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      1 /*
      2  * Copyright 2018 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 package androidx.fragment.app;
     18 
     19 import android.os.Parcelable;
     20 import android.util.Log;
     21 import android.view.View;
     22 import android.view.ViewGroup;
     23 
     24 import androidx.viewpager.widget.PagerAdapter;
     25 
     26 /**
     27  * Implementation of {@link PagerAdapter} that
     28  * represents each page as a {@link Fragment} that is persistently
     29  * kept in the fragment manager as long as the user can return to the page.
     30  *
     31  * <p>This version of the pager is best for use when there are a handful of
     32  * typically more static fragments to be paged through, such as a set of tabs.
     33  * The fragment of each page the user visits will be kept in memory, though its
     34  * view hierarchy may be destroyed when not visible.  This can result in using
     35  * a significant amount of memory since fragment instances can hold on to an
     36  * arbitrary amount of state.  For larger sets of pages, consider
     37  * {@link FragmentStatePagerAdapter}.
     38  *
     39  * <p>When using FragmentPagerAdapter the host ViewPager must have a
     40  * valid ID set.</p>
     41  *
     42  * <p>Subclasses only need to implement {@link #getItem(int)}
     43  * and {@link #getCount()} to have a working adapter.
     44  *
     45  * <p>Here is an example implementation of a pager containing fragments of
     46  * lists:
     47  *
     48  * {@sample frameworks/support/samples/Support4Demos/src/main/java/com/example/android/supportv4/app/FragmentPagerSupport.java
     49  *      complete}
     50  *
     51  * <p>The <code>R.layout.fragment_pager</code> resource of the top-level fragment is:
     52  *
     53  * {@sample frameworks/support/samples/Support4Demos/src/main/res/layout/fragment_pager.xml
     54  *      complete}
     55  *
     56  * <p>The <code>R.layout.fragment_pager_list</code> resource containing each
     57  * individual fragment's layout is:
     58  *
     59  * {@sample frameworks/support/samples/Support4Demos/src/main/res/layout/fragment_pager_list.xml
     60  *      complete}
     61  */
     62 public abstract class FragmentPagerAdapter extends PagerAdapter {
     63     private static final String TAG = "FragmentPagerAdapter";
     64     private static final boolean DEBUG = false;
     65 
     66     private final FragmentManager mFragmentManager;
     67     private FragmentTransaction mCurTransaction = null;
     68     private Fragment mCurrentPrimaryItem = null;
     69 
     70     public FragmentPagerAdapter(FragmentManager fm) {
     71         mFragmentManager = fm;
     72     }
     73 
     74     /**
     75      * Return the Fragment associated with a specified position.
     76      */
     77     public abstract Fragment getItem(int position);
     78 
     79     @Override
     80     public void startUpdate(ViewGroup container) {
     81         if (container.getId() == View.NO_ID) {
     82             throw new IllegalStateException("ViewPager with adapter " + this
     83                     + " requires a view id");
     84         }
     85     }
     86 
     87     @SuppressWarnings("ReferenceEquality")
     88     @Override
     89     public Object instantiateItem(ViewGroup container, int position) {
     90         if (mCurTransaction == null) {
     91             mCurTransaction = mFragmentManager.beginTransaction();
     92         }
     93 
     94         final long itemId = getItemId(position);
     95 
     96         // Do we already have this fragment?
     97         String name = makeFragmentName(container.getId(), itemId);
     98         Fragment fragment = mFragmentManager.findFragmentByTag(name);
     99         if (fragment != null) {
    100             if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment);
    101             mCurTransaction.attach(fragment);
    102         } else {
    103             fragment = getItem(position);
    104             if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
    105             mCurTransaction.add(container.getId(), fragment,
    106                     makeFragmentName(container.getId(), itemId));
    107         }
    108         if (fragment != mCurrentPrimaryItem) {
    109             fragment.setMenuVisibility(false);
    110             fragment.setUserVisibleHint(false);
    111         }
    112 
    113         return fragment;
    114     }
    115 
    116     @Override
    117     public void destroyItem(ViewGroup container, int position, Object object) {
    118         if (mCurTransaction == null) {
    119             mCurTransaction = mFragmentManager.beginTransaction();
    120         }
    121         if (DEBUG) Log.v(TAG, "Detaching item #" + getItemId(position) + ": f=" + object
    122                 + " v=" + ((Fragment)object).getView());
    123         mCurTransaction.detach((Fragment)object);
    124     }
    125 
    126     @SuppressWarnings("ReferenceEquality")
    127     @Override
    128     public void setPrimaryItem(ViewGroup container, int position, Object object) {
    129         Fragment fragment = (Fragment)object;
    130         if (fragment != mCurrentPrimaryItem) {
    131             if (mCurrentPrimaryItem != null) {
    132                 mCurrentPrimaryItem.setMenuVisibility(false);
    133                 mCurrentPrimaryItem.setUserVisibleHint(false);
    134             }
    135             if (fragment != null) {
    136                 fragment.setMenuVisibility(true);
    137                 fragment.setUserVisibleHint(true);
    138             }
    139             mCurrentPrimaryItem = fragment;
    140         }
    141     }
    142 
    143     @Override
    144     public void finishUpdate(ViewGroup container) {
    145         if (mCurTransaction != null) {
    146             mCurTransaction.commitNowAllowingStateLoss();
    147             mCurTransaction = null;
    148         }
    149     }
    150 
    151     @Override
    152     public boolean isViewFromObject(View view, Object object) {
    153         return ((Fragment)object).getView() == view;
    154     }
    155 
    156     @Override
    157     public Parcelable saveState() {
    158         return null;
    159     }
    160 
    161     @Override
    162     public void restoreState(Parcelable state, ClassLoader loader) {
    163     }
    164 
    165     /**
    166      * Return a unique identifier for the item at the given position.
    167      *
    168      * <p>The default implementation returns the given position.
    169      * Subclasses should override this method if the positions of items can change.</p>
    170      *
    171      * @param position Position within this adapter
    172      * @return Unique identifier for the item at position
    173      */
    174     public long getItemId(int position) {
    175         return position;
    176     }
    177 
    178     private static String makeFragmentName(int viewId, long id) {
    179         return "android:switcher:" + viewId + ":" + id;
    180     }
    181 }
    182