core/java/android/view/GestureDetector.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.os.Handler;
import android.os.Message;
import android.content.Context;

/**
 * Detects various gestures and events using the supplied {@link MotionEvent}s.
 * The {@link OnGestureListener} callback will notify users when a particular
 * motion event has occurred. This class should only be used with {@link MotionEvent}s
 * reported via touch (don't use for trackball events).
 *
 * To use this class:
 * <ul>
 *  <li>Create an instance of the {@code GestureDetector} for your {@link View}
 *  <li>In the {@link View#onTouchEvent(MotionEvent)} method ensure you call
 *          {@link #onTouchEvent(MotionEvent)}. The methods defined in your callback
 *          will be executed when the events occur.
 * </ul>
 */
public class GestureDetector {
    /**
     * The listener that is used to notify when gestures occur.
     * If you want to listen for all the different gestures then implement
     * this interface. If you only want to listen for a subset it might
     * be easier to extend {@link SimpleOnGestureListener}.
     */
    public interface OnGestureListener {

        /**
         * Notified when a tap occurs with the down {@link MotionEvent}
         * that triggered it. This will be triggered immediately for
         * every down event. All other events should be preceded by this.
         *
         * @param e The down motion event.
         */
        boolean onDown(MotionEvent e);

        /**
         * The user has performed a down {@link MotionEvent} and not performed
         * a move or up yet. This event is commonly used to provide visual
         * feedback to the user to let them know that their action has been
         * recognized i.e. highlight an element.
         *
         * @param e The down motion event
         */
        void onShowPress(MotionEvent e);

        /**
         * Notified when a tap occurs with the up {@link MotionEvent}
         * that triggered it.
         *
         * @param e The up motion event that completed the first tap
         * @return true if the event is consumed, else false
         */
        boolean onSingleTapUp(MotionEvent e);

        /**
         * Notified when a scroll occurs with the initial on down {@link MotionEvent} and the
         * current move {@link MotionEvent}. The distance in x and y is also supplied for
         * convenience.
         *
         * @param e1 The first down motion event that started the scrolling.
         * @param e2 The move motion event that triggered the current onScroll.
         * @param distanceX The distance along the X axis that has been scrolled since the last
         *              call to onScroll. This is NOT the distance between {@code e1}
         *              and {@code e2}.
         * @param distanceY The distance along the Y axis that has been scrolled since the last
         *              call to onScroll. This is NOT the distance between {@code e1}
         *              and {@code e2}.
         * @return true if the event is consumed, else false
         */
        boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY);

        /**
         * Notified when a long press occurs with the initial on down {@link MotionEvent}
         * that trigged it.
         *
         * @param e The initial on down motion event that started the longpress.
         */
        void onLongPress(MotionEvent e);

        /**
         * Notified of a fling event when it occurs with the initial on down {@link MotionEvent}
         * and the matching up {@link MotionEvent}. The calculated velocity is supplied along
         * the x and y axis in pixels per second.
         *
         * @param e1 The first down motion event that started the fling.
         * @param e2 The move motion event that triggered the current onFling.
         * @param velocityX The velocity of this fling measured in pixels per second
         *              along the x axis.
         * @param velocityY The velocity of this fling measured in pixels per second
         *              along the y axis.
         * @return true if the event is consumed, else false
         */
        boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY);
    }

    /**
     * The listener that is used to notify when a double-tap or a confirmed
     * single-tap occur.
     */
    public interface OnDoubleTapListener {
        /**
         * Notified when a single-tap occurs.
         * <p>
         * Unlike {@link OnGestureListener#onSingleTapUp(MotionEvent)}, this
         * will only be called after the detector is confident that the user's
         * first tap is not followed by a second tap leading to a double-tap
         * gesture.
         *
         * @param e The down motion event of the single-tap.
         * @return true if the event is consumed, else false
         */
        boolean onSingleTapConfirmed(MotionEvent e);
 
        /**
         * Notified when a double-tap occurs.
         *
         * @param e The down motion event of the first tap of the double-tap.
         * @return true if the event is consumed, else false
         */
        boolean onDoubleTap(MotionEvent e);

        /**
         * Notified when an event within a double-tap gesture occurs, including
         * the down, move, and up events.
         *
         * @param e The motion event that occurred during the double-tap gesture.
         * @return true if the event is consumed, else false
         */
        boolean onDoubleTapEvent(MotionEvent e);
    }

