mirror of https://github.com/M66B/FairEmail.git
151 lines
5.2 KiB
Java
151 lines
5.2 KiB
Java
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/*
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* Copyright (C) 2017 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package androidx.lifecycle;
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import androidx.annotation.MainThread;
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import androidx.annotation.NonNull;
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import androidx.annotation.RestrictTo;
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import androidx.annotation.VisibleForTesting;
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import androidx.annotation.WorkerThread;
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import androidx.arch.core.executor.ArchTaskExecutor;
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import java.util.concurrent.Executor;
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import java.util.concurrent.atomic.AtomicBoolean;
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/**
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* A LiveData class that can be invalidated & computed when there are active observers.
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* <p>
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* It can be invalidated via {@link #invalidate()}, which will result in a call to
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* {@link #compute()} if there are active observers (or when they start observing)
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* <p>
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* This is an internal class for now, might be public if we see the necessity.
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*
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* @param <T> The type of the live data
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* @hide internal
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*/
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@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP_PREFIX)
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public abstract class ComputableLiveData<T> {
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@SuppressWarnings("WeakerAccess") /* synthetic access */
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final Executor mExecutor;
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@SuppressWarnings("WeakerAccess") /* synthetic access */
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final LiveData<T> mLiveData;
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@SuppressWarnings("WeakerAccess") /* synthetic access */
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final AtomicBoolean mInvalid = new AtomicBoolean(true);
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@SuppressWarnings("WeakerAccess") /* synthetic access */
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final AtomicBoolean mComputing = new AtomicBoolean(false);
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/**
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* Creates a computable live data that computes values on the arch IO thread executor.
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*/
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@SuppressWarnings("WeakerAccess")
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public ComputableLiveData() {
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this(ArchTaskExecutor.getIOThreadExecutor());
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}
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/**
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* Creates a computable live data that computes values on the specified executor.
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*
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* @param executor Executor that is used to compute new LiveData values.
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*/
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@SuppressWarnings("WeakerAccess")
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public ComputableLiveData(@NonNull Executor executor) {
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mExecutor = executor;
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mLiveData = new LiveData<T>() {
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@Override
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protected void onActive() {
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mExecutor.execute(mRefreshRunnable);
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}
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};
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}
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/**
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* Returns the LiveData managed by this class.
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*
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* @return A LiveData that is controlled by ComputableLiveData.
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*/
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@SuppressWarnings("WeakerAccess")
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@NonNull
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public LiveData<T> getLiveData() {
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return mLiveData;
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}
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@VisibleForTesting
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final Runnable mRefreshRunnable = new Runnable() {
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@WorkerThread
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@Override
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public void run() {
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boolean computed;
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do {
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computed = false;
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// compute can happen only in 1 thread but no reason to lock others.
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if (mComputing.compareAndSet(false, true)) {
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// as long as it is invalid, keep computing.
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try {
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T value = null;
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while (mInvalid.compareAndSet(true, false)) {
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computed = true;
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value = compute();
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}
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if (computed) {
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mLiveData.postValue(value);
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}
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} finally {
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// release compute lock
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mComputing.set(false);
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}
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}
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// check invalid after releasing compute lock to avoid the following scenario.
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// Thread A runs compute()
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// Thread A checks invalid, it is false
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// Main thread sets invalid to true
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// Thread B runs, fails to acquire compute lock and skips
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// Thread A releases compute lock
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// We've left invalid in set state. The check below recovers.
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} while (computed && mInvalid.get());
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}
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};
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// invalidation check always happens on the main thread
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@VisibleForTesting
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final Runnable mInvalidationRunnable = new Runnable() {
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@MainThread
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@Override
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public void run() {
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boolean isActive = mLiveData.hasActiveObservers();
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if (mInvalid.compareAndSet(false, true)) {
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if (isActive) {
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mExecutor.execute(mRefreshRunnable);
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}
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}
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}
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};
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/**
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* Invalidates the LiveData.
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* <p>
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* When there are active observers, this will trigger a call to {@link #compute()}.
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*/
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public void invalidate() {
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ArchTaskExecutor.getInstance().executeOnMainThread(mInvalidationRunnable);
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}
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// TODO https://issuetracker.google.com/issues/112197238
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@SuppressWarnings({"WeakerAccess", "UnknownNullness"})
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@WorkerThread
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protected abstract T compute();
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}
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