* <p>
 * All providers implement this interface, but this interface may be implemented whenever
 * lazy evaluation is required.
 * </p>
 */
public interface ValueSupplier {
    /**
     * Visits the producer of the value for this supplier. This might be one or more tasks, some external location, nothing (for a fixed value) or might unknown.
     */
    ValueProducer getProducer();

import java.util.Objects;

class OrElseValueProducer implements ValueSupplier.ValueProducer {
    private final EvaluationContext.EvaluationOwner owner;
    private final ProviderInternal<?> left;
    @Nullable
    private final ProviderInternal<?> right;
    private final ValueSupplier.ValueProducer leftProducer;
    private final ValueSupplier.ValueProducer rightProducer;

    override fun calculateOwnValue(consumer: ValueSupplier.ValueConsumer): ValueSupplier.Value<out BuildWorkResult> {
        require(result != null) {
            "Cannot access the value of '${BuildWorkResult::class.simpleName}' before it becomes available!"
        }
        return ValueSupplier.Value.ofNullable(result)
    }

    override fun calculateExecutionTimeValue(): ValueSupplier.ExecutionTimeValue<out BuildWorkResult> =

                }
            }
            return finalizeOnNextGet || consumer == ValueSupplier.ValueConsumer.DisallowUnsafeRead;
        }

        @Override
        public ValueSupplier.ValueConsumer forUpstream(ValueSupplier.ValueConsumer consumer) {
            if (disallowUnsafeRead) {
                return ValueSupplier.ValueConsumer.DisallowUnsafeRead;
            } else {
                return consumer;

        when:
        def result = toList(container)

        then:
        result == iterationOrder(b, a, d, c)

        and:
        1 * provider1.calculateValue(_) >> ValueSupplier.Value.of(a)
        1 * provider2.calculateValue(_) >> ValueSupplier.Value.of(d)
        0 * _
    }

    def "provider for iterable of elements is queried when elements iterated and insertion order is retained"() {

        return decodeValue().toProvider()
    }

    suspend fun ReadContext.decodeValue(): ValueSupplier.ExecutionTimeValue<*> =
        when (readByte()) {
            1.toByte() -> ValueSupplier.ExecutionTimeValue.missing<Any>()
            2.toByte() -> ValueSupplier.ExecutionTimeValue.ofNullable(read()) // nullable because serialization may replace value with null, e.g. when using provider of Task

        if (value.isMissing()) {
            visitor.execute(ValueSupplier.ExecutionTimeValue.missing());
        } else if (value.hasFixedValue()) {
            // transform preserving side effects

import java.util.Collection;

/**
 * A {@link ValueSupplier value supplier} that supplies collection values. Instances of
 * this type are used as value suppliers for {@link AbstractCollectionProperty}.
 *
 * @param <T> the type of elements
 * @param <C> type of collection
 */
interface CollectionSupplier<T, C extends Collection<? extends T>> extends ValueSupplier {
    /**
     * Realizes and returns the (collection) value.

    @Override
    AbstractProperty<T, ? extends ValueSupplier> providerWithValue(T value) {
        return propertyWithNoValue().value(value)
    }

    @Override
    AbstractProperty<T, ? extends ValueSupplier> providerWithNoValue() {
        return propertyWithNoValue()
    }

    @Override
    abstract AbstractProperty<T, ? extends ValueSupplier> propertyWithNoValue()

        then:
        1 * valueProvider.calculateValue(_) >> ValueSupplier.Value.of(['k1': 'v1'])
        1 * putProvider.calculateValue(_) >> ValueSupplier.Value.of('v2')
        1 * putAllProvider.calculateValue(_) >> ValueSupplier.Value.of(['k3': 'v3'])
        0 * _

        when:
        property.getOrNull()
        then:
        1 * valueProvider.calculateValue(_) >> ValueSupplier.Value.of(['k1': 'v1'])