    /**
     * A convenience class to extend when you only want to listen for a subset
     * of all the gestures. This implements all methods in the
     * {@link OnGestureListener} and {@link OnDoubleTapListener} but does
     * nothing and return {@code false} for all applicable methods.
     */
    public static class SimpleOnGestureListener implements OnGestureListener, OnDoubleTapListener {
        public boolean onSingleTapUp(MotionEvent e) {
            return false;
        }

        public void onLongPress(MotionEvent e) {
        }

        public boolean onScroll(MotionEvent e1, MotionEvent e2,
                float distanceX, float distanceY) {
            return false;
        }

        public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX,
                float velocityY) {
            return false;
        }

        public void onShowPress(MotionEvent e) {
        }

        public boolean onDown(MotionEvent e) {
            return false;
        }

        public boolean onDoubleTap(MotionEvent e) {
            return false;
        }

        public boolean onDoubleTapEvent(MotionEvent e) {
            return false;
        }

        public boolean onSingleTapConfirmed(MotionEvent e) {
            return false;
        }
    }

    // TODO: ViewConfiguration
    private int mBiggerTouchSlopSquare = 20 * 20;

    private int mTouchSlopSquare;
    private int mDoubleTapSlopSquare;
    private int mMinimumFlingVelocity;

    private static final int LONGPRESS_TIMEOUT = ViewConfiguration.getLongPressTimeout();
    private static final int TAP_TIMEOUT = ViewConfiguration.getTapTimeout();
    private static final int DOUBLE_TAP_TIMEOUT = ViewConfiguration.getDoubleTapTimeout();

    // constants for Message.what used by GestureHandler below
    private static final int SHOW_PRESS = 1;
    private static final int LONG_PRESS = 2;
    private static final int TAP = 3;

    private final Handler mHandler;
    private final OnGestureListener mListener;
    private OnDoubleTapListener mDoubleTapListener;

    private boolean mStillDown;
    private boolean mInLongPress;
    private boolean mAlwaysInTapRegion;
    private boolean mAlwaysInBiggerTapRegion;

    private MotionEvent mCurrentDownEvent;
    private MotionEvent mPreviousUpEvent;

    /**
     * True when the user is still touching for the second tap (down, move, and
     * up events). Can only be true if there is a double tap listener attached.
     */
    private boolean mIsDoubleTapping;

    private float mLastMotionY;
    private float mLastMotionX;

    private boolean mIsLongpressEnabled;

    /**
     * Determines speed during touch scrolling
     */
    private VelocityTracker mVelocityTracker;

    private class GestureHandler extends Handler {
        GestureHandler() {
            super();
        }

        GestureHandler(Handler handler) {
            super(handler.getLooper());
        }

        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
            case SHOW_PRESS:
                mListener.onShowPress(mCurrentDownEvent);
                break;
                
            case LONG_PRESS:
                dispatchLongPress();
                break;
                
            case TAP:
                // If the user's finger is still down, do not count it as a tap
                if (mDoubleTapListener != null && !mStillDown) {
                    mDoubleTapListener.onSingleTapConfirmed(mCurrentDownEvent);
                }
                break;

            default:
                throw new RuntimeException("Unknown message " + msg); //never
            }
        }
    }

    /**
     * Creates a GestureDetector with the supplied listener.
     * This variant of the constructor should be used from a non-UI thread 
     * (as it allows specifying the Handler).
     * 
     * @param listener the listener invoked for all the callbacks, this must
     * not be null.
     * @param handler the handler to use
     *
     * @throws NullPointerException if either {@code listener} or
     * {@code handler} is null.
     *
     * @deprecated Use {@link #GestureDetector(android.content.Context,
     *      android.view.GestureDetector.OnGestureListener, android.os.Handler)} instead.
     */
    @Deprecated
    public GestureDetector(OnGestureListener listener, Handler handler) {
        this(null, listener, handler);
    }

    /**
     * Creates a GestureDetector with the supplied listener.
     * You may only use this constructor from a UI thread (this is the usual situation).
     * @see android.os.Handler#Handler()
     * 
     * @param listener the listener invoked for all the callbacks, this must
     * not be null.
     * 
     * @throws NullPointerException if {@code listener} is null.
     *
     * @deprecated Use {@link #GestureDetector(android.content.Context,
     *      android.view.GestureDetector.OnGestureListener)} instead.
     */
    @Deprecated
    public GestureDetector(OnGestureListener listener) {
        this(null, listener, null);
    }

    /**
     * Creates a GestureDetector with the supplied listener.
     * You may only use this constructor from a UI thread (this is the usual situation).
     * @see android.os.Handler#Handler()
     *
     * @param context the application's context
     * @param listener the listener invoked for all the callbacks, this must
     * not be null.
     *
     * @throws NullPointerException if {@code listener} is null.
     */
    public GestureDetector(Context context, OnGestureListener listener) {
        this(context, listener, null);
    }

