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<div class="breadcrumbs"><a href="/v5/">Documentation</a> » <a href="/v5/concepts/">Ethereum Basics</a> » <span class="current">Events</span></div>
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<a name="events"></a><h1 class="show-anchors"><div>Events<div class="anchors"><a class="self" href="/v5/concepts/events/#events"></a></div></div></h1><p>Explain how topics and such work</p>
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<a name="events--solidity-topics"></a><h2 class="show-anchors"><div>Solidity Topics<div class="anchors"><a class="self" href="/v5/concepts/events/#events--solidity-topics"></a></div></div></h2><p>How to compute the topic...</p>
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<a name="events--logs-and-filtering"></a><h2 class="show-anchors"><div>Logs and Filtering<div class="anchors"><a class="self" href="/v5/concepts/events/#events--logs-and-filtering"></a></div></div></h2><p>Example hog logs are used.</p>
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<p>Link to provider.getLogs and contract.on</p>
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<a name="events--logs-and-filtering--filters"></a><h3 class="show-anchors"><div>Filters<div class="anchors"><a class="self" href="/v5/concepts/events/#events--logs-and-filtering--filters"></a></div></div></h3><p>Filter are used as a way to query ... efficient, explain bloom filters lightly</p>
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<p>A filter may have up to 4 topic-sets, where each topic-set refers to a condition that must match the log topic in that position (i.e. each condition is <code class="inline">AND</code>-ed together).</p>
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<p>If a topic-set is <code class="inline">null</code>, a log topic in that position is not filtered at all and <b>any value</b> matches.</p>
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<p>If a topic-set is a single topic, a log topic in that position must match <b>that topic</b>.</p>
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<p>If a topic-set is an array of topics, a log topic in that position must match any <b>one</b> of the topics (i.e. the topic in this position are <code class="inline">OR</code>-ed).</p>
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<table class="table full"><tr><td align="center" width="25%"><b>Topic-Sets</b></td><td align="center" colspan="3" width="75%"><b>Matching Logs</b></td><td class="fix"> </td></tr><tr><td align="center" width="25%">[ A ]</td><td align="left" colspan="3" rowspan="2" width="75%">topic[0] = A</td><td class="fix"> </td></tr><tr><td align="center" width="25%">[ A, null ]</td><td class="fix"> </td></tr><tr><td align="center" width="25%">[ null, B ]</td><td align="left" colspan="3" rowspan="3" width="75%">topic[1] = B</td><td class="fix"> </td></tr><tr><td align="center" width="25%">[ null, [ B ] ]</td><td class="fix"> </td></tr><tr><td align="center" width="25%">[ null, [ B ], null ]</td><td class="fix"> </td></tr><tr><td align="center" width="25%">[ A, B ]</td><td align="left" colspan="3" rowspan="3" width="75%">(topic[0] = A) <b>AND</b> (topic[1] = B)</td><td class="fix"> </td></tr><tr><td align="center" width="25%">[ A, [ B ] ]</td><td class="fix"> </td></tr><tr><td align="center" width="25%">[ A, [ B ], null ]</td><td class="fix"> </td></tr><tr><td align="center" width="25%">[ [ A, B ] ]</td><td align="left" colspan="3" rowspan="2" width="75%">(topic[0] = A) <b>OR</b> (topic[0] = B)</td><td class="fix"> </td></tr><tr><td align="center" width="25%">[ [ A, B ], null ]</td><td class="fix"> </td></tr><tr><td align="center" width="25%">[ [ A, B ], [ C, D ] ]</td><td align="left" colspan="3" width="75%"><b>[</b> (topic[0] = A) <b>OR</b> (topic[0] = B) <b>]</b> <b>AND</b> <b>[</b> (topic[1] = C) <b>OR</b> (topic[1] = D) <b>]</b></td><td class="fix"> </td></tr><tr><td class="table-title" colspan="4">Example Log Matching</td><td class="fix"> </td></tr></table><div class="code-title"><div>ERC-20 Transfer Filter Examples</div></div><div class="code"><span class="comment">// Short example of manually creating filters for an ERC-20
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</span><span class="comment">// Transfer event.
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</span>//
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<span class="comment">// Most users should generally use the Contract API to
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</span><span class="comment">// compute filters, as it is much simpler, but this is
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</span><span class="comment">// provided as an illustration for those curious. See
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</span><span class="comment">// below for examples of the equivalent Contract API.
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</span>
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<span class="comment">// ERC-20:
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</span><span class="comment">// Transfer(address indexed src, address indexed dst, uint val)
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</span>//
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<span class="comment">// -------------------^
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</span><span class="comment">// ----------------------------------------^
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</span>//
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<span class="comment">// Notice that only *src* and *dst* are *indexed*, so ONLY they
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</span><span class="comment">// qualify for filtering.
