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Debugging a Dapp using Remix & Geth |
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========================================== |
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|
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The ultimate goal of this tutorial is to debug transactions that have |
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been created by a dapp front end. |
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It is easy in Remix to debug a transaction created from its own GUI. |
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However, setting up an environment that allows you to debug transactions |
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created outside of Remix, require a bit more of complexity. |
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We will need four tools for that : |
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> - Geth - this is the center piece and provides the blockchain |
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> environment. We will basically run geth in a dev mode. |
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> - Remix - this is the Ethereum IDE. We will use it to develop our |
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> Solidity contract. |
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> - Any code editor you want - in order to write your front end :) |
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Install the environment |
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----------------------- |
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|
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### Install Metamask |
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Basically we will run our front end in the Metamask Chrome plugin ([Metamask](http://metamask.io)). |
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### Install Geth |
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[Geth](http://github.com/ethereum/go-ethereum/releases) is the official |
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Ethereum client. |
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Running the environment |
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----------------------- |
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|
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### Run Geth |
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We will run a test node. This node will have a new empty state and will |
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not be synced to the main or ropsten network. |
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geth --ipcpath <test-chain-directory>/geth.ipc --datadir <test-chain-directory> --dev console |
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`<test-chain-directory>` is the folder where keys and chain data will be |
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stored. |
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`--ipcpath` defines the end point that other apps (like Metamask) use to |
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talk to geth. |
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`--datadir` specifies the data directory. |
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`--dev` sets the node into private chain mode and adds some debugging |
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flags. |
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Then we need to create accounts and mine a bit to generate some Ether: |
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// from the geth console : |
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personal.newAccount() // You can execute this command several time if you need more than one account. |
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miner.start() // generate some Ether. |
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miner.stop() // stop mining after 30s-60s - we could also keep mining. |
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Next time we run Geth, we will only need to mine transactions (no need |
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to recreate account). |
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### Starting Remix |
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In Mist click on `Develop` / `Open Remix IDE` |
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Remix will open in a new window. If this is the first time it is run, |
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the `Ballot` contract will be loaded. |
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Now, we need to check if Remix is connected to Mist: |
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Right panel / third tab from the left, `Injected Provider` should be |
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checked. |
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![image](remix4.png) |
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Right panel / second tab from the left, `Transaction Origin` should |
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contain accounts we have previously created in Geth. |
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![image](remix5.png) |
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Developing contract / front end |
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------------------------------- |
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### Donation contract - Dapp Back end |
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Here is a sample solidity contract. |
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Copy and paste the following inside remix: |
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``` |
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{.sourceCode .none} |
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contract Donation { |
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address owner; |
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event fundMoved(address _to, uint _amount); |
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modifier onlyowner { if (msg.sender == owner) _; } |
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address[] _giver; |
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uint[] _values; |
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function Donation() { |
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owner = msg.sender; |
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} |
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function donate() payable { |
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addGiver(msg.value); |
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} |
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function moveFund(address _to, uint _amount) onlyowner { |
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uint balance = this.balance; |
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uint amount = _amount; |
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if (_amount <= this.balance) { |
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if (_to.send(this.balance)) { |
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fundMoved(_to, _amount); |
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} else { |
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throw; |
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} |
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} else { |
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throw; |
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} |
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} |
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function addGiver(uint _amount) internal { |
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_giver.push(msg.sender); |
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_values.push(_amount); |
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} |
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} |
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``` |
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### Dapp Front end |
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and here is the front end: |
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```html |
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<div> |
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<div>Donation Contract</div> |
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<br/> |
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<input id='contractaddress' placeholder='contract address' /> |
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<br/> |
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<div> |
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<br/> |
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<input id='fromGive' placeholder='from' /><input placeholder='amount' id='valueGive' /><button id="fallbackbtn" onclick="donate()">give</button> |
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<br/> |
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<br/> |
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<input id='fromMoveFund' placeholder='from' /><input id='moveFundTo' placeholder='move to' /><input id='amountToMove' placeholder='amount' /><button id="movefundbtn" onclick="movefund()">moveFund</button> |
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<br/> |
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<br/> |
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<div id='wait' ></div> |
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</div> |
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<br/> |
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<br/> |
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<div id='log'> |
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</div> |
