Fixed LunarView class in Android SDK Sample LunarLander.

LunarView.java

/*
 * Copyright (C) 2007 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 com.example.android.lunarlander;

import android.content.Context;
import android.content.res.Resources;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.RectF;
import android.graphics.drawable.Drawable;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.util.AttributeSet;
import android.view.KeyEvent;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.View;
import android.widget.TextView;


/**
 * View that draws, takes keystrokes, etc. for a simple LunarLander game.
 * 
 * Has a mode which RUNNING, PAUSED, etc. Has a x, y, dx, dy, ... capturing the
 * current ship physics. All x/y etc. are measured with (0,0) at the lower left.
 * updatePhysics() advances the physics based on realtime. draw() renders the
 * ship, and does an invalidate() to prompt another draw() as soon as possible
 * by the system.
 */
class LunarView extends SurfaceView implements SurfaceHolder.Callback {
    class LunarThread extends Thread {
        /*
         * Difficulty setting constants
         */
        public static final int DIFFICULTY_EASY = 0;
        public static final int DIFFICULTY_HARD = 1;
        public static final int DIFFICULTY_MEDIUM = 2;
        /*
         * Physics constants
         */
        public static final int PHYS_DOWN_ACCEL_SEC = 35;
        public static final int PHYS_FIRE_ACCEL_SEC = 80;
        public static final int PHYS_FUEL_INIT = 60;
        public static final int PHYS_FUEL_MAX = 100;
        public static final int PHYS_FUEL_SEC = 10;
        public static final int PHYS_SLEW_SEC = 120; // degrees/second rotate
        public static final int PHYS_SPEED_HYPERSPACE = 180;
        public static final int PHYS_SPEED_INIT = 30;
        public static final int PHYS_SPEED_MAX = 120;
        /*
         * State-tracking constants
         */
        public static final int STATE_LOSE = 1;
        public static final int STATE_PAUSE = 2;
        public static final int STATE_READY = 3;
        public static final int STATE_RUNNING = 4;
        public static final int STATE_WIN = 5;

        /*
         * Goal condition constants
         */
        public static final int TARGET_ANGLE = 18; // > this angle means crash
        public static final int TARGET_BOTTOM_PADDING = 17; // px below gear
        public static final int TARGET_PAD_HEIGHT = 8; // how high above ground
        public static final int TARGET_SPEED = 28; // > this speed means crash
        public static final double TARGET_WIDTH = 1.6; // width of target
        /*
         * UI constants (i.e. the speed & fuel bars)
         */
        public static final int UI_BAR = 100; // width of the bar(s)
        public static final int UI_BAR_HEIGHT = 10; // height of the bar(s)
        private static final String KEY_DIFFICULTY = "mDifficulty";
        private static final String KEY_DX = "mDX";

        private static final String KEY_DY = "mDY";
        private static final String KEY_FUEL = "mFuel";
        private static final String KEY_GOAL_ANGLE = "mGoalAngle";
        private static final String KEY_GOAL_SPEED = "mGoalSpeed";
        private static final String KEY_GOAL_WIDTH = "mGoalWidth";

        private static final String KEY_GOAL_X = "mGoalX";
        private static final String KEY_HEADING = "mHeading";
        private static final String KEY_LANDER_HEIGHT = "mLanderHeight";
        private static final String KEY_LANDER_WIDTH = "mLanderWidth";
        private static final String KEY_WINS = "mWinsInARow";

        private static final String KEY_X = "mX";
        private static final String KEY_Y = "mY";

        /*
         * Member (state) fields
         */
        /** The drawable to use as the background of the animation canvas */
        private Bitmap mBackgroundImage;

        /**
         * Current height of the surface/canvas.
         * 
         * @see #setSurfaceSize
         */
        private int mCanvasHeight = 1;

        /**
         * Current width of the surface/canvas.
         * 
         * @see #setSurfaceSize
         */
        private int mCanvasWidth = 1;

        /** What to draw for the Lander when it has crashed */
        private final Drawable mCrashedImage;

        /**
         * Current difficulty -- amount of fuel, allowed angle, etc. Default is
         * MEDIUM.
         */
        private int mDifficulty;

        /** Velocity dx. */
        private double mDX;

        /** Velocity dy. */
        private double mDY;

        /** Is the engine burning? */
        private boolean mEngineFiring;

