/*
 * Copyright 2018 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package androidx.fragment.app;

import android.os.Parcelable;
import android.util.Log;
import android.view.View;
import android.view.ViewGroup;

import androidx.annotation.IntDef;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.lifecycle.Lifecycle;
import androidx.viewpager.widget.PagerAdapter;

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

/**
 * Implementation of {@link PagerAdapter} that
 * represents each page as a {@link Fragment} that is persistently
 * kept in the fragment manager as long as the user can return to the page.
 *
 * <p>This version of the pager is best for use when there are a handful of
 * typically more static fragments to be paged through, such as a set of tabs.
 * The fragment of each page the user visits will be kept in memory, though its
 * view hierarchy may be destroyed when not visible.  This can result in using
 * a significant amount of memory since fragment instances can hold on to an
 * arbitrary amount of state.  For larger sets of pages, consider
 * {@link FragmentStatePagerAdapter}.
 *
 * <p>When using FragmentPagerAdapter the host ViewPager must have a
 * valid ID set.</p>
 *
 * <p>Subclasses only need to implement {@link #getItem(int)}
 * and {@link #getCount()} to have a working adapter.
 *
 * <p>Here is an example implementation of a pager containing fragments of
 * lists:
 *
 * {@sample frameworks/support/samples/Support4Demos/src/main/java/com/example/android/supportv4/app/FragmentPagerSupport.java
 *      complete}
 *
 * <p>The <code>R.layout.fragment_pager</code> resource of the top-level fragment is:
 *
 * {@sample frameworks/support/samples/Support4Demos/src/main/res/layout/fragment_pager.xml
 *      complete}
 *
 * <p>The <code>R.layout.fragment_pager_list</code> resource containing each
 * individual fragment's layout is:
 *
 * {@sample frameworks/support/samples/Support4Demos/src/main/res/layout/fragment_pager_list.xml
 *      complete}
 */
@SuppressWarnings("deprecation")
public abstract class FragmentPagerAdapter extends PagerAdapter {
    private static final String TAG = "FragmentPagerAdapter";
    private static final boolean DEBUG = false;

    @Retention(RetentionPolicy.SOURCE)
    @IntDef({BEHAVIOR_SET_USER_VISIBLE_HINT, BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT})
    private @interface Behavior { }

    /**
     * Indicates that {@link Fragment#setUserVisibleHint(boolean)} will be called when the current
     * fragment changes.
     *
     * @deprecated This behavior relies on the deprecated
     * {@link Fragment#setUserVisibleHint(boolean)} API. Use
     * {@link #BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT} to switch to its replacement,
     * {@link FragmentTransaction#setMaxLifecycle}.
     * @see #FragmentPagerAdapter(FragmentManager, int)
     */
    @Deprecated
    public static final int BEHAVIOR_SET_USER_VISIBLE_HINT = 0;

    /**
     * Indicates that only the current fragment will be in the {@link Lifecycle.State#RESUMED}
     * state. All other Fragments are capped at {@link Lifecycle.State#STARTED}.
     *
     * @see #FragmentPagerAdapter(FragmentManager, int)
     */
    public static final int BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT = 1;

    private final FragmentManager mFragmentManager;
    private final int mBehavior;
    private FragmentTransaction mCurTransaction = null;
    private Fragment mCurrentPrimaryItem = null;

    /**
     * Constructor for {@link FragmentPagerAdapter} that sets the fragment manager for the adapter.
     * This is the equivalent of calling {@link #FragmentPagerAdapter(FragmentManager, int)} and
     * passing in {@link #BEHAVIOR_SET_USER_VISIBLE_HINT}.
     *
     * <p>Fragments will have {@link Fragment#setUserVisibleHint(boolean)} called whenever the
     * current Fragment changes.</p>
     *
     * @param fm fragment manager that will interact with this adapter
     * @deprecated use {@link #FragmentPagerAdapter(FragmentManager, int)} with
     * {@link #BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT}
     */
    @Deprecated
    public FragmentPagerAdapter(@NonNull FragmentManager fm) {
        this(fm, BEHAVIOR_SET_USER_VISIBLE_HINT);
    }

