在項目中ViewPager和Fragment接口框架已經是處處可見,但是在使用中,我們肯定不希望用戶在當前頁面時就在前後頁面的數據,加入數據量很大,而用戶又不願意左右滑動瀏覽,那麼這時候ViewPager中本來充滿善意的預加載就有點令人不爽了。我們能做的就是屏蔽掉ViewPager的預加載機制。雖然ViewPager中提供的有
setOffscreenPageLimit()來控制其預加載的數目,但是當設置爲0後我們發現其根本沒效果,這個的最小值就是1,也就是你只能最少前後各預加載一頁。那麼,這時候就得另覓方法了。
以下三種方法是我在學習和項目中嘗試過的,需求實現了,但各有千秋,可結合不同場景使用。因爲打算慢慢養成寫博客的習慣,就總結在此,也希望對他人有所借鑑。
方法一
在Fragment可見時請求數據。此方案仍預加載了前後的頁面,但是沒有請求數據,只有進入到當前Framgent時才請求數據。
優點:實現了數據的懶加載
缺點:一次仍是三個Framgment對象,不是完全意義的懶加載
public class FragmentSample extends Fragment{
...
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if (isVisibleToUser) {
requestData(); // 在此請求數據
}
}
...
}
方法二
直接修改ViewPager源碼。通過查看ViewPager源碼可知,控制其預加載的是一個常量
DEFAULT_OFFSCREEN_PAGES,其默認值爲1,表示當前頁面前後各預加載一個頁面,在這裏我們直接將其設置爲0即可,即去掉預加載。但是,這樣有一個問題,那就是在使用其他控件時需要傳入ViewPager時,這個就不能用了。
優點:完全屏蔽掉了預加載
缺點:應用太受限制,比如使用ViewPagerIndicator時需要傳入ViewPager對象,這時傻眼了。
// 注意,這是直接拷貝的ViewPager的源碼,只修改了註釋處的代碼
public class LazyViewPager extends ViewGroup {
private static final String TAG = "LazyViewPager";
private static final boolean DEBUG = false;
private static final boolean USE_CACHE = false;
// 默認爲1,即前後各預加載一個頁面,設置爲0去掉預加載
private static final int DEFAULT_OFFSCREEN_PAGES = 0;
private static final int MAX_SETTLE_DURATION = 600; // ms
static class ItemInfo {
Object object;
int position;
boolean scrolling;
}
private static final Comparator<ItemInfo> COMPARATOR = new Comparator<ItemInfo>() {
@Override
public int compare(ItemInfo lhs, ItemInfo rhs) {
return lhs.position - rhs.position;
}
};
............
}
方法三
直接繼承ViewPager,結合PagerAdapter實現懶加載。該方案是我用到的最完善的方法,完全的懶加載,每次只會建立一個Fragment對象。
優點:完全屏蔽預加載
缺點:稍微複雜,但是人家已經造好的輪子,直接用吧,很簡潔
開源庫:https://github.com/lianghanzhen/LazyViewPager
這個庫就4個類,作者通過繼承ViewPager(保證其普適性)、自定義ViewPagerAdapter和 LazyFragmentPagerAdapter以及設置懶加載的標記接口,很好的實現了懶加載。感謝作者。
在此貼出關鍵代碼,有興趣的同學可以學習下。
LazyViewPager:
public class LazyViewPager extends ViewPager {
private static final float DEFAULT_OFFSET = 0.5f;
private LazyPagerAdapter mLazyPagerAdapter;
private float mInitLazyItemOffset = DEFAULT_OFFSET;
public LazyViewPager(Context context) {
super(context);
}
public LazyViewPager(Context context, AttributeSet attrs) {
super(context, attrs);
TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.LazyViewPager);
setInitLazyItemOffset(a.getFloat(R.styleable.LazyViewPager_init_lazy_item_offset, DEFAULT_OFFSET));
a.recycle();
}
/**
* change the initLazyItemOffset
* @param initLazyItemOffset set mInitLazyItemOffset if {@code 0 < initLazyItemOffset <= 1}
*/
public void setInitLazyItemOffset(float initLazyItemOffset) {
if (initLazyItemOffset > 0 && initLazyItemOffset <= 1) {
mInitLazyItemOffset = initLazyItemOffset;
}
}
@Override
public void setAdapter(PagerAdapter adapter) {
super.setAdapter(adapter);
mLazyPagerAdapter = adapter != null && adapter instanceof LazyPagerAdapter ? (LazyPagerAdapter) adapter : null;
}
@Override
protected void onPageScrolled(int position, float offset, int offsetPixels) {
if (mLazyPagerAdapter != null) {
if (getCurrentItem() == position) {
int lazyPosition = position + 1;
if (offset >= mInitLazyItemOffset && mLazyPagerAdapter.isLazyItem(lazyPosition)) {
mLazyPagerAdapter.startUpdate(this);
mLazyPagerAdapter.addLazyItem(this, lazyPosition);
