Handling Multiple Init's

[andrewpeters9]

Issues:

1. LedgerBridgeKeyring can accidentally be initialised twice on the same client, resulting in silent failures (e.g. messages be either duplicated or resolved prematurely/incorrectly).
2. Now that set-up logic is being moved to a separate init function, there's a possibility that the same instance is init'ed twice, and thus requires appropriate handling and fail-safes

• setupIframe to throw an error if it detects a duplicate iframe
• this.currentMessageId to be removed and replaced with nanoid
  • Ensures that two different clients sending messages to the same bridge URL will always have a unique (instead of being 0-indexed and incrementing)
  • Reduces code complexity
• Prevent this.bridgeUrl from being null before the iframe is created, or at least update the iframe if the keyring is deserialized with a new bridgeUrl
  • See this ticket for more details: Elaborate Serializing and Deserializing of Data #161
• Safe-proofing delayed promise: Safe-Proofing delayedPromise #162
• this._eventListener will be mutated if the keyring is init'ed twice, which would interfere with:
  • window.removeEventListener('message', this._eventListener)
  • Solution to this may be to just remove this._eventListener completely, and replace it with something like this:

type WindowEventHandlerTuple = readonly [
    keyof WindowEventHandlersEventMap,
    (this: WindowEventHandlers, ev: WindowEventHandlersEventMap[keyof WindowEventHandlersEventMap]) => any
];

class LedgerBridgeKeyring extends EventEmitter {
    _windowEventListeners: ReadonlyArray<WindowEventHandlerTuple> = [];


  _setupListener () {
    const messageListener = ({ origin, data }) => {
      // ...
    }
    
    this._addWindowEventListener(['message', messageListener])
  }
  
  _addWindowEventListener(windowEventHandler:  WindowEventHandlerTuple){
     window.addListener(...windowEventHandler);
     this._windowEventListeners = [...this._windowEventListeners, windowEventHandler];
  }
  
  destroy(){
    for(const listener of this._windowEventListeners){
      window.removeListener(...listener);
    }
  }
}