        /** What to draw for the Lander when the engine is firing */
        private final Drawable mFiringImage;

        /** Fuel remaining */
        private double mFuel;

        /** Allowed angle. */
        private int mGoalAngle;

        /** Allowed speed. */
        private int mGoalSpeed;

        /** Width of the landing pad. */
        private int mGoalWidth;

        /** X of the landing pad. */
        private int mGoalX;

        /** Message handler used by thread to interact with TextView */
        private final Handler mHandler;

        /**
         * Lander heading in degrees, with 0 up, 90 right. Kept in the range
         * 0..360.
         */
        private double mHeading;

        /** Pixel height of lander image. */
        private int mLanderHeight;

        /** What to draw for the Lander in its normal state */
        private final Drawable mLanderImage;

        /** Pixel width of lander image. */
        private int mLanderWidth;

        /** Used to figure out elapsed time between frames */
        private long mLastTime;

        /** Paint to draw the lines on screen. */
        private final Paint mLinePaint;

        /** "Bad" speed-too-high variant of the line color. */
        private final Paint mLinePaintBad;

        /** The state of the game. One of READY, RUNNING, PAUSE, LOSE, or WIN */
        private int mMode;

        /** Currently rotating, -1 left, 0 none, 1 right. */
        private int mRotating;

        /** Indicate whether the surface has been created & is ready to draw */
        private boolean mRun = false;

        /** Scratch rect object. */
        private final RectF mScratchRect;

        /** Handle to the surface manager object we interact with */
		private final SurfaceHolder mSurfaceHolder;

        /** Number of wins in a row. */
        private int mWinsInARow;

        /** X of lander center. */
        private double mX;

        /** Y of lander center. */
        private double mY;

		private boolean mInBackground = false;

		public LunarThread(final SurfaceHolder surfaceHolder,
				final Context context,
                final Handler handler) {
            // get handles to some important objects
            mSurfaceHolder = surfaceHolder;
            mHandler = handler;
            mContext = context;

            final Resources res = context.getResources();
            // cache handles to our key sprites & other drawables
            mLanderImage = context.getResources().getDrawable(
                    R.drawable.lander_plain);
            mFiringImage = context.getResources().getDrawable(
                    R.drawable.lander_firing);
            mCrashedImage = context.getResources().getDrawable(
                    R.drawable.lander_crashed);

            // load background image as a Bitmap instead of a Drawable b/c
            // we don't need to transform it and it's faster to draw this way
            mBackgroundImage = BitmapFactory.decodeResource(res,
                    R.drawable.earthrise);

            // Use the regular lander image as the model size for all sprites
            mLanderWidth = mLanderImage.getIntrinsicWidth();
            mLanderHeight = mLanderImage.getIntrinsicHeight();

            // Initialize paints for speedometer
            mLinePaint = new Paint();
            mLinePaint.setAntiAlias(true);
            mLinePaint.setARGB(255, 0, 255, 0);

            mLinePaintBad = new Paint();
            mLinePaintBad.setAntiAlias(true);
            mLinePaintBad.setARGB(255, 120, 180, 0);

            mScratchRect = new RectF(0, 0, 0, 0);

            mWinsInARow = 0;
            mDifficulty = DIFFICULTY_MEDIUM;

            // initial show-up of lander (not yet playing)
            mX = mLanderWidth;
            mY = mLanderHeight * 2;
            mFuel = PHYS_FUEL_INIT;
            mDX = 0;
            mDY = 0;
            mHeading = 0;
            mEngineFiring = true;
        }

        /**
         * Starts the game, setting parameters for the current difficulty.
         */
        public void doStart() {
            synchronized (mSurfaceHolder) {
                // First set the game for Medium difficulty
                mFuel = PHYS_FUEL_INIT;
                mEngineFiring = false;
                mGoalWidth = (int) (mLanderWidth * TARGET_WIDTH);
                mGoalSpeed = TARGET_SPEED;
                mGoalAngle = TARGET_ANGLE;
                int speedInit = PHYS_SPEED_INIT;

