testData/compose/docs/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html - platform/tools/dokka-devsite-plugin - Gitiles

 <html devsite="true">
   <head>
     <title>SnapshotMutationPolicy</title>
 {% setvar book_path %}/reference/kotlin/androidx/_book.yaml{% endsetvar %}
 {% include "_shared/_reference-head-tags.html" %}
   </head>
   <body>
     <div id="metadata-info-block"></div>
     <h1>SnapshotMutationPolicy</h1>
     <p>
       <pre>interface <a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html">SnapshotMutationPolicy</a>&lt;T&nbsp;:&nbsp;<a href="https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/index.html">Any</a>?&gt;</pre>
     </p>
     <hr>
     <p>A policy to control how the result of <code><a href="/reference/kotlin/androidx/compose/runtime/package-summary.html#mutableStateOf(kotlin.Any,androidx.compose.runtime.SnapshotMutationPolicy)">mutableStateOf</a></code> report and merge changes to the state object.</p>
     <p>A mutation policy can be passed as an parameter to <code><a href="/reference/kotlin/androidx/compose/runtime/package-summary.html#mutableStateOf(kotlin.Any,androidx.compose.runtime.SnapshotMutationPolicy)">mutableStateOf</a></code>, and <code><a href="/reference/kotlin/androidx/compose/runtime/package-summary.html#compositionLocalOf(androidx.compose.runtime.SnapshotMutationPolicy,kotlin.Function0)">compositionLocalOf</a></code>.</p>
     <p>Typically, one of the stock policies should be used such as <code><a href="/reference/kotlin/androidx/compose/runtime/package-summary.html#referentialEqualityPolicy()">referentialEqualityPolicy</a></code>, <code><a href="/reference/kotlin/androidx/compose/runtime/package-summary.html#structuralEqualityPolicy()">structuralEqualityPolicy</a></code>, or <code><a href="/reference/kotlin/androidx/compose/runtime/package-summary.html#neverEqualPolicy()">neverEqualPolicy</a></code>. However, a custom mutation policy can be created by implementing this interface, such as a counter policy,</p>
     <pre class="prettyprint">
 import androidx.compose.runtime.mutableStateOf

 /**
  * A policy that treats an `MutableState&lt;Int&gt;` as a counter. Changing the value to the same
  * integer value will not be considered a change. When snapshots are applied the changes made by
  * the applying snapshot are added together with changes of other snapshots. Changes to a
  * [MutableState] with a counterPolicy will never cause an apply conflict.
  *
  * As the name implies, this is useful when counting things, such as tracking the amount of
  * a resource consumed or produced while in a snapshot. For example, if snapshot A produces 10
  * things and snapshot B produces 20 things, the result of applying both A and B should be that
  * 30 things were produced.
  */
 fun counterPolicy(): SnapshotMutationPolicy&lt;Int&gt; = object : SnapshotMutationPolicy&lt;Int&gt; {
     override fun equivalent(a: Int, b: Int): Boolean = a == b
     override fun merge(previous: Int, current: Int, applied: Int) =
         current + (applied - previous)
 }

 val state = mutableStateOf(0, counterPolicy())
 val snapshot1 = Snapshot.takeMutableSnapshot()
 val snapshot2 = Snapshot.takeMutableSnapshot()
 try {
     snapshot1.enter { state.value += 10 }
     snapshot2.enter { state.value += 20 }
     snapshot1.apply().check()
     snapshot2.apply().check()
 } finally {
     snapshot1.dispose()
     snapshot2.dispose()
 }

