goog.require('ol.Map');
goog.require('ol.View2D');
goog.require('ol.layer.Tile');
goog.require('ol.source.OSM');


var map = new ol.Map({
  layers: [
    new ol.layer.Tile({
      source: new ol.source.OSM()
    })
  ],
  target: 'map',
  view: new ol.View2D({
    center: [0, 0],
    zoom: 2
  })
});

var exportPNGElement = document.getElementById('export-png');

if ('download' in exportPNGElement) {
  exportPNGElement.addEventListener('click', function(e) {
    map.once('postcompose', function(event) {
      var canvas = event.context.canvas;
      exportPNGElement.href = canvas.toDataURL('image/png');
    });
    map.render();
  }, false);
} else {
  var info = document.getElementById('no-download');
  /**
   * display error message
   */
  info.style.display = '';
}