android/view/ScaleGestureDetector.java

/*
 * Copyright (C) 2010 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.content.Context;

/**
 * Detects transformation gestures involving more than one pointer ("multitouch")
 * using the supplied {@link MotionEvent}s. The {@link OnScaleGestureListener}
 * callback will notify users when a particular gesture event has occurred.
 * This class should only be used with {@link MotionEvent}s reported via touch.
 *
 * To use this class:
 * <ul>
 *  <li>Create an instance of the {@code ScaleGestureDetector} 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>
 * @hide Pending API approval
 */
public class ScaleGestureDetector {
    /**
     * The listener for receiving notifications 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 SimpleOnScaleGestureListener}.
     *
     * An application will receive events in the following order:
     * <ul>
     *  <li>One {@link OnScaleGestureListener#onScaleBegin()}
     *  <li>Zero or more {@link OnScaleGestureListener#onScale()}
     *  <li>One {@link OnScaleGestureListener#onTransformEnd()}
     * </ul>
     */
    public interface OnScaleGestureListener {
        /**
         * Responds to scaling events for a gesture in progress.
         * Reported by pointer motion.
         *
         * @param detector The detector reporting the event - use this to
         *          retrieve extended info about event state.
         * @return Whether or not the detector should consider this event
         *          as handled. If an event was not handled, the detector
         *          will continue to accumulate movement until an event is
         *          handled. This can be useful if an application, for example,
         *          only wants to update scaling factors if the change is
         *          greater than 0.01.
         */
        public boolean onScale(ScaleGestureDetector detector);

        /**
         * Responds to the beginning of a scaling gesture. Reported by
         * new pointers going down.
         *
         * @param detector The detector reporting the event - use this to
         *          retrieve extended info about event state.
         * @return Whether or not the detector should continue recognizing
         *          this gesture. For example, if a gesture is beginning
         *          with a focal point outside of a region where it makes
         *          sense, onScaleBegin() may return false to ignore the
         *          rest of the gesture.
         */
        public boolean onScaleBegin(ScaleGestureDetector detector);

        /**
         * Responds to the end of a scale gesture. Reported by existing
         * pointers going up. If the end of a gesture would result in a fling,
         * {@link onTransformFling()} is called instead.
         *
         * Once a scale has ended, {@link ScaleGestureDetector#getFocusX()}
         * and {@link ScaleGestureDetector#getFocusY()} will return the location
         * of the pointer remaining on the screen.
         *
         * @param detector The detector reporting the event - use this to
         *          retrieve extended info about event state.
         */
        public void onScaleEnd(ScaleGestureDetector detector);
    }

    /**
     * A convenience class to extend when you only want to listen for a subset
     * of scaling-related events. This implements all methods in
     * {@link OnScaleGestureListener} but does nothing.
     * {@link OnScaleGestureListener#onScale(ScaleGestureDetector)} and
     * {@link OnScaleGestureListener#onScaleBegin(ScaleGestureDetector)} return
     * {@code true}.
     */
    public class SimpleOnScaleGestureListener implements OnScaleGestureListener {

        public boolean onScale(ScaleGestureDetector detector) {
            return true;
        }

        public boolean onScaleBegin(ScaleGestureDetector detector) {
            return true;
        }

        public void onScaleEnd(ScaleGestureDetector detector) {
            // Intentionally empty
        }
    }

    private static final float PRESSURE_THRESHOLD = 0.67f;

    private Context mContext;
    private OnScaleGestureListener mListener;
    private boolean mGestureInProgress;

    private MotionEvent mPrevEvent;
    private MotionEvent mCurrEvent;

    private float mFocusX;
    private float mFocusY;
    private float mPrevFingerDiffX;
    private float mPrevFingerDiffY;
    private float mCurrFingerDiffX;
    private float mCurrFingerDiffY;
    private float mCurrLen;
    private float mPrevLen;
    private float mScaleFactor;
    private float mCurrPressure;
    private float mPrevPressure;
    private long mTimeDelta;

    public ScaleGestureDetector(Context context, OnScaleGestureListener listener) {
        mContext = context;
        mListener = listener;
    }

    public boolean onTouchEvent(MotionEvent event) {
        final int action = event.getAction();
        boolean handled = true;

        if (!mGestureInProgress) {
            if ((action == MotionEvent.ACTION_POINTER_1_DOWN ||
                    action == MotionEvent.ACTION_POINTER_2_DOWN) &&
                    event.getPointerCount() >= 2) {
                // We have a new multi-finger gesture

                // Be paranoid in case we missed an event
                reset();

                mPrevEvent = MotionEvent.obtain(event);
                mTimeDelta = 0;

                setContext(event);
                mGestureInProgress = mListener.onScaleBegin(this);
            }
        } else {
            // Transform gesture in progress - attempt to handle it
            switch (action) {
                case MotionEvent.ACTION_POINTER_1_UP:
                case MotionEvent.ACTION_POINTER_2_UP:
                    // Gesture ended
                    setContext(event);