 // State is now equals 30 as the changes made in the snapshots are added together.</pre>
     <h2>Summary</h2>
     <div class="devsite-table-wrapper">
       <table class="responsive">
         <thead>
           <tr>
             <th colspan="100%"><h3>Public functions</h3></th>
           </tr>
         </thead>
         <tbody class="list">
           <tr>
             <td width="40%"><code><a href="https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html">Boolean</a></code></td>
             <td>
               <div><code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#equivalent(kotlin.Any,kotlin.Any)">equivalent</a>(a:&nbsp;T,&nbsp;b:&nbsp;T)</code></div>
               <p>Determine if setting a state value's are equivalent and should be treated as equal.</p>
             </td>
           </tr>
           <tr>
             <td width="40%"><code>open T?</code></td>
             <td>
               <div><code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">merge</a>(previous:&nbsp;T,&nbsp;current:&nbsp;T,&nbsp;applied:&nbsp;T)</code></div>
               <p>Merge conflicting changes in snapshots.</p>
             </td>
           </tr>
         </tbody>
       </table>
     </div>
     <h2>Public functions</h2>
     <div><a name="equivalent(kotlin.Any, kotlin.Any)"></a><a name="equivalent-kotlin.Any-kotlin.Any-"></a><a name="equivalent"></a>
       <h3 class="api-name" id="equivalent(kotlin.Any,kotlin.Any)">equivalent</h3>
       <pre class="api-signature no-pretty-print">fun&nbsp;<a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#equivalent(kotlin.Any,kotlin.Any)">equivalent</a>(a:&nbsp;T,&nbsp;b:&nbsp;T):&nbsp;<a href="https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html">Boolean</a></pre>
       <p>Determine if setting a state value's are equivalent and should be treated as equal. If <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#equivalent(kotlin.Any,kotlin.Any)">equivalent</a></code> returns <code>true</code> the new value is not considered a change.</p>
     </div>
     <div><a name="merge(kotlin.Any, kotlin.Any, kotlin.Any)"></a><a name="merge-kotlin.Any-kotlin.Any-kotlin.Any-"></a><a name="merge"></a>
       <h3 class="api-name" id="merge(kotlin.Any,kotlin.Any,kotlin.Any)">merge</h3>
       <pre class="api-signature no-pretty-print">open&nbsp;fun&nbsp;<a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">merge</a>(previous:&nbsp;T,&nbsp;current:&nbsp;T,&nbsp;applied:&nbsp;T):&nbsp;T?</pre>
       <p>Merge conflicting changes in snapshots. This is only called if <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">current</a></code> and <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">applied</a></code> are not <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#equivalent(kotlin.Any,kotlin.Any)">equivalent</a></code>. If a valid merged value can be calculated then it should be returned.</p>
       <p>For example, if the state object holds an immutable data class with multiple fields, and <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">applied</a></code> has changed fields that are unmodified by <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">current</a></code> it might be valid to return a new copy of the data class that combines that changes from both <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">current</a></code> and <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">applied</a></code> allowing a snapshot to apply that would have otherwise failed.</p>
       <pre class="prettyprint">
 import androidx.compose.runtime.mutableStateOf

 /**
  * A policy that treats an `MutableState&lt;Int&gt;` as a counter. Changing the value to the same
  * integer value will not be considered a change. When snapshots are applied the changes made by
  * the applying snapshot are added together with changes of other snapshots. Changes to a
  * [MutableState] with a counterPolicy will never cause an apply conflict.
  *
  * As the name implies, this is useful when counting things, such as tracking the amount of
  * a resource consumed or produced while in a snapshot. For example, if snapshot A produces 10
  * things and snapshot B produces 20 things, the result of applying both A and B should be that
  * 30 things were produced.
  */
 fun counterPolicy(): SnapshotMutationPolicy&lt;Int&gt; = object : SnapshotMutationPolicy&lt;Int&gt; {
     override fun equivalent(a: Int, b: Int): Boolean = a == b
     override fun merge(previous: Int, current: Int, applied: Int) =
         current + (applied - previous)
 }

 val state = mutableStateOf(0, counterPolicy())
 val snapshot1 = Snapshot.takeMutableSnapshot()
 val snapshot2 = Snapshot.takeMutableSnapshot()
 try {
     snapshot1.enter { state.value += 10 }
     snapshot2.enter { state.value += 20 }
     snapshot1.apply().check()
     snapshot2.apply().check()
 } finally {
     snapshot1.dispose()
     snapshot2.dispose()
 }

