Support asynchronous blocking

[kirill-konshin]

It's really hard to implement asynchronous procedure that should delay navigation.

It would be great if blocker function could return Promise (if it's resolved navigation should proceed, if rejected — stopped) or a function that can accept an argument callback so that callback(true) will resume nav.

In this case result in https://github.com/ReactTraining/history/blob/master/modules/createTransitionManager.js#L26 treatment should be a bit different.

Change is super small and should not break anything. I propose 2 options (I have made a pull request #546 with scenario 2 because it's more user friendly):

1. Less changes: use getUserConfirmation to handle string and any other object results

  var confirmTransitionTo = function confirmTransitionTo(location, action, getUserConfirmation, callback) {
      console.log('Bar', prompt, getUserConfirmation);
    // TODO: If another transition starts while we're still confirming
    // the previous one, we may end up in a weird state. Figure out the
    // best way to handle this.
    if (prompt != null) {
      var result = typeof prompt === 'function' ? prompt(location, action) : prompt;

// NEW STUFF
      if (typeof result === 'string' || typeof result === 'object' || typeof result === 'function') {
// NEW STUFF
        if (typeof getUserConfirmation === 'function') {
          getUserConfirmation(result, callback);
        } else {
          warning(false, 'A history needs a getUserConfirmation function in order to use a prompt message');

          callback(true);
        }
      } else {
        // Return false from a transition hook to cancel the transition.
        callback(result !== false);
      }
    } else {
      callback(true);
    }
  };

2. More user friendly: handle Promise right there in transition confirmation

  var confirmTransitionTo = function confirmTransitionTo(location, action, getUserConfirmation, callback) {
      console.log('Bar', prompt, getUserConfirmation);
    // TODO: If another transition starts while we're still confirming
    // the previous one, we may end up in a weird state. Figure out the
    // best way to handle this.
    if (prompt != null) {
      var result = typeof prompt === 'function' ? prompt(location, action) : prompt;

      if (typeof result === 'string') {
        if (typeof getUserConfirmation === 'function') {
          getUserConfirmation(result, callback);
        } else {
          warning(false, 'A history needs a getUserConfirmation function in order to use a prompt message');

          callback(true);
        }
// NEW STUFF
      } else if (typeof result === 'function') {
        result(callback);
      } else if ('then' in result && 'catch' in result) { // duck typing here for simplicity
        result
          .then(function() { callback(true); })
          .catch(function() { callback(false; });
// NEW STUFF
      } else {
        // Return false from a transition hook to cancel the transition.
        callback(result !== false);
      }
    } else {
      callback(true);
    }
  };

I personally would prefer option 2 because in this case all handling can be done locally in component w/o need to configure extra getUserConfirmation in some other files:

const blocker = (({pathname}, action) => {
  return asyncFigureOutWhatToDoWith(pathname, action); // returns promise
};

or

const blocker = (({pathname}, action) => {
  return (callback) => {
    asyncFigureOutWhatToDoWith(pathname, action)
      .then(function() { callback(true); })
      .catch(function() { callback(false; });    
  };
};

[idrm]

You can pass a custom getUserConfirmation prop to your <Router/> component? E.g.

<Router
  getUserConfirmation={(message, callback) => {
    console.log('will async confirm transition', message);
    setTimeout(() => {
      callback(true);
    }, 1500);
  }}
>
...
</Router>

where you'd replace setTimeout with your custom async logic.

The message param is equal to what you pass inside <Prompt />.

If you need to pass a more complicated object inside message you will run into issues with type checking in, say, TypeScript, since the original signature for that prop is string | (Location => (string | boolean)).

My preferred async callback approach is to change the state inside the component that contains <Router/>, and display a modal dialog when the state changes. This keeps everything declarative. E.g.

class App extends Component {

  state = {
    transitioning: false
  }

  render = () => (
    <Router
      getUserConfirmation={(message, callback) => {
        this.setState({
          transitioning: true,
          transitionMessage: message,
          transitionCallback: callback,
        });
      }}
    >
      {
        this.state.transitioning ? (
         <ModalConfirmationDialog
           onConfirm={() => {
             const { transitionCallback } = this.state;
             this.setState({
               transitioning: false,
               transitionMessage: undefined,
               transitionCallback: undefined,
             }, () => transitionCallback(true));
           })
           onReject={() => {
             const { transitionCallback } = this.state;
             this.setState({
               transitioning: false,
               transitionMessage: undefined,
               transitionCallback: undefined,
             }, () => transitionCallback(false));
            }}
          />
        ) : null
      }
      ...
    </Router>
  )
}

where <ModalConfirmationDialog/> is a custom modal confirmation dialog implementation.

[kirill-konshin]

This seems to be an overly global solution to me... I need something more local, say, block transition for one page only. Moreover it could be UI-free thing, for example, do XHR and do not allow user to leave the page while this XHR is pending.

[mjackson]

The problem with a general async blocking function is that it doesn't work for the beforeunload event, which is what I was trying to generalize when I wrote the current implementation. beforeunload is synchronous. You only have one shot to return a string that will be displayed in the dialog.

The current API needs to be reworked to accommodate both use cases:

1. You want to prevent navigation in the same app
2. You want to prevent navigation to another site (or closing the tab)

[kirill-konshin]

@mjackson I see now from where it is all coming :) makes sense. I was solving first case, so maybe for that we can have separate Promise-based API?

[kirill-konshin]

Any news about this?

[mufumbo]

The proposed solution doesn't work? any work around this?

[mjackson]

You should not rely on history.block for anything at this point because the API has problems that I don't know how to resolve (see #690 for more info). I'm planning on removing it in the next major version. There is probably a much more straightforward way to accomplish what you need. I'd be happy to discuss in #690 if you can tell me your use case.