Custom Label for openlayers 3 features

Question

I have LineString in openlayers-3. I want to set 2 label for this feature In the start and end of the feature. An example is as follow:

How Can I do this labels?

Answer 1

I resolve this question own and I want to share it with others.
In first step, we define an method that get two point with a max to calculate the position of label. See image:

The code is as follow:

function getLineStylePosition(firstCoor, lastCoor, max)
{
    var deltaX = lastCoor[0] - firstCoor[0];
    var deltaY = lastCoor[1] - firstCoor[1];

    var rotation = Math.atan(deltaY/deltaX) * - 1;

    var offsetX = max * Math.cos(rotation) * (-1);
    var offsetY = max * Math.sin(rotation) * (-1);

    var textAlign = 'right';
    if(deltaX < 0)
    {
        offsetX *= -1;
        offsetY *= -1;
        textAlign = 'left';
    }

    return {
        rotation: rotation,
        textAlign: textAlign,
        offsetX: offsetX,
        offsetY: offsetY
    };
};   

Now you have the offset of style. You can define a style as follow:

var startTextStyle = new ol.style.Text(getLineStylePosition(firstCoor, lastCoor, 15));
startTextStyle.setText("label 1");
startTextStyle.setFont("Arial");
startTextStyle.setFill(new ol.style.Fill({color: 'blue'});

var startStyle = new ol.style.Style({
    text: startTextStyle,
    geometry: new ol.geom.Point(firstCoor)
});

Copy this code for Label 2.