document.getElementById('export-png').addEventListener('click', function() {
        map.once('rendercomplete', function(event) {
          var canvas = event.context.canvas;
          if (navigator.msSaveBlob) {
            navigator.msSaveBlob(canvas.msToBlob(), 'map.png');
          } else {
            canvas.toBlob(function(blob) {
              saveAs(blob, 'map.png');
            });
          }
        });
        map.renderSync();
      });
用demo的方法一点反应没有 除了install file-saver之外还有什么要注意的吗