function main() {
  const w=$env.width,h=$env.height;
  const band=Math.min(80,h);
  const r=$data.current;
  return <Group width={w} height={h}>
    <Map width="fill" height="fill" followMode={MapFollowMode.latestData}/>
    <Mask x={0} y={h-band} width={w} height={band}>
      <Material style="ultraThin"/>
      <Rectangle width="fill" height="fill" gradient={{angle:90,colors:[
        {color:'clear',location:0},{color:'#00000099',location:0.45},{color:'black',location:1}
      ]}}/>
    </Mask>
    <Text x={18} y={h-42} width={w-36} height={24} fontSize={16}
      alignment="leading" color={$env.foreground}>
      {r.speed===null?'暂无速度':Math.round(r.speed)+' '+r.speedUnit}
    </Text>
  </Group>;
}