@parity/light.js
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A high-level reactive JS library optimized for light clients
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<h1 id="rpcobservables">RpcObservables</h1>
<p>The whole library <code>@parity/light.js</code> works with the concept of RpcObservables.</p>
<p>The name <em>RpcObservable</em> might actually not be the best, because it's actually <strong>not</strong> an Observable. A RpcObservable is a function returning an Observable.</p>
<p>Here's the TypeScript type of a RpcObservable:</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">export</span> interface RpcObservable<Source, Out> {
(...args: any[]): Observable<Out>;
metadata?: Metadata<Source, Out>;
}
</code></pre>
<p>Or in simple words: a RpcObservable is a function returning an Observable, and this function has some metadata.</p>
<p>Let's have a look at <code>MetaData</code>:</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">export</span> interface Metadata<Source, Out> {
<span class="hljs-comment">// --snip--</span>
frequency?: FrequencyObservable<Source>[];
name?: string;
pipes?: (...args: any[]) => OperatorFunction<Source, Out>[];
}
</code></pre>
<p>These are the most important fields of <code>MetaData</code>, which we will explain.</p>
<h2 id="main-idea">Main Idea</h2>
<p>We believe that data streams are an intuitive way to express events happening on the Ethereum blockchain. The most obvious example is the pubsub pattern we described <a href="light-client-development.html#pubsub">before</a>, where we wanted to fetch the balance on every new block.</p>
<p>It's intuitive to have an Observable, called <code>onEveryBlock$</code>, that would fire an event each time it receives a new block from the network. Then, every time it fires, we declaratively make an JSONRPC call to <code>eth_getBalance</code>. Into code, it looks like this:</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">import</span> { <span class="hljs-keyword">from</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'rxjs'</span>;
<span class="hljs-keyword">import</span> { switchMap } <span class="hljs-keyword">from</span> <span class="hljs-string">'rxjs/operators'</span>;
<span class="hljs-comment">// Observable that makes a JSONRPC request</span>
<span class="hljs-keyword">const</span> fetchBalance = () => <span class="hljs-keyword">from</span>(api.eth.getBalance(<span class="hljs-string">'0x12..ff'</span>));
<span class="hljs-comment">// On every block, we make that JSONRPC request</span>
onEveryBlock$.pipe(switchMap(fetchBalance)));
</code></pre>
<p>Similarly, if we want to check our peer count every 5 seconds, we'd do</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">import</span> { <span class="hljs-keyword">from</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'rxjs'</span>;
<span class="hljs-keyword">import</span> { switchMap } <span class="hljs-keyword">from</span> <span class="hljs-string">'rxjs/operators'</span>;
<span class="hljs-comment">// Observable that makes a JSONRPC request</span>
<span class="hljs-keyword">const</span> checkPeerCount = () => <span class="hljs-keyword">from</span>(api.net.peerCount());
<span class="hljs-comment">// On every block, we make that JSONRPC request</span>
onEvery5Seconds$.pipe(switchMap(checkPeerCount)));
</code></pre>
<p>The main idea of <code>@parity/light.js</code> is the following: <strong>"on every [frequency], we do [something]"</strong>.</p>
<p>On every new block, we call <code>eth_balance</code>. On every 5 seconds, we call <code>net_peerCount</code>. Etc.</p>
<p>Looking back at the <code>MetaData</code> above, the <code>frequency</code> field holds the <strong>frequency</strong>, which is an array of <code>FrequencyObservable</code>s, and the <code>pipes</code> hold an array of RxJS pipe functions, and each of them does <strong>something</strong>.</p>
<h2 id="frequencyobservable"><code>FrequencyObservable</code></h2>
<p>Some examples of <code>FrequencyObservable</code> shipped with <code>@parity/light.js</code> are:</p>
<ul>
<li><code>onEveryBlock$</code></li>
<li><code>onEvery2Blocks$</code></li>
<li><code>onEverySecond$</code></li>
<li><code>onEvery5Seconds$</code></li>
<li><code>onSyncingChanged$</code> (fires when sync status changes)</li>
<li><code>onAccountsChanged$</code> (fires when you change account)</li>
<li><code>onStartup$</code> (fires once at dapp start)</li>
<li>etc.</li>
</ul>
<p>The full list can be seen in the <a href="../api/API.md">API section</a>.</p>
<h2 id="rpcobservable">RpcObservable</h2>
<p>An RpcObservable can either have a frequency, or depend on another RpcObservable which has a frequency.</p>
<h3 id="rpcobservable-with-frequencies">RpcObservable with frequencies</h3>
<p>An RpcObservable with a frequency has its <code>Metadata.frequency</code> field filled with one or more <code>FrequencyObservable</code>s. For example, <code>balanceOf$</code> has as frequency of <code>[onEveryBlock$]</code>.</p>
<p>Then, this RpcObservable has an array of pipes, which will be piped to the <code>FrequencyObservable</code>. For example, <code>balanceOf$</code> has one pipe which make a <code>eth_getBalance</code> JSONRPC request.</p>
<h3 id="rpcobservable-which-depends-on-another-rpcobservable">RpcObservable which depends on another RpcObservable</h3>
<p>Some RpcObservable don't have their own frequencies, but depend on a parent RpcObservable. This is the case of <code>defaultAccount$</code>, which depends on <code>account$</code>. In this case, the <code>Metadata.dependsOn</code> field points to the parent RpcObservable.</p>
<p>These RpcObservables still have some pipes. For example <code>defaultAccount$</code> has one pipe which takes the 1st element of the array returned by <code>accounts$</code>.</p>
<h3 id="calling-an-rpcobservable">Calling an RpcObservable</h3>
<p>As per the TypeScript signature, the RpcObservable function can take arguments. These arguments are then passed down into the pipes. For instance, the argument <code>0x123</code> in <code>balanceOf$('0x123')</code> is passed down into <code>balanceOf$</code>'s pipes, i.e. into the JSONRPC <code>eth_getBalance</code> call.</p>
<h3 id="examples-of-rpcobservable">Examples of RpcObservable</h3>
<ul>
<li><code>accounts$</code>: Returns the array of accounts managed by the node. Frequency is <code>onAccountsChanged$</code>.</li>
<li><code>defaultAccount$</code>: Returns the default active account. Depends on <code>accounts$</code>.</li>
<li><code>blockNumber$</code>: Returns the current block number. Frequency is <code>onEveryBlock$</code>.</li>
<li><code>peerCount$</code>: Returns the number of peers. Frequency is <code>onEvery5Seconds$</code>.</li>
</ul>
<p>The full list can be seen in the <a href="../api/API.md">API section</a>.</p>
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