Is there any way to wait until the DirectionsServi

问题:

I have a problem with the Google DirectionsService. I know it's asynchronous and that is the cause of my troubles. I'd like to wait until the DirectionsService returns a result instead of executing code without an answer. Here is a sample:

function snap_to_road (lat) {
    var position;

    var request = {
        origin: lat,
        destination: lat,
        travelMode: google.maps.DirectionsTravelMode.DRIVING
    };

    directionsService.route(request, function(response, status) {
        if (status == google.maps.DirectionsStatus.OK) {
            return response.routes[0].legs[0].start_location;
        }
    });
}

alert(snap_to_road(current.latLng));

The alert always shows: "undefined". Is there any way to solve this?

回答1:

I don't think that is possible. You could use a callback parameter in snap_to_road:

function snap_to_road (lat, callback) {
    var position;

    var request = {
        origin: lat,
        destination: lat,
        travelMode: google.maps.DirectionsTravelMode.DRIVING
    };

    directionsService.route(request, function(response, status) {
        if (status == google.maps.DirectionsStatus.OK) {
            callback(response.routes[0].legs[0].start_location);
        }
    });
}

snap_to_road(current.latLng, function(result) {
    alert(result);
});

回答2:

Call your alert method after google directions returns result. Like: Call snap_to_road after button click event.

function snap_to_road (lat) {
  var position;

  var request = {
    origin: lat,
    destination: lat,
    travelMode: google.maps.DirectionsTravelMode.DRIVING
  };

  directionsService.route(request, function(response, status) {
    if (status == google.maps.DirectionsStatus.OK) {
      ShowAlert(response.routes[0].legs[0].start_location);
    }
  });
}

function ShowAlert(result){
  alert(result);
}

回答3:

Here is an example of what I mentioned above:

var path = "/path/to/some/resource";

$.ajax({
        url:    path,
        type: "get",
        data: serializedData,  //<-- depends on what data you are using
        async: false, //will wait until resource has loaded or failed 

         //will be called on success
        success: function(response, textStatus, jqXHR){
            // log a message to the console
            console.log("Successfully loaded resource!");
        },
         //will be called on error
        error: function(jqXHR, textStatus, errorThrown){
            // log the error to the console
            console.log(
                "The following error occured: "+
                textStatus, errorThrown
            );
        },
        // callback handler that will be called on completion
        // which means, either on success or error
        complete: function(){
            console.log("Completed: with error or success");
        }
    });