【设计模式】原型模式——其他框架源码中的原型模式

原型模式在其他框架源码中也有广泛的应用。

Retrofit

众所周知Retrofit是OkHttp的扩展，因此Retrofit的Call接口也像OkHttp的Call类一样实现了原型模式。Call与原型模式有关的代码如下：

public interface Call<T> extends Cloneable {
  // ……代码省略……

  /**
   * Create a new, identical call to this one which can be enqueued or executed even if this call
   * has already been.
   */
  Call<T> clone();
}

Call的子类OkHttpCall与原型模式有关的源码如下：

final class OkHttpCall<T> implements Call<T> {
  // ……代码省略……

  @SuppressWarnings("CloneDoesntCallSuperClone") // We are a final type & this saves clearing state.
  @Override
  public OkHttpCall<T> clone() {
    return new OkHttpCall<>(requestFactory, args, callFactory, responseConverter);
  }
}

Gson的Excluder类

Gson的Excluder类与原型模式有关的源码如下：

public final class Excluder implements TypeAdapterFactory, Cloneable {
  // ……代码省略……

  @Override protected Excluder clone() {
    try {
      return (Excluder) super.clone();
    } catch (CloneNotSupportedException e) {
      throw new AssertionError(e);
    }
  }

  public Excluder withVersion(double ignoreVersionsAfter) {
    Excluder result = clone();
    result.version = ignoreVersionsAfter;
    return result;
  }

  public Excluder withModifiers(int... modifiers) {
    Excluder result = clone();
    result.modifiers = 0;
    for (int modifier : modifiers) {
      result.modifiers |= modifier;
    }
    return result;
  }

  public Excluder disableInnerClassSerialization() {
    Excluder result = clone();
    result.serializeInnerClasses = false;
    return result;
  }

  public Excluder excludeFieldsWithoutExposeAnnotation() {
    Excluder result = clone();
    result.requireExpose = true;
    return result;
  }

  public Excluder withExclusionStrategy(ExclusionStrategy exclusionStrategy,
      boolean serialization, boolean deserialization) {
    Excluder result = clone();
    // ……代码省略……
    return result;
  }

  // ……代码省略……
}

可见Gson的Excluder类不但采用了原型模式，而且很多方法中，还使用clone()方法生成新的实例，非常节省资源。

Protobuf-Java

Protobuf-Java的MessageLite.Builder类与原型模式有关的代码如下：

public interface MessageLite extends MessageLiteOrBuilder {

  // ……代码省略……
  
  interface Builder extends MessageLiteOrBuilder, Cloneable {

    // ……代码省略……
    
    /**
     * Clones the Builder.
     *
     * @see Object#clone()
     */
    Builder clone();

  }
}

Glide的TransitionOptions

Glide是Android专用的框架，其他方向的读者到此可以跳过。

Glide的TransitionOptions类是用于决定你的加载完成时会发生什么的过渡选项，比如View淡入等。TransitionOptions与原型模式相关的源码如下：

public abstract class TransitionOptions<
        CHILD extends TransitionOptions<CHILD, TranscodeType>, TranscodeType>
    implements Cloneable {
    
    // ……代码省略……

	@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);
    	}
  	}

}