 // State is now equals 30 as the changes made in the snapshots are added together.</pre>
     </div>
   </body>
 </html>
	<html devsite="true">
	<head>
	<title>SnapshotMutationPolicy</title>
	{% setvar book_path %}/reference/kotlin/androidx/_book.yaml{% endsetvar %}
	{% include "_shared/_reference-head-tags.html" %}
	</head>
	<body>
	<div id="metadata-info-block"></div>
	<h1>SnapshotMutationPolicy</h1>
	<p>
	<pre>interface <a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html">SnapshotMutationPolicy</a><T : <a href="https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/index.html">Any</a>?></pre>
	</p>
	<hr>
	<p>A policy to control how the result of <code><a href="/reference/kotlin/androidx/compose/runtime/package-summary.html#mutableStateOf(kotlin.Any,androidx.compose.runtime.SnapshotMutationPolicy)">mutableStateOf</a></code> report and merge changes to the state object.</p>
	<p>A mutation policy can be passed as an parameter to <code><a href="/reference/kotlin/androidx/compose/runtime/package-summary.html#mutableStateOf(kotlin.Any,androidx.compose.runtime.SnapshotMutationPolicy)">mutableStateOf</a></code>, and <code><a href="/reference/kotlin/androidx/compose/runtime/package-summary.html#compositionLocalOf(androidx.compose.runtime.SnapshotMutationPolicy,kotlin.Function0)">compositionLocalOf</a></code>.</p>
	<p>Typically, one of the stock policies should be used such as <code><a href="/reference/kotlin/androidx/compose/runtime/package-summary.html#referentialEqualityPolicy()">referentialEqualityPolicy</a></code>, <code><a href="/reference/kotlin/androidx/compose/runtime/package-summary.html#structuralEqualityPolicy()">structuralEqualityPolicy</a></code>, or <code><a href="/reference/kotlin/androidx/compose/runtime/package-summary.html#neverEqualPolicy()">neverEqualPolicy</a></code>. However, a custom mutation policy can be created by implementing this interface, such as a counter policy,</p>
	<pre class="prettyprint">
	import androidx.compose.runtime.mutableStateOf

	/**
	* A policy that treats an `MutableState<Int>` as a counter. Changing the value to the same
	* integer value will not be considered a change. When snapshots are applied the changes made by
	* the applying snapshot are added together with changes of other snapshots. Changes to a
	* [MutableState] with a counterPolicy will never cause an apply conflict.
	*
	* As the name implies, this is useful when counting things, such as tracking the amount of
	* a resource consumed or produced while in a snapshot. For example, if snapshot A produces 10
	* things and snapshot B produces 20 things, the result of applying both A and B should be that
	* 30 things were produced.
	*/
	fun counterPolicy(): SnapshotMutationPolicy<Int> = object : SnapshotMutationPolicy<Int> {
	override fun equivalent(a: Int, b: Int): Boolean = a == b
	override fun merge(previous: Int, current: Int, applied: Int) =
	current + (applied - previous)
	}

	val state = mutableStateOf(0, counterPolicy())
	val snapshot1 = Snapshot.takeMutableSnapshot()
	val snapshot2 = Snapshot.takeMutableSnapshot()
	try {
	snapshot1.enter { state.value += 10 }
	snapshot2.enter { state.value += 20 }
	snapshot1.apply().check()
	snapshot2.apply().check()
	} finally {
	snapshot1.dispose()
	snapshot2.dispose()
	}

