/*
 * Copyright (C) 2008 Google Inc.
 *
 * 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.google.inject.spi;

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

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;

A lookup of the provider for a type. Lookups are created explicitly in a module using getProvider() statements: 
    Provider<PaymentService> paymentServiceProvider
        = getProvider(PaymentService.class);
Author:
jessewilson@google.com (Jesse Wilson)
Since:
2.0/**
 * 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:
 *
 * <pre>
 *     Provider&lt;PaymentService&gt; paymentServiceProvider
 *         = getProvider(PaymentService.class);</pre>
 *
 * @author jessewilson@google.com (Jesse Wilson)
 * @since 2.0
 */
public final class ProviderLookup<T> implements Element {
  private final Object source;
  private final Dependency<T> dependency;
  private Provider<T> delegate;

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

  
Since:
4.0/** @since 4.0 */
  public ProviderLookup(Object source, Dependency<T> dependency) {
    this.source = checkNotNull(source, "source");
    this.dependency = checkNotNull(dependency, "dependency");
  }

  @Override
  public Object getSource() {
    return source;
  }

  public Key<T> getKey() {
    return dependency.getKey();
  }

  
Since:
4.0/** @since 4.0 */
  public Dependency<T> getDependency() {
    return dependency;
  }

  @Override
  public <T> T acceptVisitor(ElementVisitor<T> visitor) {
    return visitor.visit(this);
  }

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

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

  
Returns the delegate provider, or 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. /**
   * 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<T> 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 
IllegalStateException if you try to use it beforehand. /**
   * 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<T> getProvider() {
    return new ProviderWithDependencies<T>() {
      @Override
      public T get() {
        Provider<T> local = delegate;
        if (local == null) {
          throw new IllegalStateException(
              "This Provider cannot be used until the Injector has been created.");
        }
        return local.get();
      }

      @Override
      public Set<Dependency<?>> getDependencies() {
        // We depend on Provider<T>, 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.<Dependency<?>>of(Dependency.get(providerKey));
      }

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