Also how the organizer of the RAFFLE choose a block? Is he do it as the time at which the RAFFLE closes and the current block which is going on or on some other basis?
A (random) number block in the not to distant future might also work.
Lets say I start the raffle today and want it to finish some time sunday next week. So I go to e.g. blocktrail and see the current block height is 447073 and there are on average 144 blocks per day. So I go to random.org and let me get a random number between 72 and 144. It tells me 99. I add another 144*7 blocks for the week and say that block 447073 + 99 + (144*7) = 448180 will decide who wins the raffle. I would also require valid transactions to be confirmed in block 448179 or earlier.
For fun. I generates a new one with each click, yes.
No it doesnt need one, but might as well add one in the mix for fun.
but how a block is chosen?