修改Android Snake示例的奇怪问题

Question

嘿所有，这对我来说是一个非常复杂的问题，我现在一直想弄清楚。即使解释它对我来说有点困难，但我会试一试。

我正在使用Snake Android示例的内脏版本。我几乎是逐字使用TileView类，我只是试图在屏幕上显示几个图块。而不是SnakeView类我使用的是GameView类，其中我只包含了我认为在屏幕上显示图块所需的代码。这是班级

    public class GameView extends TileView {

    /**
     * Labels for the drawables that will be loaded into the TileView class
     */
    private static final int RED_STAR = 1;
    private static final int YELLOW_STAR = 2;
    private static final int GREEN_STAR = 3;

    /**
     * mMoveDelay: number of milliseconds between animations. 
     */
    private long mMoveDelay = 600;
    /**
     * mLastMove: tracks the absolute time when the last animation happened.
     */
    private long mLastMove;

    /**
     * Create a simple handler that we can use to cause animation to happen.  We
     * set ourselves as a target and we can use the sleep()
     * function to cause an update/invalidate to occur at a later date.
     */
    private RefreshHandler mRedrawHandler = new RefreshHandler();

    class RefreshHandler extends Handler {

        @Override
        public void handleMessage(Message msg) {
            GameView.this.update();
            GameView.this.invalidate();
        }

        public void sleep(long delayMillis) {
            this.removeMessages(0);
            sendMessageDelayed(obtainMessage(0), delayMillis);
        }
    };

    /**
     * Constructs a GameView based on inflation from XML
     * 
     * @param context
     * @param attrs
     */
    public GameView(Context context, AttributeSet attrs) {
        super(context, attrs);
        initGameView();
   }

    public GameView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        initGameView();
    }

    private void initGameView() {
        setFocusable(true);

        Resources r = this.getContext().getResources();

        resetTiles(4);
        loadTile(RED_STAR, r.getDrawable(R.drawable.redstar));
        loadTile(YELLOW_STAR, r.getDrawable(R.drawable.yellowstar));
        loadTile(GREEN_STAR, r.getDrawable(R.drawable.greenstar));

    }

    public void initNewGame() {       
        // set the move delay. This tells the update method how often it should
        // refresh the screen.
        mMoveDelay = 600;

        update();
    }

    public void update() {
        long now = System.currentTimeMillis();

        if (now - mLastMove > mMoveDelay) {
            // clear any tiles on the screen
            clearTiles();

            updateWalls();

            // draws the tiles storred in mCellularArray
            //updateCellularArray();

            mLastMove = now;
        }
        mRedrawHandler.sleep(mMoveDelay);

    }

    private void updateWalls() {
        for (int x = 0; x < mXTileCount; x++) {
            setTile(GREEN_STAR, x, 0);
            setTile(GREEN_STAR, x, mYTileCount - 1);
        }
        for (int y = 1; y < mYTileCount - 1; y++) {
            setTile(GREEN_STAR, 0, y);
            setTile(GREEN_STAR, mXTileCount - 1, y);
        }
    }

    private void updateCellularArray() {
        setTile(YELLOW_STAR, 6, 7);
    }
}

现在发生的事情是，当调用updateWalls（）方法时，tile被放置在View中并且出现如下：

现在，当我取消注释updateCellularArray()方法并注释掉updateWalls()程序强制关闭并抛出NullPointerException时。经过一些调试后，我发现当调用updateCellular数组方法时，TileView类中的mTileGrid数组没有被初始化，而是在调用updateWalls()时。我完全不知道为什么会这样。如果我用简单的setTile(GREEN_STAR, 3, 3);替换for循环，它仍然会强制关闭。

这是我正在使用的TileView类（同样，这与Android SDK附带的Snake示例中的相同）：

/**
 * TileView: a View-variant designed for handling arrays of "icons" or other
 * drawables.
 * 
 */
public class TileView extends View {

    /**
     * Parameters controlling the size of the tiles and their range within view.
     * Width/Height are in pixels, and Drawables will be scaled to fit to these
     * dimensions. X/Y Tile Counts are the number of tiles that will be drawn.
     */

    protected static int mTileSize;

    protected static int mXTileCount;
    protected static int mYTileCount;

    private static int mXOffset;
    private static int mYOffset;