                // Adjust difficulty params for EASY/HARD
                if (mDifficulty == DIFFICULTY_EASY) {
                    mFuel = mFuel * 3 / 2;
                    mGoalWidth = mGoalWidth * 4 / 3;
                    mGoalSpeed = mGoalSpeed * 3 / 2;
                    mGoalAngle = mGoalAngle * 4 / 3;
                    speedInit = speedInit * 3 / 4;
                } else if (mDifficulty == DIFFICULTY_HARD) {
                    mFuel = mFuel * 7 / 8;
                    mGoalWidth = mGoalWidth * 3 / 4;
                    mGoalSpeed = mGoalSpeed * 7 / 8;
                    speedInit = speedInit * 4 / 3;
                }

                // pick a convenient initial location for the lander sprite
                mX = mCanvasWidth / 2;
                mY = mCanvasHeight - mLanderHeight / 2;

                // start with a little random motion
                mDY = Math.random() * -speedInit;
                mDX = Math.random() * 2 * speedInit - speedInit;
                mHeading = 0;

                // Figure initial spot for landing, not too near center
                while (true) {
                    mGoalX = (int) (Math.random() * (mCanvasWidth - mGoalWidth));
                    if (Math.abs(mGoalX - (mX - mLanderWidth / 2)) > mCanvasHeight / 6)
                        break;
                }

                mLastTime = System.currentTimeMillis() + 100;
                setState(STATE_RUNNING);
            }
        }

        /**
         * Pauses the physics update & animation.
         */
        public void pause() {
            synchronized (mSurfaceHolder) {
                if (mMode == STATE_RUNNING) setState(STATE_PAUSE);
            }
        }

        /**
         * Restores game state from the indicated Bundle. Typically called when
         * the Activity is being restored after having been previously
         * destroyed.
         * 
         * @param savedState Bundle containing the game state
         */
        public synchronized void restoreState(final Bundle savedState) {
            synchronized (mSurfaceHolder) {
                setState(STATE_PAUSE);
                mRotating = 0;
                mEngineFiring = false;

                mDifficulty = savedState.getInt(KEY_DIFFICULTY);
                mX = savedState.getDouble(KEY_X);
                mY = savedState.getDouble(KEY_Y);
                mDX = savedState.getDouble(KEY_DX);
                mDY = savedState.getDouble(KEY_DY);
                mHeading = savedState.getDouble(KEY_HEADING);

                mLanderWidth = savedState.getInt(KEY_LANDER_WIDTH);
                mLanderHeight = savedState.getInt(KEY_LANDER_HEIGHT);
                mGoalX = savedState.getInt(KEY_GOAL_X);
                mGoalSpeed = savedState.getInt(KEY_GOAL_SPEED);
                mGoalAngle = savedState.getInt(KEY_GOAL_ANGLE);
                mGoalWidth = savedState.getInt(KEY_GOAL_WIDTH);
                mWinsInARow = savedState.getInt(KEY_WINS);
                mFuel = savedState.getDouble(KEY_FUEL);
            }
        }

        @Override
        public void run() {
			while (mRun) {

				if (mInBackground) {
					try {
						sleep(100);
					} catch (final InterruptedException e) {
					}
				} else {
					Canvas c = null;
					try {
						c = mSurfaceHolder.lockCanvas(null);
						synchronized (mSurfaceHolder) {
							if (mMode == STATE_RUNNING)
								updatePhysics();
							doDraw(c);
						}
					} finally {
						// do this in a finally so that if an exception is
						// thrown
						// during the above, we don't leave the Surface in an
						// inconsistent state
						if (c != null) {
							mSurfaceHolder.unlockCanvasAndPost(c);
						}
					}
				}
            }
        }

        /**
         * Dump game state to the provided Bundle. Typically called when the
         * Activity is being suspended.
         * 
         * @return Bundle with this view's state
         */
        public Bundle saveState(final Bundle map) {
            synchronized (mSurfaceHolder) {
                if (map != null) {
                    map.putInt(KEY_DIFFICULTY, Integer.valueOf(mDifficulty));
                    map.putDouble(KEY_X, Double.valueOf(mX));
                    map.putDouble(KEY_Y, Double.valueOf(mY));
                    map.putDouble(KEY_DX, Double.valueOf(mDX));
                    map.putDouble(KEY_DY, Double.valueOf(mDY));
                    map.putDouble(KEY_HEADING, Double.valueOf(mHeading));
                    map.putInt(KEY_LANDER_WIDTH, Integer.valueOf(mLanderWidth));
                    map.putInt(KEY_LANDER_HEIGHT, Integer
                            .valueOf(mLanderHeight));
                    map.putInt(KEY_GOAL_X, Integer.valueOf(mGoalX));
                    map.putInt(KEY_GOAL_SPEED, Integer.valueOf(mGoalSpeed));
                    map.putInt(KEY_GOAL_ANGLE, Integer.valueOf(mGoalAngle));
                    map.putInt(KEY_GOAL_WIDTH, Integer.valueOf(mGoalWidth));
                    map.putInt(KEY_WINS, Integer.valueOf(mWinsInARow));
                    map.putDouble(KEY_FUEL, Double.valueOf(mFuel));
                }
            }
            return map;
        }

