchend', () => {
      state.inFrame = true;
    });
    iframe.addEventListener('mouseup', () => {
      state.inFrame = true;
    });
    iframe.addEventListener('focus', () => {
      state.inFrame = true;
    });
  });
};

const start = () => {
  if (state.interval) {
    clearInterval(state.interval);
  }

  if (!state.added) {
    state.added = true;
    window.addEventListener('focus', () => {
      if (state.inFrame) {
        state.inFrame = false;
        return;
      } else {
        state.callbacks.forEach((callback) => callback(true));
      }
    });
  }

  state.interval = setInterval(runIFrameCheck, 500);
};

const stop = () => {
  if (!state.callbacks.length && state.interval) {
    clearInterval(state.interval);
  }
};

Nice! I'll update the gist.

It might be worth noting that this gist works only if the code runs in the main window.

I have my react app running within an iframe which is not allowed to get notified if the user focussed the main window (probably for security reasons). If I focus an element in the main window, the iframe won't get notified if I switch to another tab or come back to this tab. Only if I focus on the iframe (or an element in the iframe), I get the focus event when switching my browser tab.

From a security perspective, it makes sense that the browser (Chrome in my case) behaves like this. Leaking information between main window and iframe should only be possible via MessageEvents. For people having a similar situation, the solution would be to catch the focus event in the main window, post the message to the iframe where the focusManager can react on the event.

PS: Please note that my snippet might not work for react-query v2 because the payload of the callback changed.

const start = () => {
  if (state.interval) {
    clearInterval(state.interval)
  }

  if (!state.added) {
    state.added = true
    window.addEventListener('focus', () => {
      if (state.inFrame) {
        state.inFrame = false
        state.callbacks.forEach((callback) => callback(true))
      } else {
        state.callbacks.forEach((callback) => callback(false))
      }
    })
  }

  state.interval = setInterval(runIFrameCheck, 1000)
}

I tweaked above code a bit and it works on react-query v2.