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/*
* 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();
}
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