Java tutorial
/* * Copyright (C) 2011 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 android.support.v4.view; import android.database.DataSetObservable; import android.database.DataSetObserver; import android.os.Parcelable; import android.view.View; import android.view.ViewGroup; /** * Base class providing the adapter to populate pages inside of * a {@link ViewPager}. You will most likely want to use a more * specific implementation of this, such as * {@link android.support.v4.app.FragmentPagerAdapter} or * {@link android.support.v4.app.FragmentStatePagerAdapter}. * * <p>When you implement a PagerAdapter, you must override the following methods * at minimum:</p> * <ul> * <li>{@link #instantiateItem(ViewGroup, int)}</li> * <li>{@link #destroyItem(ViewGroup, int, Object)}</li> * <li>{@link #getCount()}</li> * <li>{@link #isViewFromObject(View, Object)}</li> * </ul> * * <p>PagerAdapter is more general than the adapters used for * {@link android.widget.AdapterView AdapterViews}. Instead of providing a * View recycling mechanism directly ViewPager uses callbacks to indicate the * steps taken during an update. A PagerAdapter may implement a form of View * recycling if desired or use a more sophisticated method of managing page * Views such as Fragment transactions where each page is represented by its * own Fragment.</p> * * <p>ViewPager associates each page with a key Object instead of working with * Views directly. This key is used to track and uniquely identify a given page * independent of its position in the adapter. A call to the PagerAdapter method * {@link #startUpdate(ViewGroup)} indicates that the contents of the ViewPager * are about to change. One or more calls to {@link #instantiateItem(ViewGroup, int)} * and/or {@link #destroyItem(ViewGroup, int, Object)} will follow, and the end * of an update will be signaled by a call to {@link #finishUpdate(ViewGroup)}. * By the time {@link #finishUpdate(ViewGroup) finishUpdate} returns the views * associated with the key objects returned by * {@link #instantiateItem(ViewGroup, int) instantiateItem} should be added to * the parent ViewGroup passed to these methods and the views associated with * the keys passed to {@link #destroyItem(ViewGroup, int, Object) destroyItem} * should be removed. The method {@link #isViewFromObject(View, Object)} identifies * whether a page View is associated with a given key object.</p> * * <p>A very simple PagerAdapter may choose to use the page Views themselves * as key objects, returning them from {@link #instantiateItem(ViewGroup, int)} * after creation and adding them to the parent ViewGroup. A matching * {@link #destroyItem(ViewGroup, int, Object)} implementation would remove the * View from the parent ViewGroup and {@link #isViewFromObject(View, Object)} * could be implemented as <code>return view == object;</code>.</p> * * <p>PagerAdapter supports data set changes. Data set changes must occur on the * main thread and must end with a call to {@link #notifyDataSetChanged()} similar * to AdapterView adapters derived from {@link android.widget.BaseAdapter}. A data * set change may involve pages being added, removed, or changing position. The * ViewPager will keep the current page active provided the adapter implements * the method {@link #getItemPosition(Object)}.</p> */ public abstract class PagerAdapter { private DataSetObservable mObservable = new DataSetObservable(); public static final int POSITION_UNCHANGED = -1; public static final int POSITION_NONE = -2; /** * Return the number of views available. */ public abstract int getCount(); /** * Called when a change in the shown pages is going to start being made. * @param container The containing View which is displaying this adapter's * page views. */ public void startUpdate(ViewGroup container) { startUpdate((View) container); } /** * Create the page for the given position. The adapter is responsible * for adding the view to the container given here, although it only * must ensure this is done by the time it returns from * {@link #finishUpdate(ViewGroup)}. * * @param container The containing View in which the page will be shown. * @param position The page position to be instantiated. * @return Returns an Object representing the new page. This does not * need to be a View, but can be some other container of the page. */ public Object instantiateItem(ViewGroup container, int position) { return instantiateItem((View) container, position); } /** * Remove a page for the given position. The adapter is responsible * for removing the view from its container, although it only must ensure * this is done by the time it returns from {@link #finishUpdate(ViewGroup)}. * * @param container The containing View from which the page will be removed. * @param position The page position to be removed. * @param object The same object that was returned by * {@link #instantiateItem(View, int)}. */ public void destroyItem(ViewGroup container, int position, Object object) { destroyItem((View) container, position, object); } /** * Called to inform the adapter of which item is currently considered to * be the "primary", that is the one show to the user as the current page. * * @param container The containing View from which the page will be removed. * @param position The page position that is now the primary. * @param object The same object that was returned by * {@link #instantiateItem(View, int)}. */ public void setPrimaryItem(ViewGroup container, int position, Object object) { setPrimaryItem((View) container, position, object); } /** * Called when the a change in the shown pages