ℹ️ NOTE: This section has been recently added to the course and is an early draft that may still be awaiting review. Caveat reader.
This section explains how to interact with bitcoind using the Swift programming language and your own RPC client.
To date, you've built all of your alternative-programming-language development environments on your Debian virtual node. However, that's not the best platform for Swift. Though there is a version of Swift available for Ubuntu platforms, it's not fully featured, and it works somewhat differently from the Mac-native Swift. A "variant" at the bottom of this section explains how to set it up, but be warned that you'll be in uncharted territory.
Instead, we suggest creating an optimal Swift environment on a Mac. There are four major steps in doing so.
You're going to need Xcode, the integrated development enviroment for Swift and Objective-C. That can be easily installed by going to the Mac App Store and Getting Xcode.
Some people advise against an App Store install because it's somewhat all-or-nothing; it also won't work if you're still using Mojave because you want to avoid Catalina's incompatibilities. In that case you can download directly from the Developer Area at Apple.
If you're using Mojave, you'll need the xip file for Xcode 10.3.1. Otherwise, get the newest one.
Once it's downloaded, you can click on the xip to extract it, then move the Xcode app to your Applications folder.
(Either way, you should have Xcode installed in your Applications folder at the end of this step.)
You're also going to need a Bitcoin node on your Mac, so that you can communicate with it. Technically, you could use a remote node and access it with the RPC login and password over the net. However, we suggest you instead install a full node directly on your Mac, because that's the safest and cleanest setup, ensuring that none of your communications leave your machine.
To easily install a full node on your Mac, use Blockchain Commons' GordianServer for MacOS. See the installation instructions in the README, but generally all you have to do is download the current dmg file, open it, and install that app in your Applications directory too.
Afterward, run the GordianServer App, and tell it to Start Testnet.
🔗 TESTNET vs. MAINNET: Or
StartMainnet.
When you want to access the bitcoin-cli created by GordianServer on your local Mac, you can find it at ~/.standup/BitcoinCore/bitcoin-VERSION/bin/bitcoin-cli, for example ~/.standup/BitcoinCore/bitcoin-0.20.1/bin/bitcoin-cli.
You may wish to create an alias for that:
alias bitcoin-cli="~/.standup/BitcoinCore/bitcoin-0.20.1/bin/bitcoin-cli -testnet"
🔗 TESTNET vs. MAINNET: Obviously, the
-testnetparameter is only required if you're running on testnet.
Finally, you'll need your rpcuser and rpcpassword information. That's in ~/Library/Application Support/Bitcoin/bitcoin.conf by default under Gordian.
$ grep rpc ~/Library/Application\ Support/Bitcoin/bitcoin.conf
rpcuser=oIjA53JC2u
rpcpassword=ebVCeSyyM0LurvgQyi0exWTqm4oU0rZU
...
At the time of this writing, there isn't an up-to-date, simple-to-use Bitcoin RPC Library that's specific for Swift, something that you can drop in and immediately start using. Thus, you're're going to do something you're never done before: build an RPC connection by hand.
This just requires writing a function that passes RPC commands on to bitcoind in the correct format:
func makeCommand(method: String, param: Any, completionHandler: @escaping (Any?) -> Void) -> Void {
RPC connections to bitcoind use the HTML protocol, which means that you need to do three things: create a URL; make a URLRequest; and initiate a URLSession.
Within the function, you need to create a URL from your IP, port, rpcuser, rpcpassword, and wallet:
let testnetRpcPort = "18332"
let nodeIp = "127.0.0.1:\(testnetRpcPort)"
let rpcusername = "oIjA53JC2u"
let rpcpassword = "ebVCeSyyM0LurvgQyi0exWTqm4oU0rZU"
let walletName = ""
The actual RPC connection to Bitcoin Core is built using a URL of the format "http://rpcusername:rpcpassword@nodeIp/walletName":
let walletUrl = "http://\(rpcusername):\(rpcpassword)@\(nodeIp)/\(walletName)"
let url = URL(string: walletUrl)
This means that your sample variables result in the following URL:
http://oIjA53JC2u:ebVCeSyyM0LurvgQyi0exWTqm4oU0rZU@127.0.0.1:18332/
Which should look a lot like the URL used in some of the previous sections for RPC connections.
