core/java/android/view/TouchDelegate.java

/*
 * Copyright (C) 2008 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.view;

import android.graphics.Rect;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewConfiguration;

/**
 * Helper class to handle situations where you want a view to have a larger touch area than its
 * actual view bounds. The view whose touch area is changed is called the delegate view. This
 * class should be used by an ancestor of the delegate. To use a TouchDelegate, first create an
 * instance that specifies the bounds that should be mapped to the delegate and the delegate
 * view itself.
 * <p>
 * The ancestor should then forward all of its touch events received in its
 * {@link android.view.View#onTouchEvent(MotionEvent)} to {@link #onTouchEvent(MotionEvent)}.
 * </p>
 */
public class TouchDelegate {
    
    /**
     * View that should receive forwarded touch events 
     */
    private View mDelegateView;
    
    /**
     * Bounds in local coordinates of the containing view that should be mapped to the delegate
     * view. This rect is used for initial hit testing.
     */
    private Rect mBounds;
    
    /**
     * mBounds inflated to include some slop. This rect is to track whether the motion events
     * should be considered to be be within the delegate view.
     */
    private Rect mSlopBounds;
    
    /**
     * True if the delegate had been targeted on a down event (intersected mBounds).
     */
    private boolean mDelegateTargeted;

    /**
     * The touchable region of the View extends above its actual extent.
     */
    public static final int ABOVE = 1;

    /**
     * The touchable region of the View extends below its actual extent.
     */
    public static final int BELOW = 2;

    /**
     * The touchable region of the View extends to the left of its
     * actual extent.
     */
    public static final int TO_LEFT = 4;

    /**
     * The touchable region of the View extends to the right of its
     * actual extent.
     */
    public static final int TO_RIGHT = 8;

    private int mSlop;

    /**
     * Constructor
     * 
     * @param bounds Bounds in local coordinates of the containing view that should be mapped to
     *        the delegate view
     * @param delegateView The view that should receive motion events
     */
    public TouchDelegate(Rect bounds, View delegateView) {
        mBounds = bounds;

        mSlop = ViewConfiguration.get(delegateView.getContext()).getScaledTouchSlop();
        mSlopBounds = new Rect(bounds);
        mSlopBounds.inset(-mSlop, -mSlop);
        mDelegateView = delegateView;
    }

    /**
     * Will forward touch events to the delegate view if the event is within the bounds
     * specified in the constructor.
     * 
     * @param event The touch event to forward
     * @return True if the event was forwarded to the delegate, false otherwise.
     */
    public boolean onTouchEvent(MotionEvent event) {
        int x = (int)event.getX();
        int y = (int)event.getY();
        boolean sendToDelegate = false;
        boolean hit = true;
        boolean handled = false;

        switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            Rect bounds = mBounds;

            if (bounds.contains(x, y)) {
                mDelegateTargeted = true;
                sendToDelegate = true;
            }
            break;
        case MotionEvent.ACTION_UP:
        case MotionEvent.ACTION_MOVE:
            sendToDelegate = mDelegateTargeted;
            if (sendToDelegate) {
                Rect slopBounds = mSlopBounds;
                if (!slopBounds.contains(x, y)) {
                    hit = false;
                }
            }
            break;
        case MotionEvent.ACTION_CANCEL:
            sendToDelegate = mDelegateTargeted;
            mDelegateTargeted = false;
            break;
        }
        if (sendToDelegate) {
            final View delegateView = mDelegateView;
            
            if (hit) {
                // Offset event coordinates to be inside the target view
                event.setLocation(delegateView.getWidth() / 2, delegateView.getHeight() / 2);
            } else {
                // Offset event coordinates to be outside the target view (in case it does
                // something like tracking pressed state)
                int slop = mSlop;
                event.setLocation(-(slop * 2), -(slop * 2));
            }
            handled = delegateView.dispatchTouchEvent(event);
        }
        return handled;
    }
}