Glide的StateVerifier类 —— 模板模式(Template Pattern) - Bert-King/glide GitHub Wiki

StateVerifier

模板模式(Template Pattern)讲解一

模板模式(Template Pattern)讲解二

模板模式(Template Pattern)讲解三

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模板模式(Template Pattern)

背景：程序员常常会遇到这种情况：设计一个系统时知道了算法所需的关键步骤，而且确定了这些步骤的执行顺序，但某些步骤的具体实现还未知，或者说某些步骤的实现与具体的环境相关。

现实场景：例如，去银行办理业务一般要经过以下4个流程：取号、排队、办理具体业务、对银行工作人员进行评分等，其中取号、排队和对银行工作人员进行评分的业务对每个客户是一样的，可以在父类中实现，但是办理具体业务却因人而异，它可能是存款、取款或者转账等，可以延迟到子类中实现。

/**
 * 抽象(模板)类 StateVerifier 定义了两个抽象方法
 * 1. throwIfRecycled()
 * 2. setRecycled()
 */
public abstract class StateVerifier {
  // 省略无关代码...

  /**
   * Throws an exception if we believe our object is recycled and inactive (i.e. is currently in an
   * object pool).
   */
  public abstract void throwIfRecycled();

  /** Sets whether or not our object is recycled. */
  abstract void setRecycled(boolean isRecycled);

  // 具体的实现类：DefaultStateVerifier
  private static class DefaultStateVerifier extends StateVerifier {
    private volatile boolean isReleased;

    @Synthetic
    DefaultStateVerifier() {}

    @Override
    public void throwIfRecycled() {
      if (isReleased) {
        throw new IllegalStateException("Already released");
      }
    }

    @Override
    public void setRecycled(boolean isRecycled) {
      this.isReleased = isRecycled;
    }
  }

  // 具体的实现类：DebugStateVerifier
  private static class DebugStateVerifier extends StateVerifier {
    // Keeps track of the stack trace where our state was set to recycled.
    private volatile RuntimeException recycledAtStackTraceException;

    @Synthetic
    DebugStateVerifier() {}

    @Override
    public void throwIfRecycled() {
      if (recycledAtStackTraceException != null) {
        throw new IllegalStateException("Already released", recycledAtStackTraceException);
      }
    }

    @Override
    void setRecycled(boolean isRecycled) {
      if (isRecycled) {
        recycledAtStackTraceException = new RuntimeException("Released");
      } else {
        recycledAtStackTraceException = null;
      }
    }
  }
}