Android sample 之模拟重力感应，加速度

   class SimulationView extends View implements SensorEventListener {
        // diameter of the balls in meters
        private static final float sBallDiameter = 0.004f;
        private static final float sBallDiameter2 = sBallDiameter * sBallDiameter;

        // friction of the virtual table and air
        private static final float sFriction = 0.1f;

        private Sensor mAccelerometer;
        private long mLastT;
        private float mLastDeltaT;

        private float mXDpi;
        private float mYDpi;
        private float mMetersToPixelsX;
        private float mMetersToPixelsY;
        private Bitmap mBitmap;
        private Bitmap mWood;
        private float mXOrigin;
        private float mYOrigin;
        private float mSensorX;
        private float mSensorY;
        private long mSensorTimeStamp;
        private long mCpuTimeStamp;
        private float mHorizontalBound;
        private float mVerticalBound;
        private final ParticleSystem mParticleSystem = new ParticleSystem();

        /*
         * Each of our particle holds its previous and current position, its
         * acceleration. for added realism each particle has its own friction
         * coefficient.
         */
        class Particle {
            private float mPosX;
            private float mPosY;
            private float mAccelX;
            private float mAccelY;
            private float mLastPosX;
            private float mLastPosY;
            private float mOneMinusFriction;

            Particle() {
                // make each particle a bit different by randomizing its
                // coefficient of friction
                final float r = ((float) Math.random() - 0.5f) * 0.2f;
                mOneMinusFriction = 1.0f - sFriction + r;
            }

            public void computePhysics(float sx, float sy, float dT, float dTC) {
                // Force of gravity applied to our virtual object
                final float m = 1000.0f; // mass of our virtual object
                final float gx = -sx * m;
                final float gy = -sy * m;

                /*
                 * F = mA <=> A = F / m We could simplify the code by
                 * completely eliminating "m" (the mass) from all the equations,
                 * but it would hide the concepts from this sample code.
                 */
                final float invm = 1.0f / m;
                final float ax = gx * invm;
                final float ay = gy * invm;

                /*
                 * Time-corrected Verlet integration The position Verlet
                 * integrator is defined as x(t+dt) = x(t) + x(t) - x(t-dt) +
                 * a(t).t^2 However, the above equation doesn't handle variable
                 * dt very well, a time-corrected version is needed: x(t+dt) =
                 * x(t) + (x(t) - x(t-dt)) * (dt/dt_prev) + a(t).t^2 We also add
                 * a simple friction term (f) to the equation: x(t+dt) = x(t) +
                 * (1-f) * (x(t) - x(t-dt)) * (dt/dt_prev) + a(t)t^2
                 */
                final float dTdT = dT * dT;
                final float x = mPosX + mOneMinusFriction * dTC * (mPosX - mLastPosX) + mAccelX
                        * dTdT;
                final float y = mPosY + mOneMinusFriction * dTC * (mPosY - mLastPosY) + mAccelY
                        * dTdT;
                mLastPosX = mPosX;
                mLastPosY = mPosY;
                mPosX = x;
                mPosY = y;
                mAccelX = ax;
                mAccelY = ay;
            }

            /*
             * Resolving constraints and collisions with the Verlet integrator
             * can be very simple, we simply need to move a colliding or
             * constrained particle in such way that the constraint is
             * satisfied.
             */
            public void resolveCollisionWithBounds() {
                final float xmax = mHorizontalBound;
                final float ymax = mVerticalBound;
                final float x = mPosX;
                final float y = mPosY;
                if (x > xmax) {
                    mPosX = xmax;
                } else if (x < -xmax) {
                    mPosX = -xmax;
                }
                if (y > ymax) {
                    mPosY = ymax;
                } else if (y < -ymax) {
                    mPosY = -ymax;
                }
            }
        }

        /*
         * A particle system is just a collection of particles
         */
        class ParticleSystem {
            static final int NUM_PARTICLES = 15;
            private Particle mBalls[] = new Particle[NUM_PARTICLES];