With that URL in you hand, you can now create a URLRequest, with the POST method and the text/plain content type. The HTTP body is then the familiar JSON object that you've been sending whenever you connect directly to Bitcoin Core's RPC ports, as first demonstrated when using Curl in §4.4.
var request = URLRequest(url: url!)
request.httpMethod = "POST"
request.setValue("text/plain", forHTTPHeaderField: "Content-Type")
request.httpBody = "{\"jsonrpc\":\"1.0\",\"id\":\"curltest\",\"method\":\"\(method)\",\"params\":[\(param)]}".data(using: .utf8)
Finally, you're ready to build a URLSession around your URLRequest.
let session = URLSession(configuration: .default)
let task = session.dataTask(with: request as URLRequest) { data, response, error in
The completion handler for dataTask needs to check for errors:
do {
if error != nil {
//Handle the error
} else {
And then parse the data that you're receiving. Here, you're pulling the JSON results into an NSDictionary:
if let urlContent = data {
do {
let json = try JSONSerialization.jsonObject(with: urlContent, options: JSONSerialization.ReadingOptions.mutableLeaves) as! NSDictionary
After that, there's more error handling and more error handling and then you can eventually return the dictionary result using the completionHandler that you defined for the new makeCommand function:
if let errorCheck = json["error"] as? NSDictionary {
if let errorMessage = errorCheck["message"] as? String {
print("FAILED")
print(errorMessage)
}
} else {
let result = json["result"]
completionHandler(result)
}
} catch {
//Handle error here
}
Of course you eventually have to tell the task to start:
task.resume()
And that's "all" there is to doing that RPC interaction by hand using a programming language such as Swift.
🙏 THANKS: Thanks to @Fonta1n3 who provided the main code for our RPC Transmitter.
Having written the makeCommand RPC function, you can send an RPC call by running it. Here's getblockchaininfo:
let method = "getblockchaininfo"
let param = ""
makeCommand(method: method,param: param) { result in
print(result!)
}
You could similarly grab the current block count from that info and use that to (reduntantly) get the hash of the current block, by using the param parameter:
let method = "getblockchaininfo"
let param = ""
makeCommand(method: method,param: param) { result in
let blockinfo = result as! NSDictionary
let block = blockinfo["blocks"] as! NSNumber
let method = "getblockhash"
makeCommand(method: method,param: block) { result in
print("Blockhash for \(block) is \(result!)")
}
}
The complete code is available in the src directory. Load it into your Xcode playground and then "Editor -> Run Playground" and you should get results like:
{
bestblockhash = 00000000000000069725608ebc5b59e520572a8088cbc57ffa5ba87b7f300ac7;
blocks = 1836745;
chain = test;
chainwork = 0000000000000000000000000000000000000000000001cc3e9f8e0bc6b71196;
difficulty = "16508683.81195478";
headers = 1836745;
initialblockdownload = 0;
mediantime = 1601416765;
pruned = 0;
"size_on_disk" = 28205538354;
softforks = {
bip34 = {
active = 1;
height = 21111;
type = buried;
};
bip65 = {
active = 1;
height = 581885;
type = buried;
};
bip66 = {
active = 1;
height = 330776;
type = buried;
};
csv = {
active = 1;
height = 770112;
type = buried;
};
segwit = {
active = 1;
height = 834624;
type = buried;
};
};
verificationprogress = "0.999999907191804";
warnings = "Warning: unknown new rules activated (versionbit 28)";
}
Blockhash for 1836745 is 00000000000000069725608ebc5b59e520572a8088cbc57ffa5ba87b7f300ac7
With your new makeCommand for RPC functions, you can similarly run a command like getwalletinfo or getbalance:
var method = "getwalletinfo"
var param = ""
makeCommand(method: method,param: param) { result in
print(result!)
}
method = "getbalance"
makeCommand(method: method,param: param) { result in
let balance = result as! NSNumber
print("Balance is \(balance)")
}
Which returns:
Balance is 0.01
{
"avoid_reuse" = 0;
balance = "0.01";
hdseedid = bf493318f548df8e25c390d6a7f70758fd6b3668;
"immature_balance" = 0;
keypoololdest = 1599723938;
keypoolsize = 999;
"keypoolsize_hd_internal" = 1000;
paytxfee = 0;
"private_keys_enabled" = 1;
scanning = 0;
txcount = 1;
"unconfirmed_balance" = 0;
walletname = "";
walletversion = 169900;
}
Creating an address is simple enough, but what about creating a legacy address with a specific label? That requires two parameters in your RPC call.
Since the simplistic makeCommand function in this section just passes on its params as the guts of a JSON Object, all you have to do is correctly format those guts. Here's one way to do so:
method = "getnewaddress"
param = "\"learning-bitcoin\", \"legacy\""
makeCommand(method: method,param: param) { result in
let address = result as! NSString
print(address)
}
Running this in the Xcode playground produces a result:
mt3ZRsmXHVMMqYQPJ8M74QjF78bmqrdHZF
That result is obviously a Legacy address; its label can then be checked from the command line:
$ bitcoin-cli getaddressesbylabel "learning-bitcoin"
{
"mt3ZRsmXHVMMqYQPJ8M74QjF78bmqrdHZF": {
"purpose": "receive"
}
}
Success!
ℹ️ NOTE: As we often say in these coding examples, a real-world program would be much more sophisticated. In particular, you'd want to be able to send an actual JSON Object as a parameter, and then have your
makeCommandprogram parse it and input it to the URLSession appropriately. What we have here maximizes readability and simplicity without focusing on ease of use.