        /**
         * Sets the current difficulty.
         * 
         * @param difficulty
         */
        public void setDifficulty(final int difficulty) {
            synchronized (mSurfaceHolder) {
                mDifficulty = difficulty;
            }
        }

        /**
         * Sets if the engine is currently firing.
         */
        public void setFiring(final boolean firing) {
            synchronized (mSurfaceHolder) {
                mEngineFiring = firing;
            }
        }

        /**
         * Used to signal the thread whether it should be running or not.
         * Passing true allows the thread to run; passing false will shut it
         * down if it's already running. Calling start() after this was most
         * recently called with false will result in an immediate shutdown.
         * 
         * @param b true to run, false to shut down
         */
        public void setRunning(final boolean b) {
            mRun = b;
        }

        /**
         * Sets the game mode. That is, whether we are running, paused, in the
         * failure state, in the victory state, etc.
         * 
         * @see #setState(int, CharSequence)
         * @param mode one of the STATE_* constants
         */
        public void setState(final int mode) {
            synchronized (mSurfaceHolder) {
                setState(mode, null);
            }
        }

        /**
         * Sets the game mode. That is, whether we are running, paused, in the
         * failure state, in the victory state, etc.
         * 
         * @param mode one of the STATE_* constants
         * @param message string to add to screen or null
         */
        public void setState(final int mode, final CharSequence message) {
            /*
             * This method optionally can cause a text message to be displayed
             * to the user when the mode changes. Since the View that actually
             * renders that text is part of the main View hierarchy and not
             * owned by this thread, we can't touch the state of that View.
             * Instead we use a Message + Handler to relay commands to the main
             * thread, which updates the user-text View.
             */
            synchronized (mSurfaceHolder) {
                mMode = mode;

                if (mMode == STATE_RUNNING) {
                    final Message msg = mHandler.obtainMessage();
                    final Bundle b = new Bundle();
                    b.putString("text", "");
                    b.putInt("viz", View.INVISIBLE);
                    msg.setData(b);
                    mHandler.sendMessage(msg);
                } else {
                    mRotating = 0;
                    mEngineFiring = false;
                    final Resources res = mContext.getResources();
                    CharSequence str = "";
                    if (mMode == STATE_READY)
                        str = res.getText(R.string.mode_ready);
                    else if (mMode == STATE_PAUSE)
                        str = res.getText(R.string.mode_pause);
                    else if (mMode == STATE_LOSE)
                        str = res.getText(R.string.mode_lose);
                    else if (mMode == STATE_WIN)
                        str = res.getString(R.string.mode_win_prefix)
                                + mWinsInARow + " "
                                + res.getString(R.string.mode_win_suffix);

                    if (message != null) {
                        str = message + "\n" + str;
                    }

                    if (mMode == STATE_LOSE) mWinsInARow = 0;

                    final Message msg = mHandler.obtainMessage();
                    final Bundle b = new Bundle();
                    b.putString("text", str.toString());
                    b.putInt("viz", View.VISIBLE);
                    msg.setData(b);
                    mHandler.sendMessage(msg);
                }
            }
        }

		public boolean isInBackground() {
			return mInBackground;
		}

		public void setInBackground(boolean b) {
			synchronized (mSurfaceHolder) {
				this.mInBackground = b;
			}
		}

        /* Callback invoked when the surface dimensions change. */
        public void setSurfaceSize(final int width, final int height) {
            // synchronized to make sure these all change atomically
            synchronized (mSurfaceHolder) {
                mCanvasWidth = width;
                mCanvasHeight = height;

