备忘录模式实现与研究 - bei1999/work GitHub Wiki

介绍

保存对象状态，之后可以再次恢复到此状态，类似于日常说的“后悔药”，为了保护状态完整性以及内部实现不向外暴露，也就是说不能被对象从外部访问。

定义

在不破坏封闭的前提下，将该对象之外保存这个状态，需要的时候可以将保存的对象恢复到原来的保存状态

场景模拟

  • android 中Activity 中的状态保存

实现

public class MainActivity09 extends BaseActivity {
    @Override
    public int getLayoutId() {
        return R.layout.activity_main;
    }

    @Override
    public void initPresenter() {
        //TODO
    }

    @Override
    public void initView() {
        //TODO
    }


    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
    }

    @Override
    protected void onRestoreInstanceState(Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
    }
}

// AppCompatActivity 中的一段代码
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        getDelegate().onSaveInstanceState(outState);
    }
  /**
     * Save all appropriate fragment state.
     */
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        Parcelable p = mFragments.saveAllState();
        if (p != null) {
            outState.putParcelable(FRAGMENTS_TAG, p);
        }
        if (mPendingFragmentActivityResults.size() > 0) {
            outState.putInt(NEXT_CANDIDATE_REQUEST_INDEX_TAG, mNextCandidateRequestIndex);

            int[] requestCodes = new int[mPendingFragmentActivityResults.size()];
            String[] fragmentWhos = new String[mPendingFragmentActivityResults.size()];
            for (int i = 0; i < mPendingFragmentActivityResults.size(); i++) {
                requestCodes[i] = mPendingFragmentActivityResults.keyAt(i);
                fragmentWhos[i] = mPendingFragmentActivityResults.valueAt(i);
            }
            outState.putIntArray(ALLOCATED_REQUEST_INDICIES_TAG, requestCodes);
            outState.putStringArray(REQUEST_FRAGMENT_WHO_TAG, fragmentWhos);
        }
    }
//跳转到Activity 中的super调用
    protected void onSaveInstanceState(Bundle outState) {
        outState.putBundle(WINDOW_HIERARCHY_TAG, mWindow.saveHierarchyState());
        Parcelable p = mFragments.saveAllState();
        if (p != null) {
            outState.putParcelable(FRAGMENTS_TAG, p);
        }
        getApplication().dispatchActivitySaveInstanceState(this, outState);
    }
    /**
     * This method is called after {@link #onStart} when the activity is
     * being re-initialized from a previously saved state, given here in
     * <var>savedInstanceState</var>.  Most implementations will simply use {@link #onCreate}
     * to restore their state, but it is sometimes convenient to do it here
     * after all of the initialization has been done or to allow subclasses to
     * decide whether to use your default implementation.  The default
     * implementation of this method performs a restore of any view state that
     * had previously been frozen by {@link #onSaveInstanceState}.
     *
     * <p>This method is called between {@link #onStart} and
     * {@link #onPostCreate}.
     *
     * @param savedInstanceState the data most recently supplied in {@link #onSaveInstanceState}.
     *
     * @see #onCreate
     * @see #onPostCreate
     * @see #onResume
     * @see #onSaveInstanceState
     */
    protected void onRestoreInstanceState(Bundle savedInstanceState) {
        if (mWindow != null) {
            Bundle windowState = savedInstanceState.getBundle(WINDOW_HIERARCHY_TAG);
            if (windowState != null) {
                mWindow.restoreHierarchyState(windowState);
            }
        }
    }

分析

onSaveInstanceState方法中，分为三个步骤

  1. 存储窗口的视图数的状态
  2. 存储了fragment 的状态
  3. 通过Activity 的ActivityLifeCallbacks dispatchActivitySaveInstanceState 方式进行了存储状态

onRestoreInstanceState方法则通过把存储的Bundle 交给系统的window 进行了恢复restoreHierarchyState

扩展

onSaveInstanceState 调用的时机 并不是每次activity 退出之前都会调用，当某个activity变得容易被系统销毁时，该activity 的onSaveInstanceState就会执行，常见如下：

  1. 用户按home键
  2. 长按home键，切换其他app
  3. 按电源（关闭屏幕）
  4. 屏幕方向切换
  5. 呼入电话时候 概括是，不是用户主动退出或者跳转七天activity就会触发，即当系统存在“未经你许可”时销毁了我们的activity，则onSaveInstanceState会执行，系统这样做的目的是提供保存用户数据的时机。

总结

备忘录模式是在不破坏封装条件下的对内部状态的存储和还原 优点：提供了一种可恢复的时机 缺点：如果需要的数据大，保存和恢复有一定的内存消耗

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