@parity/light.js
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A high-level reactive JS library optimized for light clients
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Tutorial Part 5: Work with Contracts
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<h1 id="tutorial-part-5-work-with-contracts">Tutorial Part 5: Work with Contracts</h1>
<p>Contract API basically comes in three pieces:</p>
<ul>
<li>there is inspection of the contract state through calling constant functions.</li>
<li>there's state-changing transactions like transferring tokens to a counter-party.</li>
<li>there's event reception and reporting that (usually) happen when such a state change occurs</li>
</ul>
<p>As of version v1.0.4 of <code>@parity/light.js</code>, the library only supports the two first functionalities. Let's see how they work. But before that, let's create a contract object.</p>
<h2 id="our-first-contract">Our first contract</h2>
<p>Let's deal with the ERC20 contract, which is quite possible the most famous contract. We'll use the <code>GavCoin</code> contract, a test ERC20 contract deployed on Kovan at the address <code>0x4733659a5cB7896A65c918Add6f59C5148FB5ffa</code>.</p>
<p>The ABI of a classic ERC20 can be found in a lot of places, for example within the <a href="https://github.com/paritytech/js-libs/tree/master/packages/contracts" target="_blank"><code>@parity/contracts</code></a> npm module.</p>
<p>So let's instantiate a contract object:</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">import</span> erc20Abi <span class="hljs-keyword">from</span> <span class="hljs-string">'@parity/contracts/lib/abi/eip20'</span>;
<span class="hljs-keyword">import</span> { makeContract } <span class="hljs-keyword">from</span> <span class="hljs-string">'@parity/light.js'</span>;
<span class="hljs-keyword">const</span> gavcoinContract = makeContract(
<span class="hljs-string">'0x4733659a5cB7896A65c918Add6f59C5148FB5ffa'</span>,
erc20Abi
);
</code></pre>
<p>And done! <code>gavcoinContract</code> is now a javascript object, where all the keys are the functions in the contract with a <code>$</code> sign at the end:</p>
<pre><code class="lang-javascript">{
abi: {...}, <span class="hljs-comment">// @parity/abi Abi object, only needed for advanced usage</span>
address: <span class="hljs-string">'0x4733659a5cB7896A65c918Add6f59C5148FB5ffa'</span>,
allowance$(): ...,
approve$(): ...,
balanceOf$(): ...,
contractObject: {...}, <span class="hljs-comment">// @parity/api Contract object, only needed for advanced usage</span>
totalSupply$(): ...,
transfer$(): ...,
transferFrom$(): ...
}
</code></pre>
<p>The functions defined in the ABI are defined in this object too. We won't go into details about the <code>abi</code> and <code>contractObject</code> fields, but have a look at <a href="https://github.com/paritytech/js-libs/tree/master/packages/abi" target="_blank"><code>@parity/abi</code></a> and <a href="https://github.com/paritytech/js-libs/tree/master/packages/api" target="_blank"><code>@parity/api</code></a> respectively if interested.</p>
<h2 id="calling-constant-functions">Calling constant functions</h2>
<p>In the ERC20 ABI, the <code>balanceOf</code> function is a constant function, which means that it doesn't modify the Ethereum state. Calling such a function with return a RpcObservable that will update when the token balance of a account changes.</p>
<pre><code class="lang-javascript">gavcoinContract
.balanceOf$(<span class="hljs-string">'0x407d73d8a49eeb85d32cf465507dd71d507100c1'</span>)
.subscribe(balance => <span class="hljs-built_in">console</span>.log(<span class="hljs-string">'balance'</span>, balance));
</code></pre>
<p>See this snippet in action: <a href="https://codesandbox.io/s/mq8rwkqzxp" target="_blank">https://codesandbox.io/s/mq8rwkqzxp</a>.</p>
<h2 id="calling-state-changing-function">Calling state-changing function</h2>
<p>The usage is exactly the same as with constant functions, but the returned RpcObservable is the same as the one returned by <code>post$</code> (see <a href="tutorial4-send-a-transaction.html">previous chapter</a>).</p>
<pre><code class="lang-javascript">gavcoinContract
.transfer$(
[
<span class="hljs-string">'0x407d73d8a49eeb85d32cf465507dd71d507100c1'</span>, <span class="hljs-comment">// The "to" address</span>
<span class="hljs-keyword">new</span> BigNumber(<span class="hljs-number">2.01</span>) <span class="hljs-comment">// The amount to transfer</span>
],
<span class="hljs-comment">// This 2nd argument is an options object containing the following fields:</span>
{
<span class="hljs-keyword">from</span>: <span class="hljs-string">"0x..."</span>,
passphrase: <span class="hljs-string">"mypassphrase"</span>,
gasPrice: ...
}
)
.subscribe(transfer => <span class="hljs-built_in">console</span>.log(<span class="hljs-string">'transfer$'</span>, transfer));
<span class="hljs-comment">// Logs:</span>
<span class="hljs-comment">// transfer$ { signed: '0x123...ff' }</span>
<span class="hljs-comment">// transfer$ { sent: '0x456...ff' }</span>
<span class="hljs-comment">// transfer$ { confirmed: {/* receipt object */} }</span>
</code></pre>
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