跟蹤fragment來回切換過程

從fragment-A切換到fragment-B，再從fragment-B切換到fragment-A中。跟蹤發現，切換回到fragment-A后，fragment-A的實例變量的值不會被重置，保持和上次相同，但是fragment-A的life cycle方法被重新調用。

第1次創建fragment時，生命周期方法調用順序是：

11-06 13:56:26.519: D/OrderVideoFragment(2504): ++onCreate++
11-06 13:56:26.529: D/OrderVideoFragment(2504): ++onCreateView++
11-06 13:56:26.549: D/OrderVideoFragment(2504): ++onActivityCreated++
11-06 13:56:26.549: D/OrderVideoFragment(2504): ++onStart++
11-06 13:56:26.549: D/OrderVideoFragment(2504): ++onResume++

切換到其他fragment時，查看日志

11-06 13:57:44.379: D/OrderVideoFragment(2504): ++onPause++
11-06 13:57:44.379: D/OrderVideoFragment(2504): ++onStop++

再從其他fragment切換回來，查看日志

11-06 13:58:36.429: D/OrderVideoFragment(2504): ++onCreateView++
11-06 13:58:36.469: D/OrderVideoFragment(2504): ++onActivityCreated++
11-06 13:58:36.469: D/OrderVideoFragment(2504): ++onStart++
11-06 13:58:36.469: D/OrderVideoFragment(2504): ++onResume++

從日志中可以發現，在切換到其他fragment時，并沒有調用onDestroy方法，再從其他fragment切換回來時，也沒有再次調用onCreate方法。由此可以說明，在fragment來回切換過程中，fragment并沒有銷毀，所以也沒有重新創建，只是按照fragment的設計約定執行了一些生命周期方法。

既然這樣，那么在切換回到OrderVideoFragment時，實例變量保存著原來的值就不難理解了，我們在切換回來之后，就可以利用保存在變量中的值來恢復fragment原來的界面。例如在OrderVideoFragment中，第1次創建時要從網絡加載數據，那在后來切換回來就可以利用上次從網絡加載的數據。

以下說明一下OrderVideoFragment類中的重要代碼片段和重要方法

1. 重寫onCreateView方法，創建fragment界面（界面其實表現為View組件）。

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
                                                                                                         
    Logger.i(TAG, "++onCreateView++");
    this.inflater = inflater;
    this.context = getActivity();
    setTagId();
    setTitleName();
    setBRTL();
    View view = inflater.inflate(R.layout.weiyu_fragment, null);
    // 文字標題
    TextView titleView = (TextView) view.findViewById(R.id.txtTitleName);
    if (!TextUtils.isEmpty(titleName)) {
        titleView.setVisibility(View.VISIBLE);
        titleView.setText(titleName);
    } else {
        titleView.setVisibility(View.INVISIBLE);
    }
    // 圖片標題
    ImageView titleImageView = ((ImageView) view.findViewById(R.id.titleImage));
    if (titleImageResId != 0) {
        titleImageView.setVisibility(View.VISIBLE);
        titleImageView.setImageResource(titleImageResId);
    } else {
        titleImageView.setVisibility(View.INVISIBLE);
    }
    // 菜單按鈕
    view.findViewById(R.id.btnMenu).setOnClickListener(this);
    // 搜索按鈕
    view.findViewById(R.id.btnSearch).setOnClickListener(this);
    pullToRefreshView = (PullToRefreshView) view.findViewById(R.id.pullView);
    pullToRefreshView.setFootLocked(true);
    pullToRefreshView.setOnHeaderRefreshListener(this);
                                                                                                         
    mLayout = ((LinearLayout) view.findViewById(R.id.videoGroupsLayout));
    return view;
}

2. 重新onStart方法。在onStart方法中加載數據。

@Override
public void onStart() {
    super.onStart();
    loadData();
}

3. 實現加載數據方法。

private void loadData() {
    if(mResult == null)
        loadDataFromNetWork();
    else
        loadDataFromCache();
}

4. 加載數據要分兩種情況：

（1）從實例變量中加載數據。

private void loadDataFromCache() {
    new AsyncTask<Void, Void, Void>() {
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            hideRefreshView();
            hideWaitView();
            mViewList.clear();
        }
        @Override
        protected Void doInVoid...  params) {
            for (VideoGroupData dataItem : mData) {
                mViewList.add(buildVideoGroupView(dataItem));
            }
            return null;
        }
        @Override
        protected void onPostExecute(Void result) {
            super.onPostExecute(result);
            mLayout.removeAllViews();
            for (View view : mViewList) {
                mLayout.addView(view);
            }
        }
    }.execute();
}

（2）從網絡加載數據。

private void loadDataFromNetWork() {
    IVideoFetcher.CallBack callBack = new IVideoFetcher.CallBackAdapter() {
        @Override
        public void onComplete(IVideoResult result) {
            hideWaitView();
            mResult = result;
            // 加載數據成功
            if (result.isSuccess()) {
                mData = ((WeiyuVideoResult) result).getData();
                mViewList.clear();
                for (VideoGroupData dataItem : mData) {
                    mViewList.add(buildVideoGroupView(dataItem));
                }
                mLayout.removeAllViews();
                for (View view : mViewList) {
                    mLayout.addView(view);
                }
            }
            // 加載數據失敗
            else {
                if (!isPullDownRefresh) {
                    showRefreshView();
                }
                UIUtils.showToast(context, getString(R.string.load_fail_tray_again_later));
            }
            if (isPullDownRefresh) {
                pullToRefreshView.onHeaderRefreshComplete();
                isPullDownRefresh = false;
            }
        }
        @Override
        public void onBegin() {
            hideRefreshView();
            if (!isPullDownRefresh)
                showWaitView();
        }
    };
    HashMap<String, String> params = new HashMap<String, String>();
    params.put("q", null);
    IVideoFetcher.doGet(WeiyuVideoResult.class, callBack, IVideoURL.getCloudVideoURL(), params);
}

5. 根據加載到的數據構建View界面。

private View buildVideoGroupView(VideoGroupData videoGroupData) {
    int resid = bRTL ? R.layout.video_group2_rtl : R.layout.video_group2;
    View view = (RelativeLayout) inflater.inflate(resid, null);
      
    TextView titleView = ((TextView) view.findViewById(R.id.groupTitleName));
    titleView.setText(videoGroupData.getTitle());
    View titleLayout = view.findViewById(R.id.groupTitleLayout);
    titleLayout.setOnClickListener(this);
    titleLayout.setTag(cidTagId, videoGroupData.getId());
    titleLayout.setTag(cnameTagId, videoGroupData.getTitle());
    List<VideoItemData> items = videoGroupData.getData();
    int n = items.size() / mColumns * mColumns; // 取偶數個item
    items = CommonUtils.getFirstNItems(items, n);
      
    VideoGridAdapter videoAdapter = new VideoGridAdapter(context, items, false);
    videoAdapter.setBRTL(bRTL);
    FixedGridLayout fixGridLayout = (FixedGridLayout) view.findViewById(R.id.fixGridLayout);
    int cellWidth = UIUtils.getScreenWidthPixels(context) / mColumns;
    fixGridLayout.setRowParams(cellWidth, mColumns);
    fixGridLayout.setAdapter(videoAdapter);
    return view;
}