    /**
     * Creates a GestureDetector with the supplied listener.
     * You may only use this constructor from a UI thread (this is the usual situation).
     * @see android.os.Handler#Handler()
     *
     * @param context the application's context
     * @param listener the listener invoked for all the callbacks, this must
     * not be null.
     * @param handler the handler to use     
     *
     * @throws NullPointerException if {@code listener} is null.
     */
    public GestureDetector(Context context, OnGestureListener listener, Handler handler) {
        if (handler != null) {
            mHandler = new GestureHandler(handler);
        } else {
            mHandler = new GestureHandler();
        }
        mListener = listener;
        if (listener instanceof OnDoubleTapListener) {
            setOnDoubleTapListener((OnDoubleTapListener) listener);
        }
        init(context);
    }

    private void init(Context context) {
        if (mListener == null) {
            throw new NullPointerException("OnGestureListener must not be null");
        }
        mIsLongpressEnabled = true;

        // Fallback to support pre-donuts releases
        int touchSlop, doubleTapSlop;
        if (context == null) {
            //noinspection deprecation
            touchSlop = ViewConfiguration.getTouchSlop();
            doubleTapSlop = ViewConfiguration.getDoubleTapSlop();
            //noinspection deprecation
            mMinimumFlingVelocity = ViewConfiguration.getMinimumFlingVelocity();
        } else {
            final ViewConfiguration configuration = ViewConfiguration.get(context);
            touchSlop = configuration.getScaledTouchSlop();
            doubleTapSlop = configuration.getScaledDoubleTapSlop();
            mMinimumFlingVelocity = configuration.getScaledMinimumFlingVelocity();
        }
        mTouchSlopSquare = touchSlop * touchSlop;
        mDoubleTapSlopSquare = doubleTapSlop * doubleTapSlop;
    }

    /**
     * Sets the listener which will be called for double-tap and related
     * gestures.
     * 
     * @param onDoubleTapListener the listener invoked for all the callbacks, or
     *        null to stop listening for double-tap gestures.
     */
    public void setOnDoubleTapListener(OnDoubleTapListener onDoubleTapListener) {
        mDoubleTapListener = onDoubleTapListener;
    }

    /**
     * Set whether longpress is enabled, if this is enabled when a user
     * presses and holds down you get a longpress event and nothing further.
     * If it's disabled the user can press and hold down and then later
     * moved their finger and you will get scroll events. By default
     * longpress is enabled.
     *
     * @param isLongpressEnabled whether longpress should be enabled.
     */
    public void setIsLongpressEnabled(boolean isLongpressEnabled) {
        mIsLongpressEnabled = isLongpressEnabled;
    }

    /**
     * @return true if longpress is enabled, else false.
     */
    public boolean isLongpressEnabled() {
        return mIsLongpressEnabled;
    }

    /**
     * Analyzes the given motion event and if applicable triggers the
     * appropriate callbacks on the {@link OnGestureListener} supplied.
     *
     * @param ev The current motion event.
     * @return true if the {@link OnGestureListener} consumed the event,
     *              else false.
     */
    public boolean onTouchEvent(MotionEvent ev) {
        final int action = ev.getAction();
        final float y = ev.getY();
        final float x = ev.getX();

        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(ev);

        boolean handled = false;

        switch (action) {
        case MotionEvent.ACTION_DOWN:
            if (mDoubleTapListener != null) {
                boolean hadTapMessage = mHandler.hasMessages(TAP);
                if (hadTapMessage) mHandler.removeMessages(TAP);
                if ((mCurrentDownEvent != null) && (mPreviousUpEvent != null) && hadTapMessage &&
                        isConsideredDoubleTap(mCurrentDownEvent, mPreviousUpEvent, ev)) {
                    // This is a second tap
                    mIsDoubleTapping = true;
                    // Give a callback with the first tap of the double-tap
                    handled |= mDoubleTapListener.onDoubleTap(mCurrentDownEvent);
                    // Give a callback with down event of the double-tap
                    handled |= mDoubleTapListener.onDoubleTapEvent(ev);
                } else {
                    // This is a first tap
                    mHandler.sendEmptyMessageDelayed(TAP, DOUBLE_TAP_TIMEOUT);
                }
            }

            mLastMotionX = x;
            mLastMotionY = y;
            mCurrentDownEvent = MotionEvent.obtain(ev);
            mAlwaysInTapRegion = true;
            mAlwaysInBiggerTapRegion = true;
            mStillDown = true;
            mInLongPress = false;
            
