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package com.google.inject.spi;

import com.google.common.collect.ImmutableSet;
import com.google.inject.Binder;
import com.google.inject.Key;
import com.google.inject.Provider;
import com.google.inject.util.Types;

import java.util.Set;

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

/**
 * A lookup of the provider for a type. Lookups are created explicitly in a module using
 * {@link com.google.inject.Binder#getProvider(Class) getProvider()} statements:
 *  *     Provider<PaymentService> paymentServiceProvider
 *         = getProvider(PaymentService.class);
 *
 */
public final class ProviderLookup implements Element {
    private final Object source;
    private final Dependency dependency;
    private Provider delegate;

    public ProviderLookup(Object source, Key key) {
        this(source, Dependency.get(checkNotNull(key, "key")));
    }

    public ProviderLookup(Object source, Dependency dependency) {
        this.source = checkNotNull(source, "source");
        this.dependency = checkNotNull(dependency, "dependency");
    }

    public Object getSource() {
        return source;
    }

    public Key getKey() {
        return dependency.getKey();
    }

    public Dependency getDependency() {
        return dependency;
    }

    public  T acceptVisitor(ElementVisitor visitor) {
        return visitor.visit(this);
    }

    /**
     * Sets the actual provider.
     *
     * @throws IllegalStateException if the delegate is already set
     */
    public void initializeDelegate(Provider delegate) {
        checkState(this.delegate == null, "delegate already initialized");
        this.delegate = checkNotNull(delegate, "delegate");
    }

    public void applyTo(Binder binder) {
        initializeDelegate(binder.withSource(getSource()).getProvider(dependency));
    }

    /**
     * Returns the delegate provider, or {@code null} if it has not yet been initialized. The delegate
     * will be initialized when this element is processed, or otherwise used to create an injector.
     */
    public Provider getDelegate() {
        return delegate;
    }

    /**
     * Returns the looked up provider. The result is not valid until this lookup has been initialized,
     * which usually happens when the injector is created. The provider will throw an {@code
     * IllegalStateException} if you try to use it beforehand.
     */
    public Provider getProvider() {
        return new ProviderWithDependencies() {
            public T get() {
                checkState(delegate != null,
                        "This Provider cannot be used until the Injector has been created.");
                return delegate.get();
            }

            public Set> getDependencies() {
                // We depend on Provider, not T directly.  This is an important distinction
                // for dependency analysis tools that short-circuit on providers.
                Key providerKey = getKey().ofType(Types.providerOf(getKey().getTypeLiteral().getType()));
                return ImmutableSet.>of(Dependency.get(providerKey));
            }

            @Override
            public String toString() {
                return "Provider<" + getKey().getTypeLiteral() + ">";
            }
        };
    }
}