            ParticleSystem() {
                /*
                 * Initially our particles have no speed or acceleration
                 */
                for (int i = 0; i < mBalls.length; i++) {
                    mBalls[i] = new Particle();
                }
            }

            /*
             * Update the position of each particle in the system using the
             * Verlet integrator.
             */
            private void updatePositions(float sx, float sy, long timestamp) {
                final long t = timestamp;
                if (mLastT != 0) {
                    final float dT = (float) (t - mLastT) * (1.0f / 1000000000.0f);
                    if (mLastDeltaT != 0) {
                        final float dTC = dT / mLastDeltaT;
                        final int count = mBalls.length;
                        for (int i = 0; i < count; i++) {
                            Particle ball = mBalls[i];
                            ball.computePhysics(sx, sy, dT, dTC);
                        }
                    }
                    mLastDeltaT = dT;
                }
                mLastT = t;
            }

            /*
             * Performs one iteration of the simulation. First updating the
             * position of all the particles and resolving the constraints and
             * collisions.
             */
            public void update(float sx, float sy, long now) {
                // update the system's positions
                updatePositions(sx, sy, now);

                // We do no more than a limited number of iterations
                final int NUM_MAX_ITERATIONS = 10;

                /*
                 * Resolve collisions, each particle is tested against every
                 * other particle for collision. If a collision is detected the
                 * particle is moved away using a virtual spring of infinite
                 * stiffness.
                 */
                boolean more = true;
                final int count = mBalls.length;
                for (int k = 0; k < NUM_MAX_ITERATIONS && more; k++) {
                    more = false;
                    for (int i = 0; i < count; i++) {
                        Particle curr = mBalls[i];
                        for (int j = i + 1; j < count; j++) {
                            Particle ball = mBalls[j];
                            float dx = ball.mPosX - curr.mPosX;
                            float dy = ball.mPosY - curr.mPosY;
                            float dd = dx * dx + dy * dy;
                            // Check for collisions
                            if (dd <= sBallDiameter2) {
                                /*
                                 * add a little bit of entropy, after nothing is
                                 * perfect in the universe.
                                 */
                                dx += ((float) Math.random() - 0.5f) * 0.0001f;
                                dy += ((float) Math.random() - 0.5f) * 0.0001f;
                                dd = dx * dx + dy * dy;
                                // simulate the spring
                                final float d = (float) Math.sqrt(dd);
                                final float c = (0.5f * (sBallDiameter - d)) / d;
                                curr.mPosX -= dx * c;
                                curr.mPosY -= dy * c;
                                ball.mPosX += dx * c;
                                ball.mPosY += dy * c;
                                more = true;
                            }
                        }
                        /*
                         * Finally make sure the particle doesn't intersects
                         * with the walls.
                         */
                        curr.resolveCollisionWithBounds();
                    }
                }
            }

            public int getParticleCount() {
                return mBalls.length;
            }

            public float getPosX(int i) {
                return mBalls[i].mPosX;
            }

            public float getPosY(int i) {
                return mBalls[i].mPosY;
            }
        }

        public void startSimulation() {
            /*
             * It is not necessary to get accelerometer events at a very high
             * rate, by using a slower rate (SENSOR_DELAY_UI), we get an
             * automatic low-pass filter, which "extracts" the gravity component
             * of the acceleration. As an added benefit, we use less power and
             * CPU resources.
             */
            mSensorManager.registerListener(this, mAccelerometer, SensorManager.SENSOR_DELAY_UI);
        }

        public void stopSimulation() {
            mSensorManager.unregisterListener(this);
        }

        public SimulationView(Context context) {
            super(context);
            mAccelerometer = mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);

