<grid name="grid1" ...>
  <rowDefinition value={0.5} />
  <rowDefinition value={0.5} />
  <columnDefinition value={100} unit={ValueAndUnit.UNITMODE_PIXEL} />
  <columnDefinition value={100} unit={ValueAndUnit.UNITMODE_PIXEL} />
  <rectangle
    name="rect-0-1"
    gridRow={0}
    gridColumn={1}
  />
  ...
</grid>