Glide源码阅读之原型模式【TransitionOptions】【BaseRequestOptions】【RequestBuilder】

定义

菜鸟教程

原型模式（Prototype Pattern）是用于创建重复的对象，同时又能保证性能。这种类型的设计模式属于创建型模式，它提供了一种创建对象的最佳方式。

这种模式是实现了一个原型接口，该接口用于创建当前对象的克隆。当直接创建对象的代价比较大时，则采用这种模式。例如，一个对象需要在一个高代价的数据库操作之后被创建。我们可以缓存该对象，在下一个请求时返回它的克隆，在需要的时候更新数据库，以此来减少数据库调用

大话设计模式

用原型实例指定创建对象的种类，并且通过拷贝这些原型创建新的对象

《Android源码设计模式解析与实践》

特点

最大的特点是：implements Cloneable；但凡实现Cloneable接口的一般都满足原型模式。当然也不排除有其他变种情况，不过目前没有碰到。如果有请留言分享，或者后期我碰到后再补充。

注意事项：

与通过对一个类进行实例化来构造新对象不同的是，原型模式是通过拷贝一个现有对象生成新对象的。浅拷贝实现 Cloneable，重写，深拷贝是通过实现 Serializable 读取二进制流。

使用场景

1. 简历模板
2. 历史文档修改记录（比如：飞书、腾讯等在线编辑文档）

浅拷贝、深拷贝 浅拷贝

被复制对象的所有变量都含有与原来的对象相同的值，而所有的对其他对象的引用都仍然指向原来的对象。

深拷贝

把引用对象的变量指向复制过的新对象，而不是原有的被引用的对象

Glide原型模式应用

包路径：com.bumptech.glide.TransitionOptions

/**
 * A base class for setting a transition to use on a resource when a load completes.
 *
 * @param  The implementation of this class to return to chain methods.
 * @param  The type of resource that will be animated.
 */
public abstract class TransitionOptions
    implements Cloneable {
  private TransitionFactory transitionFactory = NoTransition.getFactory();

  /**
   * Removes any existing animation put on the builder. Will be overridden by subsequent calls that
   * put an animation.
   *
   * @return This request builder.
   */
  @NonNull
  public final CHILD dontTransition() {
    return transition(NoTransition.getFactory());
  }

  /**
   * Sets an {@link android.view.animation.Animation} to run on the wrapped target when an resource
   * load finishes. Will only be run if the resource was loaded asynchronously (i.e. was not in the
   * memory cache).
   *
   * @param viewAnimationId The resource id of the {@link android.view.animation} to use as the
   *     transition.
   * @return This request builder.
   */
  @NonNull
  public final CHILD transition(int viewAnimationId) {
    return transition(new ViewAnimationFactory(viewAnimationId));
  }

  /**
   * Sets an animator to run a {@link android.view.ViewPropertyAnimator} on a view that the target
   * may be wrapping when a resource load finishes. Will only be run if the load was loaded
   * asynchronously (i.e. was not in the memory cache).
   *
   * @param animator The {@link com.bumptech.glide.request.transition.ViewPropertyTransition
   *     .Animator} to run.
   * @return This request builder.
   */
  @NonNull
  public final CHILD transition(@NonNull ViewPropertyTransition.Animator animator) {
    return transition(new ViewPropertyAnimationFactory(animator));
  }

  /**
   * Uses the given {@link TransitionFactory} to build a {@link
   * com.bumptech.glide.request.transition.Transition} for each request started with these {@code
   * TransitionOptions}.
   *
   * @return This request builder.
   */
  @NonNull
  public final CHILD transition(
      @NonNull TransitionFactory transitionFactory) {
    this.transitionFactory = Preconditions.checkNotNull(transitionFactory);
    return self();
  }

  @SuppressWarnings({
    // cast to CHILD is safe given the generic argument represents the object's runtime class
    "unchecked",
    // CHILD is the correct class name.
    "PMD.CloneMethodReturnTypeMustMatchClassName",
    // we don't want to throw to be user friendly
    "PMD.CloneThrowsCloneNotSupportedException"
  })
  @Override
  public final CHILD clone() {
    try {
      return (CHILD) super.clone();
    } catch (CloneNotSupportedException e) {
      throw new RuntimeException(e);
    }
  }

  final TransitionFactory getTransitionFactory() {
    return transitionFactory;
  }

  @SuppressWarnings("unchecked")
  private CHILD self() {
    return (CHILD) this;
  }
}

被RequestBuilder调用，同时RequestBuilder也是原型模式，两者结合正好说明浅拷贝和深拷贝。TransitionOptions属于浅拷贝那种 RequestBuilder调用了TransitionOptions的clone()属于深拷贝那种。当然深浅拷贝取舍也是根据实际业务场景设定的。

/**
 * A generic class that can handle setting options and staring loads for generic resource types.
 *
 * @param  The type of resource that will be delivered to the {@link
 *     com.bumptech.glide.request.target.Target}.
 */
// Public API.
@SuppressWarnings({"unused", "WeakerAccess"})
public class RequestBuilder extends BaseRequestOptions
    implements Cloneable, ModelTypes {
  。。。
   @NonNull
  @SuppressWarnings("unchecked")
  private TransitionOptions transitionOptions;
  /**
   * Returns a copy of this request builder with all of the options put so far on this builder.
   *
   * 
This method returns a "deep" copy in that all non-immutable arguments are copied such that
   * changes to one builder will not affect the other builder. However, in addition to immutable
   * arguments, the current model is not copied so changes to the model will affect both builders.
   */
  @SuppressWarnings({
    // we don't want to throw to be user friendly
    "PMD.CloneThrowsCloneNotSupportedException"
  })
  @CheckResult
  @Override
  public RequestBuilder clone() {
    RequestBuilder result = super.clone();
    result.transitionOptions = result.transitionOptions.clone();
    if (result.requestListeners != null) {
      result.requestListeners = new ArrayList(result.requestListeners);
    }
    if (result.thumbnailBuilder != null) {
      result.thumbnailBuilder = result.thumbnailBuilder.clone();
    }
    if (result.errorBuilder != null) {
      result.errorBuilder = result.errorBuilder.clone();
    }
    return result;
  }

再补充一个BaseRequestOptions

/**
 * A base object to allow method sharing between {@link RequestOptions} and {@link
 * com.bumptech.glide.RequestBuilder}.
 *
 * 
This class is not meant for general use and may change at any time.
 *
 * @param  The particular child implementation
 */
@SuppressWarnings({"PMD.UseUtilityClass", "unused"})
public abstract class BaseRequestOptions implements Cloneable {
  private static final int UNSET = -1;
  private static final int SIZE_MULTIPLIER = 1