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The Support Package includes static "support libraries" that you can add to your Android application in order to use APIs that are either not available for older platform versions or that offer "utility" APIs that aren't a part of the framework APIs.
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/*
 * 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.os.Build;
import android.support.v4.view.accessibility.AccessibilityNodeInfoCompat;
import android.view.View;
import android.view.ViewGroup;
import android.view.accessibility.AccessibilityEvent;

/**
 * Helper for accessing {@link View.AccessibilityDelegate} introduced after
 * API level 4 in a backwards compatible fashion.
 */
public class AccessibilityDelegateCompat {

    static interface AccessibilityDelegateImpl {
        public Object newAccessiblityDelegateDefaultImpl();
        public Object newAccessiblityDelegateBridge(AccessibilityDelegateCompat listener);
        public boolean dispatchPopulateAccessibilityEvent(Object delegate, View host,
                AccessibilityEvent event);
        public void onInitializeAccessibilityEvent(Object delegate, View host,
                AccessibilityEvent event);
        public void onInitializeAccessibilityNodeInfo(Object delegate, View host,
                AccessibilityNodeInfoCompat info);
        public void onPopulateAccessibilityEvent(Object delegate, View host,
                AccessibilityEvent event);
        public boolean onRequestSendAccessibilityEvent(Object delegate, ViewGroup host, View child,
                AccessibilityEvent event);
        public void sendAccessibilityEvent(Object delegate, View host, int eventType);
        public void sendAccessibilityEventUnchecked(Object delegate, View host,
                AccessibilityEvent event);
    }

    static class AccessibilityDelegateStubImpl implements AccessibilityDelegateImpl {
        public Object newAccessiblityDelegateDefaultImpl() {
            return null;
        }

        public Object newAccessiblityDelegateBridge(AccessibilityDelegateCompat listener) {
            return null;
        }

        public boolean dispatchPopulateAccessibilityEvent(Object delegate, View host,
                AccessibilityEvent event) {
            return false;
        }

        public void onInitializeAccessibilityEvent(Object delegate, View host,
                AccessibilityEvent event) {

        }

        public void onInitializeAccessibilityNodeInfo(Object delegate, View host,
                AccessibilityNodeInfoCompat info) {

        }

        public void onPopulateAccessibilityEvent(Object delegate, View host,
                AccessibilityEvent event) {

        }

        public boolean onRequestSendAccessibilityEvent(Object delegate, ViewGroup host, View child,
                AccessibilityEvent event) {
            return true;
        }

        public void sendAccessibilityEvent(Object delegate, View host, int eventType) {

        }

        public void sendAccessibilityEventUnchecked(Object delegate, View host,
                AccessibilityEvent event) {

        }
    }

    static class AccessibilityDelegateIcsImpl extends AccessibilityDelegateStubImpl {
        @Override
        public Object newAccessiblityDelegateDefaultImpl() {
            return AccessibilityDelegateCompatIcs.newAccessibilityDelegateDefaultImpl();
        }

        @Override
        public Object newAccessiblityDelegateBridge(final AccessibilityDelegateCompat compat) {
            return AccessibilityDelegateCompatIcs.newAccessibilityDelegateBridge(
                    new AccessibilityDelegateCompatIcs.AccessibilityDelegateBridge() {
                @Override
                public boolean dispatchPopulateAccessibilityEvent(View host,
                        AccessibilityEvent event) {
                    return compat.dispatchPopulateAccessibilityEvent(host, event);
                }

                @Override
                public void onInitializeAccessibilityEvent(View host, AccessibilityEvent event) {
                    compat.onInitializeAccessibilityEvent(host, event);
                }

                @Override
                public void onInitializeAccessibilityNodeInfo(View host, Object info) {
                    compat.onInitializeAccessibilityNodeInfo(host,
                            new AccessibilityNodeInfoCompat(info));
                }

                @Override
                public void onPopulateAccessibilityEvent(View host, AccessibilityEvent event) {
                    compat.onPopulateAccessibilityEvent(host, event);
                }

                @Override
                public boolean onRequestSendAccessibilityEvent(ViewGroup host, View child,
                        AccessibilityEvent event) {
                    return compat.onRequestSendAccessibilityEvent(host, child, event);
                }

                @Override
                public void sendAccessibilityEvent(View host, int eventType) {
                    compat.sendAccessibilityEvent(host, eventType);
                }

                @Override
                public void sendAccessibilityEventUnchecked(View host, AccessibilityEvent event) {
                    compat.sendAccessibilityEventUnchecked(host, event);
                }
            });
        }