mLazyPagerAdapter.finishUpdate(this);
}
} else if (getCurrentItem() > position) {
int lazyPosition = position;
if (1 - offset >= mInitLazyItemOffset && mLazyPagerAdapter.isLazyItem(lazyPosition)) {
mLazyPagerAdapter.startUpdate(this);
mLazyPagerAdapter.addLazyItem(this, lazyPosition);
mLazyPagerAdapter.finishUpdate(this);
}
}
}
super.onPageScrolled(position, offset, offsetPixels);
}
}
public abstract class LazyFragmentPagerAdapter extends LazyPagerAdapter<Fragment> {
private static final String TAG = "LazyFragmentPagerAdapter";
private static final boolean DEBUG = false;
private final FragmentManager mFragmentManager;
private FragmentTransaction mCurTransaction = null;
public LazyFragmentPagerAdapter(FragmentManager fm) {
mFragmentManager = fm;
}
@Override
public void startUpdate(ViewGroup container) {
}
@Override
public Object instantiateItem(ViewGroup container, int position) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
final long itemId = getItemId(position);
// Do we already have this fragment?
String name = makeFragmentName(container.getId(), itemId);
Fragment fragment = mFragmentManager.findFragmentByTag(name);
if (fragment != null) {
if (DEBUG)
Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment);
mCurTransaction.attach(fragment);
} else {
fragment = getItem(container, position);
if (fragment instanceof Laziable) {
mLazyItems.put(position, fragment);
} else {
mCurTransaction.add(container.getId(), fragment, name);
}
}
if (fragment != getCurrentItem()) {
fragment.setMenuVisibility(false);
fragment.setUserVisibleHint(false);
}
return fragment;
}
@Override
public void destroyItem(ViewGroup container, int position, Object object) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
if (DEBUG)
Log.v(TAG, "Detaching item #" + getItemId(position) + ": f=" + object + " v=" + ((Fragment) object).getView());
final long itemId = getItemId(position);
String name = makeFragmentName(container.getId(), itemId);
if (mFragmentManager.findFragmentByTag(name) == null) {
mCurTransaction.detach((Fragment) object);
} else {
mLazyItems.remove(position);
}
}
@Override
public Fragment addLazyItem(ViewGroup container, int position) {
Fragment fragment = mLazyItems.get(position);
if (fragment == null)
return null;
final long itemId = getItemId(position);
String name = makeFragmentName(container.getId(), itemId);
if (mFragmentManager.findFragmentByTag(name) == null) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
mCurTransaction.add(container.getId(), fragment, name);
mLazyItems.remove(position);
}
return fragment;
}
@Override
public void finishUpdate(ViewGroup container) {
if (mCurTransaction != null) {
mCurTransaction.commitAllowingStateLoss();
mCurTransaction = null;
mFragmentManager.executePendingTransactions();
}
}
@Override
public boolean isViewFromObject(View view, Object object) {
return ((Fragment) object).getView() == view;
}
public long getItemId(int position) {
return position;
}
private static String makeFragmentName(int viewId, long id) {
return "android:switcher:" + viewId + ":" + id;
}
/**
* mark the fragment can be added lazily
*/
public interface Laziable {
}
}
以上。