/*
 * Copyright 2016, 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 com.example.android.architecture.blueprints.todoapp.util;

import android.support.test.espresso.IdlingResource;

import java.util.concurrent.atomic.AtomicInteger;

import static com.google.common.base.Preconditions.checkNotNull;

/**
 * An simple counter implementation of {@link IdlingResource} that determines idleness by
 * maintaining an internal counter. When the counter is 0 - it is considered to be idle, when it is
 * non-zero it is not idle. This is very similar to the way a {@link java.util.concurrent.Semaphore}
 * behaves.
 * <p>
 * This class can then be used to wrap up operations that while in progress should block tests from
 * accessing the UI.
 */
public final class SimpleCountingIdlingResource implements IdlingResource {

    private final String mResourceName;

    private final AtomicInteger counter = new AtomicInteger(0);

    // written from main thread, read from any thread.
    private volatile ResourceCallback resourceCallback;

    /**
     * Creates a SimpleCountingIdlingResource
     *
     * @param resourceName the resource name this resource should report to Espresso.
     */
    public SimpleCountingIdlingResource(String resourceName) {
        mResourceName = checkNotNull(resourceName);
    }

    @Override
    public String getName() {
        return mResourceName;
    }

    @Override
    public boolean isIdleNow() {
        return counter.get() == 0;
    }

    @Override
    public void registerIdleTransitionCallback(ResourceCallback resourceCallback) {
        this.resourceCallback = resourceCallback;
    }

    /**
     * Increments the count of in-flight transactions to the resource being monitored.
     */
    public void increment() {
        counter.getAndIncrement();
    }

    /**
     * Decrements the count of in-flight transactions to the resource being monitored.
     *
     * If this operation results in the counter falling below 0 - an exception is raised.
     *
     * @throws IllegalStateException if the counter is below 0.
     */
    public void decrement() {
        int counterVal = counter.decrementAndGet();
        if (counterVal == 0) {
            // we've gone from non-zero to zero. That means we're idle now! Tell espresso.
            if (null != resourceCallback) {
                resourceCallback.onTransitionToIdle();
            }
        }

        if (counterVal < 0) {
            throw new IllegalArgumentException("Counter has been corrupted!");
        }
    }
}