                    // Set focus point to the remaining finger
                    int id = (((action & MotionEvent.ACTION_POINTER_ID_MASK)
                            >> MotionEvent.ACTION_POINTER_ID_SHIFT) == 0) ? 1 : 0;
                    mFocusX = event.getX(id);
                    mFocusY = event.getY(id);

                    mListener.onScaleEnd(this);
                    mGestureInProgress = false;

                    reset();
                    break;

                case MotionEvent.ACTION_CANCEL:
                    mListener.onScaleEnd(this);
                    mGestureInProgress = false;

                    reset();
                    break;

                case MotionEvent.ACTION_MOVE:
                    setContext(event);

                    // Only accept the event if our relative pressure is within
                    // a certain limit - this can help filter shaky data as a
                    // finger is lifted.
                    if (mCurrPressure / mPrevPressure > PRESSURE_THRESHOLD) {
                        final boolean updatePrevious = mListener.onScale(this);

                        if (updatePrevious) {
                            mPrevEvent.recycle();
                            mPrevEvent = MotionEvent.obtain(event);
                        }
                    }
                    break;
            }
        }
        return handled;
    }

    private void setContext(MotionEvent curr) {
        if (mCurrEvent != null) {
            mCurrEvent.recycle();
        }
        mCurrEvent = MotionEvent.obtain(curr);

        mCurrLen = -1;
        mPrevLen = -1;
        mScaleFactor = -1;

        final MotionEvent prev = mPrevEvent;

        final float px0 = prev.getX(0);
        final float py0 = prev.getY(0);
        final float px1 = prev.getX(1);
        final float py1 = prev.getY(1);
        final float cx0 = curr.getX(0);
        final float cy0 = curr.getY(0);
        final float cx1 = curr.getX(1);
        final float cy1 = curr.getY(1);

        final float pvx = px1 - px0;
        final float pvy = py1 - py0;
        final float cvx = cx1 - cx0;
        final float cvy = cy1 - cy0;
        mPrevFingerDiffX = pvx;
        mPrevFingerDiffY = pvy;
        mCurrFingerDiffX = cvx;
        mCurrFingerDiffY = cvy;

        mFocusX = cx0 + cvx * 0.5f;
        mFocusY = cy0 + cvy * 0.5f;
        mTimeDelta = curr.getEventTime() - prev.getEventTime();
        mCurrPressure = curr.getPressure(0) + curr.getPressure(1);
        mPrevPressure = prev.getPressure(0) + prev.getPressure(1);
    }

    private void reset() {
        if (mPrevEvent != null) {
            mPrevEvent.recycle();
            mPrevEvent = null;
        }
        if (mCurrEvent != null) {
            mCurrEvent.recycle();
            mCurrEvent = null;
        }
    }

    /**
     * Returns {@code true} if a two-finger scale gesture is in progress.
     * @return {@code true} if a scale gesture is in progress, {@code false} otherwise.
     */
    public boolean isInProgress() {
        return mGestureInProgress;
    }

    /**
     * Get the X coordinate of the current gesture's focal point.
     * If a gesture is in progress, the focal point is directly between
     * the two pointers forming the gesture.
     * If a gesture is ending, the focal point is the location of the
     * remaining pointer on the screen.
     * If {@link isInProgress()} would return false, the result of this
     * function is undefined.
     *
     * @return X coordinate of the focal point in pixels.
     */
    public float getFocusX() {
        return mFocusX;
    }

    /**
     * Get the Y coordinate of the current gesture's focal point.
     * If a gesture is in progress, the focal point is directly between
     * the two pointers forming the gesture.
     * If a gesture is ending, the focal point is the location of the
     * remaining pointer on the screen.
     * If {@link isInProgress()} would return false, the result of this
     * function is undefined.
     *
     * @return Y coordinate of the focal point in pixels.
     */
    public float getFocusY() {
        return mFocusY;
    }

    /**
     * Return the current distance between the two pointers forming the
     * gesture in progress.
     *
     * @return Distance between pointers in pixels.
     */
    public float getCurrentSpan() {
        if (mCurrLen == -1) {
            final float cvx = mCurrFingerDiffX;
            final float cvy = mCurrFingerDiffY;
            mCurrLen = (float)Math.sqrt(cvx*cvx + cvy*cvy);
        }
        return mCurrLen;
    }

    /**
     * Return the previous distance between the two pointers forming the
     * gesture in progress.
     *
     * @return Previous distance between pointers in pixels.
     */
    public float getPreviousSpan() {
        if (mPrevLen == -1) {
            final float pvx = mPrevFingerDiffX;
            final float pvy = mPrevFingerDiffY;
            mPrevLen = (float)Math.sqrt(pvx*pvx + pvy*pvy);
        }
        return mPrevLen;
    }

    /**
     * Return the scaling factor from the previous scale event to the current
     * event. This value is defined as
     * ({@link getCurrentSpan()} / {@link getPreviousSpan()}).
     *
     * @return The current scaling factor.
     */
    public float getScaleFactor() {
        if (mScaleFactor == -1) {
            mScaleFactor = getCurrentSpan() / getPreviousSpan();
        }
        return mScaleFactor;
    }

    /**
     * Return the time difference in milliseconds between the previous
     * accepted scaling event and the current scaling event.
     *
     * @return Time difference since the last scaling event in milliseconds.
     */
    public long getTimeDelta() {
        return mTimeDelta;
    }

    /**
     * Return the event time of the current event being processed.
     *
     * @return Current event time in milliseconds.
     */
    public long getEventTime() {
        return mCurrEvent.getEventTime();
    }
}