| /* |
| * Copyright (C) 2017 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 android.arch.lifecycle; |
| |
| import android.arch.core.executor.ArchTaskExecutor; |
| import android.support.annotation.MainThread; |
| import android.support.annotation.NonNull; |
| import android.support.annotation.RestrictTo; |
| import android.support.annotation.VisibleForTesting; |
| import android.support.annotation.WorkerThread; |
| |
| import java.util.concurrent.atomic.AtomicBoolean; |
| |
| /** |
| * A LiveData class that can be invalidated & computed on demand. |
| * <p> |
| * This is an internal class for now, might be public if we see the necessity. |
| * |
| * @param <T> The type of the live data |
| * @hide internal |
| */ |
| @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP) |
| public abstract class ComputableLiveData<T> { |
| |
| private final LiveData<T> mLiveData; |
| |
| private AtomicBoolean mInvalid = new AtomicBoolean(true); |
| private AtomicBoolean mComputing = new AtomicBoolean(false); |
| |
| /** |
| * Creates a computable live data which is computed when there are active observers. |
| * <p> |
| * It can also be invalidated via {@link #invalidate()} which will result in a call to |
| * {@link #compute()} if there are active observers (or when they start observing) |
| */ |
| @SuppressWarnings("WeakerAccess") |
| public ComputableLiveData() { |
| mLiveData = new LiveData<T>() { |
| @Override |
| protected void onActive() { |
| // TODO if we make this class public, we should accept an executor |
| ArchTaskExecutor.getInstance().executeOnDiskIO(mRefreshRunnable); |
| } |
| }; |
| } |
| |
| /** |
| * Returns the LiveData managed by this class. |
| * |
| * @return A LiveData that is controlled by ComputableLiveData. |
| */ |
| @SuppressWarnings("WeakerAccess") |
| @NonNull |
| public LiveData<T> getLiveData() { |
| return mLiveData; |
| } |
| |
| @VisibleForTesting |
| final Runnable mRefreshRunnable = new Runnable() { |
| @WorkerThread |
| @Override |
| public void run() { |
| boolean computed; |
| do { |
| computed = false; |
| // compute can happen only in 1 thread but no reason to lock others. |
| if (mComputing.compareAndSet(false, true)) { |
| // as long as it is invalid, keep computing. |
| try { |
| T value = null; |
| while (mInvalid.compareAndSet(true, false)) { |
| computed = true; |
| value = compute(); |
| } |
| if (computed) { |
| mLiveData.postValue(value); |
| } |
| } finally { |
| // release compute lock |
| mComputing.set(false); |
| } |
| } |
| // check invalid after releasing compute lock to avoid the following scenario. |
| // Thread A runs compute() |
| // Thread A checks invalid, it is false |
| // Main thread sets invalid to true |
| // Thread B runs, fails to acquire compute lock and skips |
| // Thread A releases compute lock |
| // We've left invalid in set state. The check below recovers. |
| } while (computed && mInvalid.get()); |
| } |
| }; |
| |
| // invalidation check always happens on the main thread |
| @VisibleForTesting |
| final Runnable mInvalidationRunnable = new Runnable() { |
| @MainThread |
| @Override |
| public void run() { |
| boolean isActive = mLiveData.hasActiveObservers(); |
| if (mInvalid.compareAndSet(false, true)) { |
| if (isActive) { |
| // TODO if we make this class public, we should accept an executor. |
| ArchTaskExecutor.getInstance().executeOnDiskIO(mRefreshRunnable); |
| } |
| } |
| } |
| }; |
| |
| /** |
| * Invalidates the LiveData. |
| * <p> |
| * When there are active observers, this will trigger a call to {@link #compute()}. |
| */ |
| public void invalidate() { |
| ArchTaskExecutor.getInstance().executeOnMainThread(mInvalidationRunnable); |
| } |
| |
| @SuppressWarnings("WeakerAccess") |
| @WorkerThread |
| protected abstract T compute(); |
| } |