            DisplayMetrics metrics = new DisplayMetrics();
            getWindowManager().getDefaultDisplay().getMetrics(metrics);
            mXDpi = metrics.xdpi;
            mYDpi = metrics.ydpi;
            mMetersToPixelsX = mXDpi / 0.0254f;
            mMetersToPixelsY = mYDpi / 0.0254f;

            // rescale the ball so it's about 0.5 cm on screen
            Bitmap ball = BitmapFactory.decodeResource(getResources(), R.drawable.ball);
            final int dstWidth = (int) (sBallDiameter * mMetersToPixelsX + 0.5f);
            final int dstHeight = (int) (sBallDiameter * mMetersToPixelsY + 0.5f);
            mBitmap = Bitmap.createScaledBitmap(ball, dstWidth, dstHeight, true);

            Options opts = new Options();
            opts.inDither = true;
            opts.inPreferredConfig = Bitmap.Config.RGB_565;
            mWood = BitmapFactory.decodeResource(getResources(), R.drawable.wood, opts);
        }

        @Override
        protected void onSizeChanged(int w, int h, int oldw, int oldh) {
            // compute the origin of the screen relative to the origin of
            // the bitmap
            mXOrigin = (w - mBitmap.getWidth()) * 0.5f;
            mYOrigin = (h - mBitmap.getHeight()) * 0.5f;
            mHorizontalBound = ((w / mMetersToPixelsX - sBallDiameter) * 0.5f);
            mVerticalBound = ((h / mMetersToPixelsY - sBallDiameter) * 0.5f);
        }

        @Override
        public void onSensorChanged(SensorEvent event) {
            if (event.sensor.getType() != Sensor.TYPE_ACCELEROMETER)
                return;
            /*
             * record the accelerometer data, the event's timestamp as well as
             * the current time. The latter is needed so we can calculate the
             * "present" time during rendering. In this application, we need to
             * take into account how the screen is rotated with respect to the
             * sensors (which always return data in a coordinate space aligned
             * to with the screen in its native orientation).
             */

            switch (mDisplay.getRotation()) {
                case Surface.ROTATION_0:
                    mSensorX = event.values[0];
                    mSensorY = event.values[1];
                    break;
                case Surface.ROTATION_90:
                    mSensorX = -event.values[1];
                    mSensorY = event.values[0];
                    break;
                case Surface.ROTATION_180:
                    mSensorX = -event.values[0];
                    mSensorY = -event.values[1];
                    break;
                case Surface.ROTATION_270:
                    mSensorX = event.values[1];
                    mSensorY = -event.values[0];
                    break;
            }

            mSensorTimeStamp = event.timestamp;
            mCpuTimeStamp = System.nanoTime();
        }

        @Override
        protected void onDraw(Canvas canvas) {

            /*
             * draw the background
             */

            canvas.drawBitmap(mWood, 0, 0, null);

            /*
             * compute the new position of our object, based on accelerometer
             * data and present time.
             */

            final ParticleSystem particleSystem = mParticleSystem;
            final long now = mSensorTimeStamp + (System.nanoTime() - mCpuTimeStamp);
            final float sx = mSensorX;
            final float sy = mSensorY;

            particleSystem.update(sx, sy, now);

            final float xc = mXOrigin;
            final float yc = mYOrigin;
            final float xs = mMetersToPixelsX;
            final float ys = mMetersToPixelsY;
            final Bitmap bitmap = mBitmap;
            final int count = particleSystem.getParticleCount();
            for (int i = 0; i < count; i++) {
                /*
                 * We transform the canvas so that the coordinate system matches
                 * the sensors coordinate system with the origin in the center
                 * of the screen and the unit is the meter.
                 */

                final float x = xc + particleSystem.getPosX(i) * xs;
                final float y = yc - particleSystem.getPosY(i) * ys;
                canvas.drawBitmap(bitmap, x, y, null);
            }

            // and make sure to redraw asap
            invalidate();
        }

        @Override
        public void onAccuracyChanged(Sensor sensor, int accuracy) {
        }
    }