            if (mIsLongpressEnabled) {
                mHandler.removeMessages(LONG_PRESS);
                mHandler.sendEmptyMessageAtTime(LONG_PRESS, mCurrentDownEvent.getDownTime()
                        + TAP_TIMEOUT + LONGPRESS_TIMEOUT);
            }
            mHandler.sendEmptyMessageAtTime(SHOW_PRESS, mCurrentDownEvent.getDownTime() + TAP_TIMEOUT);
            handled |= mListener.onDown(ev);
            break;

        case MotionEvent.ACTION_MOVE:
            if (mInLongPress) {
                break;
            }
            final float scrollX = mLastMotionX - x;
            final float scrollY = mLastMotionY - y;
            if (mIsDoubleTapping) {
                // Give the move events of the double-tap
                handled |= mDoubleTapListener.onDoubleTapEvent(ev);
            } else if (mAlwaysInTapRegion) {
                final int deltaX = (int) (x - mCurrentDownEvent.getX());
                final int deltaY = (int) (y - mCurrentDownEvent.getY());
                int distance = (deltaX * deltaX) + (deltaY * deltaY);
                if (distance > mTouchSlopSquare) {
                    handled = mListener.onScroll(mCurrentDownEvent, ev, scrollX, scrollY);
                    mLastMotionX = x;
                    mLastMotionY = y;
                    mAlwaysInTapRegion = false;
                    mHandler.removeMessages(TAP);
                    mHandler.removeMessages(SHOW_PRESS);
                    mHandler.removeMessages(LONG_PRESS);
                }
                if (distance > mBiggerTouchSlopSquare) {
                    mAlwaysInBiggerTapRegion = false;
                }
            } else if ((Math.abs(scrollX) >= 1) || (Math.abs(scrollY) >= 1)) {
                handled = mListener.onScroll(mCurrentDownEvent, ev, scrollX, scrollY);
                mLastMotionX = x;
                mLastMotionY = y;
            }
            break;

        case MotionEvent.ACTION_UP:
            mStillDown = false;
            MotionEvent currentUpEvent = MotionEvent.obtain(ev);
            if (mIsDoubleTapping) {
                // Finally, give the up event of the double-tap
                handled |= mDoubleTapListener.onDoubleTapEvent(ev);
            } else if (mInLongPress) {
                mHandler.removeMessages(TAP);
                mInLongPress = false;
            } else if (mAlwaysInTapRegion) {
                handled = mListener.onSingleTapUp(ev);
            } else {

                // A fling must travel the minimum tap distance
                final VelocityTracker velocityTracker = mVelocityTracker;
                velocityTracker.computeCurrentVelocity(1000);
                final float velocityY = velocityTracker.getYVelocity();
                final float velocityX = velocityTracker.getXVelocity();

                if ((Math.abs(velocityY) > mMinimumFlingVelocity)
                        || (Math.abs(velocityX) > mMinimumFlingVelocity)){
                    handled = mListener.onFling(mCurrentDownEvent, currentUpEvent, velocityX, velocityY);
                }
            }
            mPreviousUpEvent = MotionEvent.obtain(ev);
            mVelocityTracker.recycle();
            mVelocityTracker = null;
            mIsDoubleTapping = false;
            mHandler.removeMessages(SHOW_PRESS);
            mHandler.removeMessages(LONG_PRESS);
            break;
        case MotionEvent.ACTION_CANCEL:
            mHandler.removeMessages(SHOW_PRESS);
            mHandler.removeMessages(LONG_PRESS);
            mHandler.removeMessages(TAP);
            mVelocityTracker.recycle();
            mVelocityTracker = null;
            mIsDoubleTapping = false;
            mStillDown = false;
            if (mInLongPress) {
                mInLongPress = false;
                break;
            }
        }
        return handled;
    }

    private boolean isConsideredDoubleTap(MotionEvent firstDown, MotionEvent firstUp,
            MotionEvent secondDown) {
        if (!mAlwaysInBiggerTapRegion) {
            return false;
        }

        if (secondDown.getEventTime() - firstUp.getEventTime() > DOUBLE_TAP_TIMEOUT) {
            return false;
        }

        int deltaX = (int) firstDown.getX() - (int) secondDown.getX();
        int deltaY = (int) firstDown.getY() - (int) secondDown.getY();
        return (deltaX * deltaX + deltaY * deltaY < mDoubleTapSlopSquare);
    }

    private void dispatchLongPress() {
        mHandler.removeMessages(TAP);
        mInLongPress = true;
        mListener.onLongPress(mCurrentDownEvent);
    }
}