引子

最近在做应用内存优化，首要考虑优化的就是首页推荐位的图片。

首页.png

上图Tab页是通过ViewPager+Fragment实现的，推荐页使用了Scrollview可以往下滚动

首页_滚动.png

因此推荐页有大量的图片，占用了大量的内存，需要做如下优化

优化项

1.进入推荐页时，只加载Scrollview中可见的图片
2.离开推荐页时，释放Scrollview中不可见的图片
3.点击图片进入详情页后，释放全部图片
4.Scrollview滚动时只加载可见的图片

技术点

1.如何判断Fragment是否对用户可见

最容易想到的就是通过生命周期函数onPause()和onResume()来判断，但是在此viewpager中切换fragment，却未触发这两个生命周期函数。因为viewpager会默认加载当前页的前后页，而且还可通过setOffscreenPageLimit来设置默认加载多页。那有什么办法知道Fragment是否对用户可见呢？有的，具体分几种情况

• ViewPager+Fragment
  因为Fragment懒加载机制的原因，切换时Fragment并未销毁，不会触发onPause()，因此需要用setUserVisibleHint()来判断

 /**
     * Set a hint to the system about whether this fragment's UI is currently visible
     * to the user. This hint defaults to true and is persistent across fragment instance
     * state save and restore.
     *
     * <p>An app may set this to false to indicate that the fragment's UI is
     * scrolled out of visibility or is otherwise not directly visible to the user.
     * This may be used by the system to prioritize operations such as fragment lifecycle updates
     * or loader ordering behavior.</p>
     *
     * <p><strong>Note:</strong> This method may be called outside of the fragment lifecycle.
     * and thus has no ordering guarantees with regard to fragment lifecycle method calls.</p>
     *
     * @param isVisibleToUser true if this fragment's UI is currently visible to the user (default),
     *                        false if it is not.
     */
    public void setUserVisibleHint(boolean isVisibleToUser) {
        if (!mUserVisibleHint && isVisibleToUser && mState < STARTED
                && mFragmentManager != null && isAdded()) {
            mFragmentManager.performPendingDeferredStart(this);
        }
        mUserVisibleHint = isVisibleToUser;
        mDeferStart = mState < STARTED && !isVisibleToUser;
    }

ViewPager初始化时不会触发setUserVisibleHint()，只会在切换Fragment时

• Activity+Fragment
  xml引入fragment，或者通过addFragment、replaceFragment，直接判断onPause()和onResume()即可
• FragmentManger show() hide()
  fragment没有被销毁，虽然不可见，但是不会触发onPause，需要用onHiddenChanged()来判断

@Override
public void onHiddenChanged(boolean hidden) {
    super.onHiddenChanged(hidden);
    if(hidden){
        //TODO now visible to user
    } else {
        //TODO now invisible to user
    }
}

2.如何判断View是否可见

判断View是否在屏幕中可见，可使用如下函数

/**
     * 判断视图是否显示在屏幕上
     * @param context
     * @param view
     * @return
     */
    public static boolean checkIsVisible(Context context, View view) {
        int screenWidth = getScreenMetrics(context).x;
        int screenHeight = getScreenMetrics(context).y;
        Rect rect = new Rect(0, 0, screenWidth, screenHeight);
        int[] location = new int[2];
        view.getLocationInWindow(location);
        if (view.getLocalVisibleRect(rect)) {
            return true;
        } else {
            //view已不在屏幕可见区域;
            return false;
        }
    }

3.如何判断View是否在ScrollView中可见

判断View是否在ScrollView中可见，可使用如下函数

private boolean isVisibleInScrollView(ScrollView scroll, View view) {
        Rect bounds = new Rect();
        view.getHitRect(bounds);
        Rect scrollBounds = new Rect(scroll.getScrollX(), scroll.getScrollY(),
                scroll.getScrollX() + scroll.getWidth(), scroll.getScrollY() + scroll.getHeight());
        if (Rect.intersects(scrollBounds, bounds)) {
            return true;
        } else {
            return false;
        }
    }

4.如何控制图片加载

控制图片加载我们不能直接使用View.setVisibility(View.VISIBLE|VIew.GONE)，否则会视图需要重新渲染，引起操作时的卡顿。
我们可以通过发送dispatchDisplayHint(int hint)，然后在View中处理onDisplayHint(int hint)来控制图片加载

