public static class Builder {
  private final AlertController.AlertParams P;

  public Builder(Context context) {
    this(context, resolveDialogTheme(context, 0));
  }


  public Builder(Context context, int themeResId) {
    P = new AlertController.AlertParams(new ContextThemeWrapper(
        context, resolveDialogTheme(context, themeResId)));
  }

  //各种set参数方法
  setTitle()
  ...
  ...
  ...

  /**
   * Creates an {@link AlertDialog} with the arguments supplied to this
   * builder.
   * <p>
   * Calling this method does not display the dialog. If no additional
   * processing is needed, {@link #show()} may be called instead to both
   * create and display the dialog.
   */
  public AlertDialog create() {
    // Context has already been wrapped with the appropriate theme.
    final AlertDialog dialog = new AlertDialog(P.mContext, 0, false);
    P.apply(dialog.mAlert);
    dialog.setCancelable(P.mCancelable);
    if (P.mCancelable) {
      dialog.setCanceledOnTouchOutside(true);
    }
    dialog.setOnCancelListener(P.mOnCancelListener);
    dialog.setOnDismissListener(P.mOnDismissListener);
    if (P.mOnKeyListener != null) {
      dialog.setOnKeyListener(P.mOnKeyListener);
    }
    return dialog;
  }


  // 这个show方法是builder对象的，里面封装了create（）和show（）可以直接调取创建并显示对话框
  public AlertDialog show() {
    final AlertDialog dialog = create();
    dialog.show();
    return dialog;
  }
}

final AlertDialog dialog = new AlertDialog(P.mContext, 0, false);
P.apply(dialog.mAlert); // mAlert是外部类中的

public class AlertDialog extends Dialog implements DialogInterface {
  // 这个对象用来承接builder内部所设置的参数
  private AlertController mAlert;

  //以下几个构造方法决定只能通过内部类builder来构建
  protected AlertDialog(Context context) {
    this(context, 0);
  }

  protected AlertDialog(Context context, boolean cancelable,   OnCancelListener cancelListener) {
    this(context, 0);
    setCancelable(cancelable);
    setOnCancelListener(cancelListener);
  }


  protected AlertDialog(Context context, @StyleRes int themeResId) {
    this(context, themeResId, true);
  }

  AlertDialog(Context context, @StyleRes int themeResId, boolean createContextThemeWrapper) {
    super(context, createContextThemeWrapper ? resolveDialogTheme(context, themeResId) : 0,
      createContextThemeWrapper);

    mWindow.alwaysReadCloseOnTouchAttr();
    mAlert = new AlertController(getContext(), this, getWindow());
    }
}