                // don't forget to resize the background image
				mBackgroundImage = Bitmap.createScaledBitmap(mBackgroundImage,
						width, height, true);
            }
        }

        /**
         * Resumes from a pause.
         */
		public void unpause() {
			// Move the real time clock up to now
            synchronized (mSurfaceHolder) {
                mLastTime = System.currentTimeMillis() + 100;
            }
            setState(STATE_RUNNING);
        }

        /**
         * Handles a key-down event.
         * 
         * @param keyCode the key that was pressed
         * @param msg the original event object
         * @return true
         */
        boolean doKeyDown(final int keyCode, final KeyEvent msg) {
            synchronized (mSurfaceHolder) {
                boolean okStart = false;
                if (keyCode == KeyEvent.KEYCODE_DPAD_UP) okStart = true;
                if (keyCode == KeyEvent.KEYCODE_DPAD_DOWN) okStart = true;
                if (keyCode == KeyEvent.KEYCODE_S) okStart = true;

                final boolean center = (keyCode == KeyEvent.KEYCODE_DPAD_UP);

                if (okStart
                        && (mMode == STATE_READY || mMode == STATE_LOSE || mMode == STATE_WIN)) {
                    // ready-to-start -> start
                    doStart();
                    return true;
                } else if (mMode == STATE_PAUSE && okStart) {
                    // paused -> running
                    unpause();
                    return true;
                } else if (mMode == STATE_RUNNING) {
                    // center/space -> fire
                    if (keyCode == KeyEvent.KEYCODE_DPAD_CENTER
                            || keyCode == KeyEvent.KEYCODE_SPACE) {
                        setFiring(true);
                        return true;
                        // left/q -> left
                    } else if (keyCode == KeyEvent.KEYCODE_DPAD_LEFT
                            || keyCode == KeyEvent.KEYCODE_Q) {
                        mRotating = -1;
                        return true;
                        // right/w -> right
                    } else if (keyCode == KeyEvent.KEYCODE_DPAD_RIGHT
                            || keyCode == KeyEvent.KEYCODE_W) {
                        mRotating = 1;
                        return true;
                        // up -> pause
                    } else if (keyCode == KeyEvent.KEYCODE_DPAD_UP) {
                        pause();
                        return true;
                    }
                }

                return false;
            }
        }

        /**
         * Handles a key-up event.
         * 
         * @param keyCode the key that was pressed
         * @param msg the original event object
         * @return true if the key was handled and consumed, or else false
         */
        boolean doKeyUp(final int keyCode, final KeyEvent msg) {
            boolean handled = false;

            synchronized (mSurfaceHolder) {
                if (mMode == STATE_RUNNING) {
                    if (keyCode == KeyEvent.KEYCODE_DPAD_CENTER
                            || keyCode == KeyEvent.KEYCODE_SPACE) {
                        setFiring(false);
                        handled = true;
                    } else if (keyCode == KeyEvent.KEYCODE_DPAD_LEFT
                            || keyCode == KeyEvent.KEYCODE_Q
                            || keyCode == KeyEvent.KEYCODE_DPAD_RIGHT
                            || keyCode == KeyEvent.KEYCODE_W) {
                        mRotating = 0;
                        handled = true;
                    }
                }
            }

            return handled;
        }

        /**
         * Draws the ship, fuel/speed bars, and background to the provided
         * Canvas.
         */
        private void doDraw(final Canvas canvas) {
            // Draw the background image. Operations on the Canvas accumulate
            // so this is like clearing the screen.
            canvas.drawBitmap(mBackgroundImage, 0, 0, null);

            final int yTop = mCanvasHeight - ((int) mY + mLanderHeight / 2);
            final int xLeft = (int) mX - mLanderWidth / 2;

            // Draw the fuel gauge
            final int fuelWidth = (int) (UI_BAR * mFuel / PHYS_FUEL_MAX);
            mScratchRect.set(4, 4, 4 + fuelWidth, 4 + UI_BAR_HEIGHT);
            canvas.drawRect(mScratchRect, mLinePaint);

