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Is there any way, other than importaddress, to watch balances of specific addresses?
If not, how it is possible for wallets and explorers to give information (balance, transactions) about specific addresses even though importaddress takes some time? Because we have to rescan sooner or later.
Is there any way, other than
importaddress, to watch balances of specific addresses?
You can import a wallet that was exported in the dumpwallet format using the importwallet command. You can also import addresses using importmulti and importpubkey. But, in general, no, there is no way to watch balances of specific addresses in Bitcoin Core without using an import* command.
With Bitcoin Core 0.17, you can use the scantxoutset command which will scan the UTXO set for the UTXOs pertaining to an output script (defined in various ways).
If not, how it is possible for wallets and explorers to give information (balance, transactions) about specific addresses even though importaddress takes some time? Because we have to rescan sooner or later.
Wallets keep track of this information themselves. They do so in the same way that the import* command work. import* commands are wallet commands to tell a wallet to keep track of certain output scripts. They keep track of these in the same way: scan incoming blocks and transactions for outputs that match what they are looking for.
Blockchain explorers do not use Bitcoin Core nor do they use the wallet for this sort of stuff. They maintain their own databases that store the information for all transactions and their outputs. They do so in a similar way to wallets, but instead of looking for specific output scripts, they just store all output scripts.
Is there any way, other than importaddress, to watch balances of specific addresses?
No, creating a watch-only address is the way to do this using a wallet such as Bitcoin-core.
how it is possible for wallets and explorers to give information ... even though importaddress takes some time?
Once the blockchain has been initially scanned for transactions involving an imported address, the wallet need only check new blocks for mentions of that address as it receives the new blocks, there should be no need to rescan the blockchain.
Blockchain explorers can maintain an index per address and continuously keep track of balances for all addresses. They don't need to recompute this from scratch for every new enquiry.
