ViewPager在開發(fā)中的使用頻率非常的高，所以在此做個總結(jié)。
一、簡介
1.ViewPager的簡介和作用
ViewPager是android擴(kuò)展包v4包中的類，這個類可以讓用戶左右切換當(dāng)前的view1）ViewPager類直接繼承了ViewGroup類，所有它是一個容器類，可以在其中添加其他的view類。2）ViewPager類需要一個PagerAdapter適配器類給它提供數(shù)據(jù)。3）ViewPager經(jīng)常和Fragment一起使用，并且提供了專門的FragmentPagerAdapter和FragmentStatePagerAdapter類供Fragment中的ViewPager使用。
2.ViewPager的適配器
簡介中提到了PagerAdapter，和ListView等控件使用一樣,需要ViewPager設(shè)置PagerAdapter來完成頁面和數(shù)據(jù)的綁定，這個PagerAdapter是一個基類適配器，我們經(jīng)常用它來實(shí)現(xiàn)app引導(dǎo)圖，它的子類有FragmentPagerAdapter和FragmentStatePagerAdapter,這兩個子類適配器用于和Fragment一起使用，在安卓應(yīng)用中它們就像listview一樣出現(xiàn)的頻繁。
二、Adapter的工具類
這里只是做了最簡單的封裝，可以根據(jù)需要調(diào)整
PagerAdapter工具類
public class QuickPageAdapter<T extends View> extends PagerAdapter {
private List<T> mList;
public QuickPageAdapter(List<T> mList) {
this.mList = mList;
}
@Override
public int getCount() {
return mList.size();
}
@Override
public boolean isViewFromObject(View view, Object object) {
return object == view;
}
@Override
public Object instantiateItem(ViewGroup container, int position) {
container.addView(mList.get(position));
return mList.get(position);
}
@Override
public void destroyItem(ViewGroup container, int position, Object object) {
container.removeView(mList.get(position));
}
}
使用
List<View> views = new ArrayList<>();
mViewPager.setAdapter(new QuickPageAdapter<View>(views));
FragmentPagerAdapter工具類
public class QuickFragmentPageAdapter<T extends Fragment> extends FragmentPagerAdapter {
private List<T> mList;
private String[] mStrings;
/**
* @param fm
* @param list
* @param titles PageTitles
*/
public QuickFragmentPageAdapter(FragmentManager fm, List<T> list, String[] titles) {
super(fm);
mList = list;
mStrings = titles;
}
@Override
public Fragment getItem(int position) {
return mList.get(position);
}
@Override
public int getCount() {
return mList.size();
}
@Override
public CharSequence getPageTitle(int position) {
return mStrings == null ? super.getPageTitle(position) : mStrings[position];
}
}
FragmentStatePagerAdapter封裝類似FragmentPagerAdapter就不寫了，基本使用講完了。
三、源碼淺析
FragmentPagerAdapter和FragmentStatePagerAdapter的區(qū)別，你造嗎？
先上源碼
<ul>
<li> FragmentStatePagerAdapter</li>
</ul>
@Override
public Object instantiateItem(ViewGroup container, int position) {
// If we already have this item instantiated, there is nothing
// to do. This can happen when we are restoring the entire pager
// from its saved state, where the fragment manager has already
// taken care of restoring the fragments we previously had instantiated.
if (mFragments.size() > position) {
Fragment f = mFragments.get(position);//fragment被釋放后這里得到的null值
if (f != null) {
return f;
}
}
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
Fragment fragment = getItem(position);//fragment被釋放后或者是初次進(jìn)入頁面拿到新的Fragment實(shí)例
if (DEBUG) Log.v(TAG, "Adding item #" + position + ": f=" + fragment);
if (mSavedState.size() > position) {
Fragment.SavedState fss = mSavedState.get(position);
if (fss != null) {
fragment.setInitialSavedState(fss);
}
}
while (mFragments.size() <= position) {
mFragments.add(null);
}
fragment.setMenuVisibility(false);
fragment.setUserVisibleHint(false);
mFragments.set(position, fragment);
mCurTransaction.add(container.getId(), fragment);//新的Fragment實(shí)例 是add上去的
return fragment;
}
@Override
public void destroyItem(ViewGroup container, int position, Object object) {
Fragment fragment = (Fragment) object;
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
if (DEBUG) Log.v(TAG, "Removing item #" + position + ": f=" + object
+ " v=" + ((Fragment)object).getView());
while (mSavedState.size() <= position) {
mSavedState.add(null);
}
mSavedState.set(position, fragment.isAdded()
? mFragmentManager.saveFragmentInstanceState(fragment) : null);
mFragments.set(position, null);//真正釋放了fragment實(shí)例
mCurTransaction.remove(fragment);
}
<ul>
<li> FragmentPagerAdapter</li>
</ul>
@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);//因?yàn)閒ragment實(shí)例沒有被真正釋放，所以可以直接attach效率高
} else {
fragment = getItem(position);//初始化頁面的時候拿到fragment的實(shí)例
if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
mCurTransaction.add(container.getId(), fragment,
makeFragmentName(container.getId(), itemId));//add上去
}
if (fragment != mCurrentPrimaryItem) {
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());
mCurTransaction.detach((Fragment)object);//并沒有真正釋放fragment對象只是detach
}
小結(jié)
?????從源碼中我們可以看出FragmentStatePagerAdapter中fragment實(shí)例在destroyItem的時候被真正釋放，所以FragmentStatePagerAdapter省內(nèi)存。FragmentPagerAdapter中的fragment實(shí)例在destroyItem的時候并沒有真正釋放fragment對象只是detach，也就是說，F(xiàn)ragmentPagerAdapter銷毀的是Fragment的視圖，而FragmentStatePagerAdapter銷毀的是實(shí)例對象，所以FragmentPagerAdapter消耗更多的內(nèi)存，帶來的好處就是效率更高一些。所以得出這樣的結(jié)論：FragmentPagerAdapter適用于頁面比較少的情況，F(xiàn)ragmentStatePagerAdapter適用于頁面比較多的情況，因此不同的場合選擇合適的適配器才是正確的做法
