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();
}