[GIS] How to programmatically start drawing in OpenLayers 4._

eventsjavascriptmap-drawingopenlayers

In openlayers there are 2 paradigms for drawing features one is to create a "draw interaction" that lets the user draw a feature and the other is to create the geometry and feature yourself–then add it to the layer.

I would like to programmatically start a polygon "interaction" drawing for the user by placing the first point. After which the user will place the remaining points. It therefore must be an "interaction".

This means I must simulate a user click or draw event that the openlayers api accepts as valid.

I've tried..

map.dispatchEvent(<clickevent>)
map.dispatchEvent(<drawstartevent>)

/*draw being ol.interaction.Draw*/
draw.dispatchEvent(<clickevent>)
draw.dispatchEvent(<drawstartevent>)

Best Answer

In non minified (ol-debug) version you can use function startDrawing_ (doc is here) which accepts event as a parametr. It uses only coordinates from that event, so it could be replaced by a simple object. Problem is I dont know how is this function named in minified version and also that name could change in time. Also it is not a very clean approach...

var draw = new ol.interaction.Draw({features:new ol.Collection([]), type:"Polygon"});
map.addInteraction(draw);
draw.startDrawing_({coordinates:[111,222]});

Related Solutions

[GIS] How to clear a vector layer before drawstart in openlayers 3

Points are a bit tricky because the drawstart and drawend events are driven from the same mouse click. To support all geometry types, have a look at the sample code below.

If you only ever need to support the ol.geom.Point type, then just use the drawend event and don't worry about the rest.

Sample Code

var geomType, lastFeature;

var source = new ol.source.Vector({
    wrapX: false
});

var draw = new ol.interaction.Draw({
    source: source,
    type: geomType
});

// removes the last feature from the vector source.
var removeLastFeature = function() {
    if (lastFeature)
      source.removeFeature(lastFeature);
};

// On draw start for polygon, line and circle types we need to 
// remove the previous feature so that it doesn't show.
draw.on('drawstart', function (e) {

    // Filter on all BUT points
    if (geomType !== 'Point')) {
       source.clear();  // implicit remove of last feature.
    }

});

// When a drawing ends for a point feature, remove the last feature
draw.on('drawend', function (e) {

    // Filter on points ONLY
    if (geomType === 'Point') {
        removeLastFeature();
    }
    lastFeature = e.feature;

});

map.addInteraction(draw);

jsFiddle

See a complete example here that handles all geometry types: https://jsfiddle.net/abb54n79/1/

[GIS] OpenLayers 3 – Modify Feature – Delete vertex on keypress

I think there is a problem with freehandCondition in Draw interaction, which is ShiftKeyOnly by default. When I changed it to ol.events.condition.never I can delete vertex without creating new feature. This solution deactivates freehand drawing. Example:

draw = new ol.interaction.Draw({
features:features,
type: ('Polygon'),
freehandCondition: function(event){
return ol.events.condition.never(event);
}

Best Answer

Related Solutions

[GIS] How to clear a vector layer before drawstart in openlayers 3

[GIS] OpenLayers 3 – Modify Feature – Delete vertex on keypress

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