        @Override
        public boolean dispatchPopulateAccessibilityEvent(Object delegate, View host,
                AccessibilityEvent event) {
            return AccessibilityDelegateCompatIcs.dispatchPopulateAccessibilityEvent(delegate,
                    host, event);
        }

        @Override
        public void onInitializeAccessibilityEvent(Object delegate, View host,
                AccessibilityEvent event) {
            AccessibilityDelegateCompatIcs.onInitializeAccessibilityEvent(delegate, host, event);
        }

        @Override
        public void onInitializeAccessibilityNodeInfo(Object delegate, View host,
                AccessibilityNodeInfoCompat info) {
            AccessibilityDelegateCompatIcs.onInitializeAccessibilityNodeInfo(delegate, host,
                    info.getImpl());
        }

        @Override
        public void onPopulateAccessibilityEvent(Object delegate, View host,
                AccessibilityEvent event) {
            AccessibilityDelegateCompatIcs.onPopulateAccessibilityEvent(delegate, host, event);
        }

        @Override
        public boolean onRequestSendAccessibilityEvent(Object delegate, ViewGroup host, View child,
                AccessibilityEvent event) {
            return AccessibilityDelegateCompatIcs.onRequestSendAccessibilityEvent(delegate, host,
                    child, event);
        }

        @Override
        public void sendAccessibilityEvent(Object delegate, View host, int eventType) {
            AccessibilityDelegateCompatIcs.sendAccessibilityEvent(delegate, host, eventType);
        }

        @Override
        public void sendAccessibilityEventUnchecked(Object delegate, View host,
                AccessibilityEvent event) {
            AccessibilityDelegateCompatIcs.sendAccessibilityEventUnchecked(delegate, host, event);
        }
    }

    private static final AccessibilityDelegateImpl IMPL;
    private static final Object DEFAULT_DELEGATE;

    static {
        if (Build.VERSION.SDK_INT >= 14) { // ICS
            IMPL = new AccessibilityDelegateIcsImpl();
        } else {
            IMPL = new AccessibilityDelegateStubImpl();
        }
        DEFAULT_DELEGATE = IMPL.newAccessiblityDelegateDefaultImpl();
    }

    final Object mBridge;

    /**
     * Creates a new instance.
     */
    public AccessibilityDelegateCompat() {
        mBridge = IMPL.newAccessiblityDelegateBridge(this);
    }

    /**
     * @return The wrapped bridge implementation.
     */
    Object getBridge() {
        return mBridge;
    }

    /**
     * Sends an accessibility event of the given type. If accessibility is not
     * enabled this method has no effect.
     * 
     * The default implementation behaves as {@link View#sendAccessibilityEvent(int)
     * View#sendAccessibilityEvent(int)} for the case of no accessibility delegate
     * been set.
     * 
     *
     * @param host The View hosting the delegate.
     * @param eventType The type of the event to send.
     *
     * @see View#sendAccessibilityEvent(int) View#sendAccessibilityEvent(int)
     */
    public void sendAccessibilityEvent(View host, int eventType) {
        IMPL.sendAccessibilityEvent(DEFAULT_DELEGATE, host, eventType);
    }

    /**
     * Sends an accessibility event. This method behaves exactly as
     * {@link #sendAccessibilityEvent(View, int)} but takes as an argument an
     * empty {@link AccessibilityEvent} and does not perform a check whether
     * accessibility is enabled.
     * 
     * The default implementation behaves as
     * {@link View#sendAccessibilityEventUnchecked(AccessibilityEvent)
     * View#sendAccessibilityEventUnchecked(AccessibilityEvent)} for
     * the case of no accessibility delegate been set.
     * 
     *
     * @param host The View hosting the delegate.
     * @param event The event to send.
     *
     * @see View#sendAccessibilityEventUnchecked(AccessibilityEvent)
     *      View#sendAccessibilityEventUnchecked(AccessibilityEvent)
     */
    public void sendAccessibilityEventUnchecked(View host, AccessibilityEvent event) {
        IMPL.sendAccessibilityEventUnchecked(DEFAULT_DELEGATE, host, event);
    }