    /**
     * A hash that maps integer handles specified by the subclasser to the
     * drawable that will be used for that reference
     */
    private Bitmap[] mTileArray; 

    /**
     * A two-dimensional array of integers in which the number represents the
     * index of the tile that should be drawn at that locations
     */
    private int[][] mTileGrid;

    private final Paint mPaint = new Paint();

    public TileView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);

        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.TileView);

        mTileSize = a.getInt(R.styleable.TileView_tileSize, 12);

        a.recycle();
    }

    public TileView(Context context, AttributeSet attrs) {
        super(context, attrs);

        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.TileView);

        mTileSize = a.getInt(R.styleable.TileView_tileSize, 12);

        a.recycle();
    }



    /**
     * Rests the internal array of Bitmaps used for drawing tiles, and
     * sets the maximum index of tiles to be inserted
     * 
     * @param tilecount
     */

    public void resetTiles(int tilecount) {
     mTileArray = new Bitmap[tilecount];
    }


    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        mXTileCount = (int) Math.floor(w / mTileSize);
        mYTileCount = (int) Math.floor(h / mTileSize);

        mXOffset = ((w - (mTileSize * mXTileCount)) / 2);
        mYOffset = ((h - (mTileSize * mYTileCount)) / 2);

        Log.d("Tomek", "TileGrid array dimensions are: " + mXTileCount + "," + mYTileCount);
        mTileGrid = new int[mXTileCount][mYTileCount];
        clearTiles();
    }

    /**
     * Function to set the specified Drawable as the tile for a particular
     * integer key.
     * 
     * @param key
     * @param tile
     */
    public void loadTile(int key, Drawable tile) {
        Bitmap bitmap = Bitmap.createBitmap(mTileSize, mTileSize, Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bitmap);
        tile.setBounds(0, 0, mTileSize, mTileSize);
        tile.draw(canvas);

        mTileArray[key] = bitmap;
    }

    /**
     * Resets all tiles to 0 (empty)
     * 
     */
    public void clearTiles() {
        for (int x = 0; x < mXTileCount; x++) {
            for (int y = 0; y < mYTileCount; y++) {
                setTile(0, x, y);
            }
        }
    }

    /**
     * Used to indicate that a particular tile (set with loadTile and referenced
     * by an integer) should be drawn at the given x/y coordinates during the
     * next invalidate/draw cycle.
     * 
     * @param tileindex
     * @param x
     * @param y
     */
    public void setTile(int tileindex, int x, int y) {
     //Log.d("Tomek", "Attempting to set tile: " + x + "," + y);
     //Log.d("Tomek", "The current value at " + x + "," + y + " is " + mTileGrid[x][y]);
     //Log.d("Tomek", "It will be changed to " + tileindex);
        mTileGrid[x][y] = tileindex;
    }


    @Override
    public void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        for (int x = 0; x < mXTileCount; x += 1) {
            for (int y = 0; y < mYTileCount; y += 1) {
                if (mTileGrid[x][y] > 0) {
                    canvas.drawBitmap(mTileArray[mTileGrid[x][y]], 
                      mXOffset + x * mTileSize,
                      mYOffset + y * mTileSize,
                      mPaint);
                }
            }
        }

    }

}

很抱歉发布所有这些代码。任何帮助将不胜感激。

非常感谢，

托梅克

编辑：简化的GameView类

编辑2：将updateCellularArray()方法修改为以下内容后正好：

private void updateCellularArray() {
    for (int x = 0; x < mXTileCount; x++) {
        setTile(GREEN_STAR, 12, 12);
    }
}

它最终把瓷砖放在我想要的地方...... alt text http://img188.imageshack.us/img188/1148/device2z.png

我开始怀疑它是否与RefreshHandler有关。可能是在GameView类的update方法中调用RedrawHandler的sleep方法时。我不太确定它是如何工作的，所以我会试着玩它，看看我能想出什么。

Answer 1

我忘了实例化mLastMove变量。将声明修改为：

private long mLastMove = System.currentTimeMillis();

修复了问题。

似乎与RefreshHandler一起使用的方法虽然很聪明，但已经过时了？发现这篇文章：

http://www.mail-archive.com/android-beginners@googlegroups.com/msg07352.html

托梅克