            // Draw the speed gauge, with a two-tone effect
            final double speed = Math.sqrt(mDX * mDX + mDY * mDY);
            final int speedWidth = (int) (UI_BAR * speed / PHYS_SPEED_MAX);

            if (speed <= mGoalSpeed) {
                mScratchRect.set(4 + UI_BAR + 4, 4,
                        4 + UI_BAR + 4 + speedWidth, 4 + UI_BAR_HEIGHT);
                canvas.drawRect(mScratchRect, mLinePaint);
            } else {
                // Draw the bad color in back, with the good color in front of
                // it
                mScratchRect.set(4 + UI_BAR + 4, 4,
                        4 + UI_BAR + 4 + speedWidth, 4 + UI_BAR_HEIGHT);
                canvas.drawRect(mScratchRect, mLinePaintBad);
                final int goalWidth = (UI_BAR * mGoalSpeed / PHYS_SPEED_MAX);
                mScratchRect.set(4 + UI_BAR + 4, 4, 4 + UI_BAR + 4 + goalWidth,
                        4 + UI_BAR_HEIGHT);
                canvas.drawRect(mScratchRect, mLinePaint);
            }

            // Draw the landing pad
            canvas.drawLine(mGoalX, 1 + mCanvasHeight - TARGET_PAD_HEIGHT,
                    mGoalX + mGoalWidth, 1 + mCanvasHeight - TARGET_PAD_HEIGHT,
                    mLinePaint);


            // Draw the ship with its current rotation
            canvas.save();
            canvas.rotate((float) mHeading, (float) mX, mCanvasHeight
                    - (float) mY);
            if (mMode == STATE_LOSE) {
                mCrashedImage.setBounds(xLeft, yTop, xLeft + mLanderWidth, yTop
                        + mLanderHeight);
                mCrashedImage.draw(canvas);
            } else if (mEngineFiring) {
                mFiringImage.setBounds(xLeft, yTop, xLeft + mLanderWidth, yTop
                        + mLanderHeight);
                mFiringImage.draw(canvas);
            } else {
                mLanderImage.setBounds(xLeft, yTop, xLeft + mLanderWidth, yTop
                        + mLanderHeight);
                mLanderImage.draw(canvas);
            }
            canvas.restore();
        }

        /**
         * Figures the lander state (x, y, fuel, ...) based on the passage of
         * realtime. Does not invalidate(). Called at the start of draw().
         * Detects the end-of-game and sets the UI to the next state.
         */
        private void updatePhysics() {
            final long now = System.currentTimeMillis();

            // Do nothing if mLastTime is in the future.
            // This allows the game-start to delay the start of the physics
            // by 100ms or whatever.
            if (mLastTime > now) return;

            final double elapsed = (now - mLastTime) / 1000.0;

            // mRotating -- update heading
            if (mRotating != 0) {
                mHeading += mRotating * (PHYS_SLEW_SEC * elapsed);

                // Bring things back into the range 0..360
                if (mHeading < 0)
                    mHeading += 360;
                else if (mHeading >= 360) mHeading -= 360;
            }

            // Base accelerations -- 0 for x, gravity for y
            double ddx = 0.0;
            double ddy = -PHYS_DOWN_ACCEL_SEC * elapsed;

            if (mEngineFiring) {
                // taking 0 as up, 90 as to the right
                // cos(deg) is ddy component, sin(deg) is ddx component
                double elapsedFiring = elapsed;
                double fuelUsed = elapsedFiring * PHYS_FUEL_SEC;

                // tricky case where we run out of fuel partway through the
                // elapsed
                if (fuelUsed > mFuel) {
                    elapsedFiring = mFuel / fuelUsed * elapsed;
                    fuelUsed = mFuel;

                    // Oddball case where we adjust the "control" from here
                    mEngineFiring = false;
                }

                mFuel -= fuelUsed;

                // have this much acceleration from the engine
                final double accel = PHYS_FIRE_ACCEL_SEC * elapsedFiring;

                final double radians = 2 * Math.PI * mHeading / 360;
                ddx = Math.sin(radians) * accel;
                ddy += Math.cos(radians) * accel;
            }

            final double dxOld = mDX;
            final double dyOld = mDY;

            // figure speeds for the end of the period
            mDX += ddx;
            mDY += ddy;

            // figure position based on average speed during the period
            mX += elapsed * (mDX + dxOld) / 2;
            mY += elapsed * (mDY + dyOld) / 2;

            mLastTime = now;

