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A LiveData class that can be invalidated & computed on demand (based on an internal architecture components class)
package be.digitalia.arch.lifecycle;
import android.arch.lifecycle.LiveData;
import android.arch.lifecycle.MutableLiveData;
import android.os.AsyncTask;
import android.support.annotation.MainThread;
import android.support.annotation.NonNull;
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.
*
* @param <T> The type of the live data
*/
public abstract class ComputableLiveData<T> {
private final MutableLiveData<T> mLiveData;
private final AtomicBoolean mInvalid = new AtomicBoolean(true);
private final 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)
*/
public ComputableLiveData() {
mLiveData = new MutableLiveData<T>() {
@Override
protected void onActive() {
AsyncTask.THREAD_POOL_EXECUTOR.execute(mRefreshRunnable);
}
};
}
/**
* Returns the LiveData managed by this class.
*
* @return A LiveData that is controlled by ComputableLiveData.
*/
@NonNull
public LiveData<T> getLiveData() {
return mLiveData;
}
@VisibleForTesting
final Runnable mRefreshRunnable = new Runnable() {
@WorkerThread
@Override
public void run() {
boolean computed;
do {
computed = false;
if (mComputing.compareAndSet(false, true)) {
try {
T value = null;
while (mInvalid.compareAndSet(true, false)) {
computed = true;
value = compute();
}
if (computed) {
mLiveData.postValue(value);
}
} finally {
mComputing.set(false);
}
}
} while (computed && mInvalid.get());
}
};
/**
* Invalidates the LiveData.
* <p>
* When there are active observers, this will trigger a call to {@link #compute()}.
*/
@MainThread
public void invalidate() {
boolean isActive = mLiveData.hasActiveObservers();
if (mInvalid.compareAndSet(false, true)) {
if (isActive) {
AsyncTask.THREAD_POOL_EXECUTOR.execute(mRefreshRunnable);
}
}
}
@SuppressWarnings("WeakerAccess")
@WorkerThread
protected abstract T compute();
}
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