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</span>//
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<span class="comment">// Also, note that in Solidity an Event uses the first topic to
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</span><span class="comment">// identify the Event name; for Transfer this will be:
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</span><span class="comment">// id("Transfer(address,address,uint256)")
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</span>//
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<span class="comment">// Other Notes:
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</span><span class="comment">// - A topic must be 32 bytes; so shorter types must be padded
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</span>
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<span class="comment">// List all token transfers *from* myAddress
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</span>filter = {
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address: tokenAddress,
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topics: [
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id("Transfer(address,address,uint256)"),
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hexZeroPad(myAddress, 32)
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]
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}
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<span class="comment">// List all token transfers *to* myAddress:
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</span>filter = {
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address: tokenAddress,
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topics: [
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id("Transfer(address,address,uint256)"),
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null,
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hexZeroPad(myAddress, 32)
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]
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}
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<span class="comment">// List all token transfers *to* myAddress or myOtherAddress:
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</span>filter = {
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address: tokenAddress,
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topics: [
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id("Transfer(address,address,uint256)"),
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null,
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[
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hexZeroPad(myAddress, 32),
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hexZeroPad(myOtherAddress, 32),
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]
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]
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}
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</div><p>To simplify life, ..., explain here, the contract API</p>
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<div class="code-title"><div>ERC-20 Contract Filter Examples</div></div><div class="code">const abi = [
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"event Transfer(address indexed src, address indexed dst, uint val)"
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];
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const contract = new Contract(tokenAddress, abi, provider);
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<span class="comment">// List all token transfers *from* myAddress
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</span>contract.filters.Transfer(myAddress)
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<span class="result ok">// {
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</span><span class="result ok">// address: '0x6B175474E89094C44Da98b954EedeAC495271d0F',
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</span><span class="result ok">// topics: [
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</span><span class="result ok">// '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef',
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</span><span class="result ok">// '0x0000000000000000000000008ba1f109551bd432803012645ac136ddd64dba72'
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</span><span class="result ok">// ]
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</span><span class="result ok">// }
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</span>
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<span class="comment">// List all token transfers *to* myAddress:
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</span>contract.filters.Transfer(null, myAddress)
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<span class="result ok">// {
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</span><span class="result ok">// address: '0x6B175474E89094C44Da98b954EedeAC495271d0F',
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</span><span class="result ok">// topics: [
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</span><span class="result ok">// '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef',
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</span><span class="result ok">// null,
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</span><span class="result ok">// '0x0000000000000000000000008ba1f109551bd432803012645ac136ddd64dba72'
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</span><span class="result ok">// ]
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</span><span class="result ok">// }
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</span>
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<span class="comment">// List all token transfers *from* myAddress *to* otherAddress:
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</span>contract.filters.Transfer(myAddress, otherAddress)
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<span class="result ok">// {
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</span><span class="result ok">// address: '0x6B175474E89094C44Da98b954EedeAC495271d0F',
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</span><span class="result ok">// topics: [
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</span><span class="result ok">// '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef',
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</span><span class="result ok">// '0x0000000000000000000000008ba1f109551bd432803012645ac136ddd64dba72',
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</span><span class="result ok">// '0x000000000000000000000000ea517d5a070e6705cc5467858681ed953d285eb9'
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</span><span class="result ok">// ]
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</span><span class="result ok">// }
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</span>
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<span class="comment">// List all token transfers *to* myAddress OR otherAddress:
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</span>contract.filters.Transfer(null, [ myAddress, otherAddress ])
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<span class="result ok">// {
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</span><span class="result ok">// address: '0x6B175474E89094C44Da98b954EedeAC495271d0F',
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</span><span class="result ok">// topics: [
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</span><span class="result ok">// '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef',
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</span><span class="result ok">// null,
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</span><span class="result ok">// [
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</span><span class="result ok">// '0x0000000000000000000000008ba1f109551bd432803012645ac136ddd64dba72',
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</span><span class="result ok">// '0x000000000000000000000000ea517d5a070e6705cc5467858681ed953d285eb9'
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</span><span class="result ok">// ]
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</span><span class="result ok">// ]
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</span><span class="result ok">// }
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</span></div><a name="events--logs-and-filtering--other-things-todo"></a><h3 class="show-anchors"><div>Other Things? TODO<div class="anchors"><a class="self" href="/v5/concepts/events/#events--logs-and-filtering--other-things-todo"></a></div></div></h3><p>Explain what happens to strings and bytes, how to filter and retain the value</p>
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