/**
     * Dispatch a hint about whether this view is displayed. For instance, when
     * a View moves out of the screen, it might receives a display hint indicating
     * the view is not displayed. Applications should not <em>rely</em> on this hint
     * as there is no guarantee that they will receive one.
     *
     * @param hint A hint about whether or not this view is displayed:
     * {@link #VISIBLE} or {@link #INVISIBLE}.
     */
    public void dispatchDisplayHint(int hint) {
        onDisplayHint(hint);
    }
    /**
     * Gives this view a hint about whether is displayed or not. For instance, when
     * a View moves out of the screen, it might receives a display hint indicating
     * the view is not displayed. Applications should not <em>rely</em> on this hint
     * as there is no guarantee that they will receive one.
     *
     * @param hint A hint about whether or not this view is displayed:
     * {@link #VISIBLE} or {@link #INVISIBLE}.
     */
    protected void onDisplayHint(int hint) {
    }

具体优化工作

1.进入推荐页时，只加载Scrollview中可见的图片

   @Override
    public void onResume() {
        super.onResume();
        Logger.getLogger().d("onResume");
        if (getUserVisibleHint()) {
            setScrollVisibleView();
        }
    }
    private void setScrollVisibleView() {
        if (mRecommendContainerLayout == null) {
            return;
        }
        for (int i = 0; i < mRecommendContainerLayout.getChildCount(); i++) {
            final RecommendContainer recommendContainer = (RecommendContainer) mRecommendContainerLayout.getChildAt(i);
            if (isVisibleInScrollView(mScrollView, recommendContainer)) {
                Logger.getLogger().d("setScrollVisibleView v=%s,vi=%s", recommendContainer, View.VISIBLE);
                recommendContainer.dispatchDisplayHint(View.VISIBLE);
            } else {
                //Logger.getLogger().d("setScrollVisibleView v=%s,vi=%s", recommendContainer, View.GONE);
                //recommendContainer.dispatchDisplayHint(View.INVISIBLE);
            }
        }
    }

2.离开推荐页时，释放Scrollview中不可见的图片

    @Override
    public void setUserVisibleHint(final boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        Logger.getLogger().d("setUserVisibleHint, isVisibleToUser=%s", isVisibleToUser);
        if (mRecommendContainerLayout == null) {
            return;
        }
        for (int i = 0; i < mRecommendContainerLayout.getChildCount(); i++) {
            final RecommendContainer recommendContainer = (RecommendContainer) mRecommendContainerLayout.getChildAt(i);
            final boolean isVisibleInScrollView = isVisibleInScrollView(mScrollView, recommendContainer);
            Logger.getLogger().d("setUserVisibleHint v=%s,isVisibleToUser=%s,isVisibleInScrollView=%s," +
                            "checkIsVisible=%s", recommendContainer.getId(), isVisibleToUser, isVisibleInScrollView,
                    ViewUtils.checkIsVisible(getContext(), recommendContainer));
            recommendContainer.dispatchDisplayHint( isVisibleInScrollView ? View.VISIBLE :
                    View.GONE);
        }
    }

3.点击图片进入详情页后，释放全部图片

    @Override
    public void onPause() {
        super.onPause();
        Logger.getLogger().d("onPause");
        hideAllViews();
    }
    private void hideAllViews() {
        if (mRecommendContainerLayout == null) {
            return;
        }
        for (int i = 0; i < mRecommendContainerLayout.getChildCount(); i++) {
            final RecommendContainer recommendContainer = (RecommendContainer) mRecommendContainerLayout.getChildAt(i);
            Logger.getLogger().d("hideAllViews v=%s,vi=%s", recommendContainer, View.GONE);
            mRecommendContainerLayout.postDelayed(new Runnable() {
                @Override
                public void run() {
                    recommendContainer.dispatchDisplayHint(View.INVISIBLE);
                }
            }, 300);
        }
    }

4.Scrollview滚动时只加载可见的图片

    @Override
    public void onScrollChanged(ScrollView scrollView, int x, int y, int oldx, int oldy) {
        if (Math.abs(oldy - y) > 50) {
            //Logger.getLogger().d("recommendFragment setScrollVisibleView");
            setScrollVisibleView();
        }
    }

参考

关于fragment到底是否可见的问题
如何判断Fragment是否对用户可见
Android: how to check if a View inside of ScrollView is visible?