	// State is now equals 30 as the changes made in the snapshots are added together.</pre>
	<h2>Summary</h2>
	<div class="devsite-table-wrapper">
	<table class="responsive">
	<thead>
	<tr>
	<th colspan="100%"><h3>Public functions</h3></th>
	</tr>
	</thead>
	<tbody class="list">
	<tr>
	<td width="40%"><code><a href="https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html">Boolean</a></code></td>
	<td>
	<div><code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#equivalent(kotlin.Any,kotlin.Any)">equivalent</a>(a: T, b: T)</code></div>
	<p>Determine if setting a state value's are equivalent and should be treated as equal.</p>
	</td>
	</tr>
	<tr>
	<td width="40%"><code>open T?</code></td>
	<td>
	<div><code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">merge</a>(previous: T, current: T, applied: T)</code></div>
	<p>Merge conflicting changes in snapshots.</p>
	</td>
	</tr>
	</tbody>
	</table>
	</div>
	<h2>Public functions</h2>
	<div><a name="equivalent(kotlin.Any, kotlin.Any)"></a><a name="equivalent-kotlin.Any-kotlin.Any-"></a><a name="equivalent"></a>
	<h3 class="api-name" id="equivalent(kotlin.Any,kotlin.Any)">equivalent</h3>
	<pre class="api-signature no-pretty-print">fun <a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#equivalent(kotlin.Any,kotlin.Any)">equivalent</a>(a: T, b: T): <a href="https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html">Boolean</a></pre>
	<p>Determine if setting a state value's are equivalent and should be treated as equal. If <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#equivalent(kotlin.Any,kotlin.Any)">equivalent</a></code> returns <code>true</code> the new value is not considered a change.</p>
	</div>
	<div><a name="merge(kotlin.Any, kotlin.Any, kotlin.Any)"></a><a name="merge-kotlin.Any-kotlin.Any-kotlin.Any-"></a><a name="merge"></a>
	<h3 class="api-name" id="merge(kotlin.Any,kotlin.Any,kotlin.Any)">merge</h3>
	<pre class="api-signature no-pretty-print">open fun <a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">merge</a>(previous: T, current: T, applied: T): T?</pre>
	<p>Merge conflicting changes in snapshots. This is only called if <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">current</a></code> and <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">applied</a></code> are not <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#equivalent(kotlin.Any,kotlin.Any)">equivalent</a></code>. If a valid merged value can be calculated then it should be returned.</p>
	<p>For example, if the state object holds an immutable data class with multiple fields, and <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">applied</a></code> has changed fields that are unmodified by <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">current</a></code> it might be valid to return a new copy of the data class that combines that changes from both <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">current</a></code> and <code><a href="/reference/kotlin/androidx/compose/runtime/SnapshotMutationPolicy.html#merge(kotlin.Any,kotlin.Any,kotlin.Any)">applied</a></code> allowing a snapshot to apply that would have otherwise failed.</p>
	<pre class="prettyprint">
	import androidx.compose.runtime.mutableStateOf

	/**
	* A policy that treats an `MutableState<Int>` as a counter. Changing the value to the same
	* integer value will not be considered a change. When snapshots are applied the changes made by
	* the applying snapshot are added together with changes of other snapshots. Changes to a
	* [MutableState] with a counterPolicy will never cause an apply conflict.
	*
	* As the name implies, this is useful when counting things, such as tracking the amount of
	* a resource consumed or produced while in a snapshot. For example, if snapshot A produces 10
	* things and snapshot B produces 20 things, the result of applying both A and B should be that
	* 30 things were produced.
	*/
	fun counterPolicy(): SnapshotMutationPolicy<Int> = object : SnapshotMutationPolicy<Int> {
	override fun equivalent(a: Int, b: Int): Boolean = a == b
	override fun merge(previous: Int, current: Int, applied: Int) =
	current + (applied - previous)
	}

	val state = mutableStateOf(0, counterPolicy())
	val snapshot1 = Snapshot.takeMutableSnapshot()
	val snapshot2 = Snapshot.takeMutableSnapshot()
	try {
	snapshot1.enter { state.value += 10 }
	snapshot2.enter { state.value += 20 }
	snapshot1.apply().check()
	snapshot2.apply().check()
	} finally {
	snapshot1.dispose()
	snapshot2.dispose()
	}

	// State is now equals 30 as the changes made in the snapshots are added together.</pre>
	</div>
	</body>
	</html>