    /**
     * Constructor for {@link FragmentPagerAdapter}.
     *
     * If {@link #BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT} is passed in, then only the current
     * Fragment is in the {@link Lifecycle.State#RESUMED} state. All other fragments are capped at
     * {@link Lifecycle.State#STARTED}. If {@link #BEHAVIOR_SET_USER_VISIBLE_HINT} is passed, all
     * fragments are in the {@link Lifecycle.State#RESUMED} state and there will be callbacks to
     * {@link Fragment#setUserVisibleHint(boolean)}.
     *
     * @param fm fragment manager that will interact with this adapter
     * @param behavior determines if only current fragments are in a resumed state
     */
    public FragmentPagerAdapter(@NonNull FragmentManager fm,
            @Behavior int behavior) {
        mFragmentManager = fm;
        mBehavior = behavior;
    }

    /**
     * Return the Fragment associated with a specified position.
     */
    @NonNull
    public abstract Fragment getItem(int position);

    @Override
    public void startUpdate(@NonNull ViewGroup container) {
        if (container.getId() == View.NO_ID) {
            throw new IllegalStateException("ViewPager with adapter " + this
                    + " requires a view id");
        }
    }

    @SuppressWarnings({"ReferenceEquality", "deprecation"})
    @NonNull
    @Override
    public Object instantiateItem(@NonNull 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(position);
            if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
            mCurTransaction.add(container.getId(), fragment,
                    makeFragmentName(container.getId(), itemId));
        }
        if (fragment != mCurrentPrimaryItem) {
            fragment.setMenuVisibility(false);
            if (mBehavior == BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
                mCurTransaction.setMaxLifecycle(fragment, Lifecycle.State.STARTED);
            } else {
                fragment.setUserVisibleHint(false);
            }
        }

        return fragment;
    }

    @Override
    public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        Fragment fragment = (Fragment) object;

        if (mCurTransaction == null) {
            mCurTransaction = mFragmentManager.beginTransaction();
        }
        if (DEBUG) Log.v(TAG, "Detaching item #" + getItemId(position) + ": f=" + object
                + " v=" + (fragment.getView()));
        mCurTransaction.detach(fragment);
        if (fragment == mCurrentPrimaryItem) {
            mCurrentPrimaryItem = null;
        }
    }

    @SuppressWarnings({"ReferenceEquality", "deprecation"})
    @Override
    public void setPrimaryItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        Fragment fragment = (Fragment)object;
        if (fragment != mCurrentPrimaryItem) {
            if (mCurrentPrimaryItem != null) {
                mCurrentPrimaryItem.setMenuVisibility(false);
                if (mBehavior == BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
                    if (mCurTransaction == null) {
                        mCurTransaction = mFragmentManager.beginTransaction();
                    }
                    mCurTransaction.setMaxLifecycle(mCurrentPrimaryItem, Lifecycle.State.STARTED);
                } else {
                    mCurrentPrimaryItem.setUserVisibleHint(false);
                }
            }
            fragment.setMenuVisibility(true);
            if (mBehavior == BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
                if (mCurTransaction == null) {
                    mCurTransaction = mFragmentManager.beginTransaction();
                }
                mCurTransaction.setMaxLifecycle(fragment, Lifecycle.State.RESUMED);
            } else {
                fragment.setUserVisibleHint(true);
            }

            mCurrentPrimaryItem = fragment;
        }
    }

    @Override
    public void finishUpdate(@NonNull ViewGroup container) {
        if (mCurTransaction != null) {
            mCurTransaction.commitNowAllowingStateLoss();
            mCurTransaction = null;
        }
    }

    @Override
    public boolean isViewFromObject(@NonNull View view, @NonNull Object object) {
        return ((Fragment)object).getView() == view;
    }

    @Override
    @Nullable
    public Parcelable saveState() {
        return null;
    }

    @Override
    public void restoreState(@Nullable Parcelable state, @Nullable ClassLoader loader) {
    }

    /**
     * Return a unique identifier for the item at the given position.
     *
     * <p>The default implementation returns the given position.
     * Subclasses should override this method if the positions of items can change.</p>
     *
     * @param position Position within this adapter
     * @return Unique identifier for the item at position
     */
    public long getItemId(int position) {
        return position;
    }

    private static String makeFragmentName(int viewId, long id) {
        return "android:switcher:" + viewId + ":" + id;
    }
}