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</div> |
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<script type="text/javascript"> |
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function donate () { |
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var donation = contractspec.at(document.getElementById('contractaddress').value) |
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donation.donate({ |
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from: document.getElementById('fromGive').value, |
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value: document.getElementById('valueGive').value |
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}, function (error, txHash) { |
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tryTillResponse(txHash, function (error, receipt) { |
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alert('done ' + txHash) |
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}) |
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}) |
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} |
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function movefund () { |
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var donation = contractspec.at(document.getElementById('contractaddress').value) |
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donation.moveFund( |
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document.getElementById('moveFundTo').value, |
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document.getElementById('amountToMove').value, |
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function (error, txHash) { |
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tryTillResponse(txHash, function (error, receipt) { |
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alert('done ' + txHash) |
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}) |
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}) |
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} |
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var contractspec = web3.eth.contract([{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_amount","type":"uint256"}],"name":"moveFund","outputs":[],"payable":false,"type":"function"},{"constant":false,"inputs":[],"name":"donate","outputs":[],"payable":true,"type":"function"},{"inputs":[],"payable":false,"type":"constructor"},{"anonymous":false,"inputs":[{"indexed":false,"name":"_to","type":"address"},{"indexed":false,"name":"_amount","type":"uint256"}],"name":"fundMoved","type":"event"}]); |
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function tryTillResponse (txhash, done) { |
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document.getElementById('wait').innerHTML = 'waiting for the transaction to be mined ...' |
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web3.eth.getTransactionReceipt(txhash, function (err, result) { |
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if (!err && !result) { |
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// Try again with a bit of delay |
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setTimeout(function () { tryTillResponse(txhash, done) }, 500) |
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} else { |
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document.getElementById('wait').innerHTML = '' |
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var log = document.createElement("div") |
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log.innerHTML = JSON.stringify(result) |
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document.getElementById('log').appendChild(log) |
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done(err,result) |
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} |
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}) |
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} |
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</script> |
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``` |
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I would suggest serving this file using `http-serve`, but you can use |
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any web server you like. |
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Example: Dapp Front End <https://github.com/ltfschoen/dapp_front_end> |
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### Important notice ! |
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The variable `contractspec` contains the abi of the `donation` contract. |
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This means that if you change something in the contract interface |
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(function names, parameters, ...) you need to copy the new abi from |
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remix to the front end. |
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Deploying |
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--------- |
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Right panel / Red button `Create` |
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![image](remix1.png) |
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This creates a new transaction that deploys the `Donation` contract |
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(Mist will ask for the usual passphrase check). |
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Wait for the transaction to be mined (don't forget to activate mining |
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`miner.start()`). Once this is done, you can use it by executing the |
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`moveFund` and `donate` function. But this is not what we want to |
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achieve. We want to run and debug those functions from the front end. |
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Remix also display the address of the contract. Save it, we'll need this |
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address later. |
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![image](remix2.png) |
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Debugging |
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--------- |
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From Mist, browse the above front end. In the first field, paste the |
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address of the newly created contract. Now, let's call the first |
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function (label `give`). |
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You will need an account and a value. |
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The account could be any account that is declared in the Wallet section |
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of Mist. This is the sender of the transaction that we are going to |
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create. The value should be no more than the actual balance of the |
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account - the unit is in wei, so just put `100` (100 wei), that should |
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be fine. |
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Click on `Give` and wait for the transaction to be mined. |
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The HTML block with id `log` is filled by all the transactions created |
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from the front end. It was easier for the purpose of this tutorial to |
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just log transactions in a div but you can have your own logging |
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mechanism. |
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There is only one field that we need, this is the `transactionHash`. |
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Copy it and switch to Remix. On the right side, the fifth panel shows a |
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small "bug" icon, that is the debugger. |
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Paste the hash into the transaction field and click on the `play` |
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button. |
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![image](remix3.png) |
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You are now entering a debug session for the call to `donate`. |
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Debugging in Remix is easier than with common tools like gdb because you |
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can freely move in time. Use the slider to change the current step and |
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click on the panels below to expand them and explore the curret state, |
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local variables, etc. There are also breakpoints to move between |
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sections of the code quickly, but more on all that later. |
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At the time of this writing, there is an issue that could break the |
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contract creation. The a workaround for that at |
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<https://github.com/ethereum/go-ethereum/issues/3653> . Please follow |
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the workaround or wait for this issue to be closed. |
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Also, although retrieving a contract's storage when Remix is using the |
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JavaScript VM is working well, there is still work to be done when Remix |
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is using eth or geth as backend. |
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