            // Evaluate if we have landed ... stop the game
            final double yLowerBound = TARGET_PAD_HEIGHT + mLanderHeight / 2
                    - TARGET_BOTTOM_PADDING;
            if (mY <= yLowerBound) {
                mY = yLowerBound;

                int result = STATE_LOSE;
                CharSequence message = "";
                final Resources res = mContext.getResources();
                final double speed = Math.sqrt(mDX * mDX + mDY * mDY);
                final boolean onGoal = (mGoalX <= mX - mLanderWidth / 2 && mX
                        + mLanderWidth / 2 <= mGoalX + mGoalWidth);

                // "Hyperspace" win -- upside down, going fast,
                // puts you back at the top.
                if (onGoal && Math.abs(mHeading - 180) < mGoalAngle
                        && speed > PHYS_SPEED_HYPERSPACE) {
                    result = STATE_WIN;
                    mWinsInARow++;
                    doStart();

                    return;
                    // Oddball case: this case does a return, all other cases
                    // fall through to setMode() below.
                } else if (!onGoal) {
                    message = res.getText(R.string.message_off_pad);
                } else if (!(mHeading <= mGoalAngle || mHeading >= 360 - mGoalAngle)) {
                    message = res.getText(R.string.message_bad_angle);
                } else if (speed > mGoalSpeed) {
                    message = res.getText(R.string.message_too_fast);
                } else {
                    result = STATE_WIN;
                    mWinsInARow++;
                }

                setState(result, message);
            }
        }
    }

    /** Handle to the application context, used to e.g. fetch Drawables. */
    private Context mContext;

    /** Pointer to the text view to display "Paused.." etc. */
    private TextView mStatusText;

    /** The thread that actually draws the animation */
    private final LunarThread thread;

    public LunarView(final Context context, final AttributeSet attrs) {
        super(context, attrs);

        // register our interest in hearing about changes to our surface
        final SurfaceHolder holder = getHolder();
        holder.addCallback(this);

        // create thread only; it's started in surfaceCreated()
        thread = new LunarThread(holder, context, new Handler() {
            @Override
            public void handleMessage(final Message m) {
                mStatusText.setVisibility(m.getData().getInt("viz"));
                mStatusText.setText(m.getData().getString("text"));
            }
        });

        setFocusable(true); // make sure we get key events
    }

    /**
     * Fetches the animation thread corresponding to this LunarView.
     * 
     * @return the animation thread
     */
    public LunarThread getThread() {
        return thread;
    }

    /**
     * Standard override to get key-press events.
     */
    @Override
    public boolean onKeyDown(final int keyCode, final KeyEvent msg) {
        return thread.doKeyDown(keyCode, msg);
    }

    /**
     * Standard override for key-up. We actually care about these, so we can
     * turn off the engine or stop rotating.
     */
    @Override
    public boolean onKeyUp(final int keyCode, final KeyEvent msg) {
        return thread.doKeyUp(keyCode, msg);
    }

    /**
     * Standard window-focus override. Notice focus lost so we can pause on
     * focus lost. e.g. user switches to take a call.
     */
    @Override
    public void onWindowFocusChanged(final boolean hasWindowFocus) {
        if (!hasWindowFocus) thread.pause();
    }

    /**
     * Installs a pointer to the text view used for messages.
     */
    public void setTextView(final TextView textView) {
        mStatusText = textView;
    }

    /* Callback invoked when the surface dimensions change. */
    @Override
	public void surfaceChanged(final SurfaceHolder holder, final int format, final int width,
            final int height) {
        thread.setSurfaceSize(width, height);
    }

    /*
     * Callback invoked when the Surface has been created and is ready to be
     * used.
     */
    @Override
	public void surfaceCreated(final SurfaceHolder holder) {
		if (thread.isInBackground()) {
			thread.setInBackground(false);
		} else {
			thread.setRunning(true);
			thread.start();
		}
    }

    /*
     * Callback invoked when the Surface has been destroyed and must no longer
     * be touched. WARNING: after this method returns, the Surface/Canvas must
     * never be touched again!
     */
    @Override
	public void surfaceDestroyed(final SurfaceHolder holder) {
		thread.pause();
		thread.setInBackground(true);
    }
}

Added mInBackground variavle.
Added isInBackground() and setInBackground().
Changed run() to check isInBackground() and sleep(100) if it is true.
Changed surfaceDestroyed() so that it does not stop the thread.
Changed surfaceCreated() to check isInBackground().