As usual, sending a transaction (the hard way) is a multi-step process:
- Generate or receive a receiving address
- Find an unspent UTXO
- Create a raw transaction
- Sign the raw transaction
- Send the raw transaction
You'll use the address that you generated in the previous step as your recipient.
The listunspent RPC lets you find your UTXO:
method = "listunspent"
param = ""
makeCommand(method: method,param: param) { result in
let unspent = result as! NSArray
let utxo = unspent[0] as! NSDictionary
let txid = utxo["txid"] as! NSString
let vout = utxo["vout"] as! NSInteger
let amount = utxo["amount"] as! NSNumber
let new_amount = amount.floatValue - 0.0001
As in other examples, you're going to arbitrarily grab the 0th UTXO, and pull the txid, vout, and amount from it.
:information_source NOTE: Once again, a real-life program would be much more sophisticated.
Creating a raw transaction is the trickiest thing because you need to get all of your JSON objects, arrays, and quotes right. Here's how to do so in Swift, using the transmitter's very basic param formatting:
method = "createrawtransaction"
param="[ { \"txid\": \"\(txid)\", \"vout\": \(vout) } ], { \"\(address)\": \(new_amount)}"
makeCommand(method: method,param: param) { result in
let hex = result as! NSString
Signing your transaction just requires you to run the signrawtransactionwithwallet RPC, using your new hex:
method = "signrawtransactionwithwallet"
param = "\"\(hex)\""
makeCommand(method: method,param: param) { result in
let signedhexinfo = result as! NSDictionary
let signedhex = signedhexinfo["hex"] as! NSString
Sending your transaction is equally simple:
method = "sendrawtransaction"
param = "\"\(signedhex)\""
makeCommand(method: method,param: param) { result in
let new_txid = result as! NSString
print("TXID: \(new_txid)")
}
}
}
}
}
The code for this transaction sender can be found in the src directory.
That covers our usual discussions of programming Bitcoin RPC in a language, but Swift is a particularly important language since it can be deployed on mobile devices, one of the prime venues for wallets. As such, you may wish to consider a few other libraries:
- The Blockchain Commons ios-Bitcoin framework converts the Libbitcoin library from C++ to Swift
- Libwally Swift is a Swift wrapper for Libwally
Swift is a robust modern programming language that unfortunately doesn't yet have any easy-to-use RPC libraries ... which just gave us the opportunity to write an RPC-access function of our own. With that in hand, you can interact with bitcoind on a Mac or build companion applications over on an iPhone, which is a perfect combination for airgapped Bitcoin work.
Learn about Lightning in Chapter 18: Understanding Your Lightning Setup.
If you prefer to deploy Swift on Ubuntu, you can do so, though the functionality isn't the same. Some of the code in this chapter will likely generate errors that you'll need to resolve, and you'll also need to do more work to link in C libraries.
To get started, install some required Debian libraries:
$ sudo apt-get install clang
$ sudo apt-get install libcurl4 libpython2.7 libpython2.7-dev
If you're using Debian 10 or higher (and you really should be), you'll also need to backdate a few libraries to get older versions:
$ sudo apt-get install libtinfo5 libncurses5
Afteward you can download and install Swift:
$ wget https://swift.org/builds/swift-5.1.3-release/ubuntu1804/swift-5.1.3-RELEASE/swift-5.1.3-RELEASE-ubuntu18.04.tar.gz
$ tar xzfv swift-5.1.3-RELEASE-ubuntu18.04.tar.gz
$ sudo mv swift-5.1.3-RELEASE-ubuntu18.04 /usr/share/swift
To be able to use your new Swift setup, you need to update your PATH in your .bashrc:
$ echo "export PATH=/usr/share/swift/usr/bin:$PATH" >> ~/.bashrc
$ source ~/.bashrc
You can now test Swift out with the --version argument:
$ swift --version
Swift version 5.1.3 (swift-5.1.3-RELEASE)
Target: x86_64-unknown-linux-gnu
Once you've installed Swift on your Ubuntu machine, you can create projects with the package init command:
$ mkdir swift-project
$ cd swift-project/
/swift-project$ swift package init --type executable
Creating executable package: swift-project
Creating Package.swift
Creating README.md
Creating .gitignore
Creating Sources/
Creating Sources/swift-project/main.swift
Creating Tests/
Creating Tests/LinuxMain.swift
Creating Tests/swift-projectTests/
Creating Tests/swift-projectTests/swift_projectTests.swift
Creating Tests/swift-projectTests/XCTestManifests.swift
You'll then edit Sources/.../main.swift and when you're ready to compile, you can use the build command:
$ swift build
[4/4] Linking swift-project
Finally, you'll be able to run the program from the .build/debug directory:
$ .build/debug/swift-project
Hello, world!
Good luck!