has been completed. At this * point you must ensure that all of the pages have actually been added or * removed from the container as appropriate. * @param container The containing View which is displaying this adapter's * page views. */ public void finishUpdate(ViewGroup container) { finishUpdate((View) container); } /** * Called when a change in the shown pages is going to start being made. * @param container The containing View which is displaying this adapter's * page views. * * @deprecated Use {@link #startUpdate(ViewGroup)} */ public void startUpdate(View container) { } /** * Create the page for the given position. The adapter is responsible * for adding the view to the container given here, although it only * must ensure this is done by the time it returns from * {@link #finishUpdate(ViewGroup)}. * * @param container The containing View in which the page will be shown. * @param position The page position to be instantiated. * @return Returns an Object representing the new page. This does not * need to be a View, but can be some other container of the page. * * @deprecated Use {@link #instantiateItem(ViewGroup, int)} */ public Object instantiateItem(View container, int position) { throw new UnsupportedOperationException("Required method instantiateItem was not overridden"); } /** * Remove a page for the given position. The adapter is responsible * for removing the view from its container, although it only must ensure * this is done by the time it returns from {@link #finishUpdate(View)}. * * @param container The containing View from which the page will be removed. * @param position The page position to be removed. * @param object The same object that was returned by * {@link #instantiateItem(View, int)}. * * @deprecated Use {@link #destroyItem(ViewGroup, int, Object)} */ public void destroyItem(View container, int position, Object object) { throw new UnsupportedOperationException("Required method destroyItem was not overridden"); } /** * Called to inform the adapter of which item is currently considered to * be the "primary", that is the one show to the user as the current page. * * @param container The containing View from which the page will be removed. * @param position The page position that is now the primary. * @param object The same object that was returned by * {@link #instantiateItem(View, int)}. * * @deprecated Use {@link #setPrimaryItem(ViewGroup, int, Object)} */ public void setPrimaryItem(View container, int position, Object object) { } /** * Called when the a change in the shown pages has been completed. At this * point you must ensure that all of the pages have actually been added or * removed from the container as appropriate. * @param container The containing View which is displaying this adapter's * page views. * * @deprecated Use {@link #setPrimaryItem(ViewGroup, int, Object)} */ public void finishUpdate(View container) { } /** * Determines whether a page View is associated with a specific key object * as returned by {@link #instantiateItem(ViewGroup, int)}. This method is * required for a PagerAdapter to function properly. * * @param view Page View to check for association with <code>object</code> * @param object Object to check for association with <code>view</code> * @return true if <code>view</code> is associated with the key object <code>object</code> */ public abstract boolean isViewFromObject(View view, Object object); /** * Save any instance state associated with this adapter and its pages that should be * restored if the current UI state needs to be reconstructed. * * @return Saved state for this adapter */ public Parcelable saveState() { return null; } /** * Restore any instance state associated with this adapter and its pages * that was previously saved by {@link #saveState()}. * * @param state State previously saved by a call to {@link #saveState()} * @param loader A ClassLoader that should be used to instantiate any restored objects */ public void restoreState(Parcelable state, ClassLoader loader) { } /** * Called when the host view is attempting to determine if an item's position * has changed. Returns {@link #POSITION_UNCHANGED} if the position of the given * item has not changed or {@link #POSITION_NONE} if the item is no longer present * in the adapter. * * <p>The default implementation assumes that items will never * change position and always returns {@link #POSITION_UNCHANGED}. * * @param object Object representing an item, previously returned by a call to * {@link #instantiateItem(View, int)}. * @return object's new position index from [0, {@link #getCount()}), * {@link #POSITION_UNCHANGED} if the object's position has not changed, * or {@link #POSITION_NONE} if the item is no longer present. */ public int getItemPosition(Object object) { return POSITION_UNCHANGED; } /** * This method should be called by the application if the data backing this adapter has changed * and associated views should update. */ public void notifyDataSetChanged() { mObservable.notifyChanged(); } void registerDataSetObserver(DataSetObserver observer) { mObservable.registerObserver(observer); } void unregisterDataSetObserver(DataSetObserver observer) { mObservable.unregisterObserver(observer); } /** * This method may be called by the ViewPager to obtain a title string * to describe the specified page. This method may return null * indicating no title for this page. The default implementation returns * null. * * @param position The position of the title requested * @return A title for the requested page */ public CharSequence getPageTitle(int position) { return null; } }