    /**
     * Dispatches an {@link AccessibilityEvent} to the host {@link View} first and then
     * to its children for adding their text content to the event.
     * 
     * The default implementation behaves as
     * {@link View#dispatchPopulateAccessibilityEvent(AccessibilityEvent)
     * View#dispatchPopulateAccessibilityEvent(AccessibilityEvent)} for
     * the case of no accessibility delegate been set.
     * 
     *
     * @param host The View hosting the delegate.
     * @param event The event.
     * @return True if the event population was completed.
     *
     * @see View#dispatchPopulateAccessibilityEvent(AccessibilityEvent)
     *      View#dispatchPopulateAccessibilityEvent(AccessibilityEvent)
     */
    public boolean dispatchPopulateAccessibilityEvent(View host, AccessibilityEvent event) {
        return IMPL.dispatchPopulateAccessibilityEvent(DEFAULT_DELEGATE, host, event);
    }

    /**
     * Gives a chance to the host View to populate the accessibility event with its
     * text content.
     * 
     * The default implementation behaves as
     * {@link ViewCompat#onPopulateAccessibilityEvent(View, AccessibilityEvent)
     * ViewCompat#onPopulateAccessibilityEvent(AccessibilityEvent)} for
     * the case of no accessibility delegate been set.
     * 
     *
     * @param host The View hosting the delegate.
     * @param event The accessibility event which to populate.
     *
     * @see ViewCompat#onPopulateAccessibilityEvent(View ,AccessibilityEvent)
     *      ViewCompat#onPopulateAccessibilityEvent(View, AccessibilityEvent)
     */
    public void onPopulateAccessibilityEvent(View host, AccessibilityEvent event) {
        IMPL.onPopulateAccessibilityEvent(DEFAULT_DELEGATE, host, event);
    }

    /**
     * Initializes an {@link AccessibilityEvent} with information about the
     * the host View which is the event source.
     * 
     * The default implementation behaves as
     * {@link ViewCompat#onInitializeAccessibilityEvent(View v, AccessibilityEvent event)
     * ViewCompat#onInitalizeAccessibilityEvent(View v, AccessibilityEvent event)} for
     * the case of no accessibility delegate been set.
     * 
     *
     * @param host The View hosting the delegate.
     * @param event The event to initialize.
     *
     * @see ViewCompat#onInitializeAccessibilityEvent(View, AccessibilityEvent)
     *      ViewCompat#onInitializeAccessibilityEvent(View, AccessibilityEvent)
     */
    public void onInitializeAccessibilityEvent(View host, AccessibilityEvent event) {
        IMPL.onInitializeAccessibilityEvent(DEFAULT_DELEGATE, host, event);
    }

    /**
     * Initializes an {@link AccessibilityNodeInfoCompat} with information about the host view.
     * 
     * The default implementation behaves as
     * {@link ViewCompat#onInitializeAccessibilityNodeInfo(View, AccessibilityNodeInfoCompat)
     * ViewCompat#onInitializeAccessibilityNodeInfo(View, AccessibilityNodeInfoCompat)} for
     * the case of no accessibility delegate been set.
     * 
     *
     * @param host The View hosting the delegate.
     * @param info The instance to initialize.
     *
     * @see ViewCompat#onInitializeAccessibilityNodeInfo(View, AccessibilityNodeInfoCompat)
     *      ViewCompat#onInitializeAccessibilityNodeInfo(View, AccessibilityNodeInfoCompat)
     */
    public void onInitializeAccessibilityNodeInfo(View host, AccessibilityNodeInfoCompat info) {
        IMPL.onInitializeAccessibilityNodeInfo(DEFAULT_DELEGATE, host, info);
    }

    /**
     * Called when a child of the host View has requested sending an
     * {@link AccessibilityEvent} and gives an opportunity to the parent (the host)
     * to augment the event.
     * 
     * The default implementation behaves as
     * {@link ViewGroupCompat#onRequestSendAccessibilityEvent(ViewGroup, View, AccessibilityEvent)
     * ViewGroupCompat#onRequestSendAccessibilityEvent(ViewGroup, View, AccessibilityEvent)} for
     * the case of no accessibility delegate been set.
     * 
     *
     * @param host The View hosting the delegate.
     * @param child The child which requests sending the event.
     * @param event The event to be sent.
     * @return True if the event should be sent
     *
     * @see ViewGroupCompat#onRequestSendAccessibilityEvent(ViewGroup, View, AccessibilityEvent)
     *      ViewGroupCompat#onRequestSendAccessibilityEvent(ViewGroup, View, AccessibilityEvent)
     */
    public boolean onRequestSendAccessibilityEvent(ViewGroup host, View child,
            AccessibilityEvent event) {
        return IMPL.onRequestSendAccessibilityEvent(DEFAULT_DELEGATE, host, child, event);
    }
}