| /* |
| * Copyright 2020 The Android Open Source Project |
| * |
| * Licensed under the Apache License, Version 2.0 (the "License"); |
| * you may not use this file except in compliance with the License. |
| * You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software |
| * distributed under the License is distributed on an "AS IS" BASIS, |
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| * See the License for the specific language governing permissions and |
| * limitations under the License. |
| */ |
| |
| package androidx.compose.samples |
| |
| import androidx.annotation.Sampled |
| import androidx.compose.ExperimentalComposeApi |
| import androidx.compose.MutableState |
| import androidx.compose.SnapshotMutationPolicy |
| import androidx.compose.mutableStateOf |
| import androidx.compose.snapshots.takeMutableSnapshot |
| |
| @Sampled |
| @OptIn(ExperimentalComposeApi::class) |
| fun counterSample() { |
| /** |
| * 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 = takeMutableSnapshot() |
| val snapshot2 = 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. |
| } |