View的可见性与图片加载优化
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.
*
*
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.
*
*
Note: This method may be called outside of the fragment lifecycle.
* and thus has no ordering guarantees with regard to fragment lifecycle method calls.
*
* @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 rely 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 rely 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
Android: how to check if a View inside of ScrollView is visible?
作者：碎语说
链接：http://www.jianshu.com/p/a9da7297e9d3
來源：简书
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