java - 2D Array grid on drawing canvas

Question

I am writing an pixel art app that paints images the user draws on the screen to pixel look. There are two ways to approach it. Either pixelate the image after saving or have a grid before hand so the user draws the pixel image. I don't find anything on the later method. So my issue with the first is drawing a grid where if a cell is touched I would change the color of it. I tried drawing rectangles on a canvas but that was pointless because i couldn't control the cells.

I was thinking about nested for loops that creates a bitmap at each cell?

Answer 1

The following is just a simple, illustrative example. It is not optimized, implements no exception handling, etc.

public class PixelGridView extends View {
    private int numColumns, numRows;
    private int cellWidth, cellHeight;
    private Paint blackPaint = new Paint();
    private boolean[][] cellChecked;

    public PixelGridView(Context context) {
        this(context, null);
    }

    public PixelGridView(Context context, AttributeSet attrs) {
        super(context, attrs);
        blackPaint.setStyle(Paint.Style.FILL_AND_STROKE);
    }

    public void setNumColumns(int numColumns) {
        this.numColumns = numColumns;
        calculateDimensions();
    }

    public int getNumColumns() {
        return numColumns;
    }

    public void setNumRows(int numRows) {
        this.numRows = numRows;
        calculateDimensions();
    }

    public int getNumRows() {
        return numRows;
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        calculateDimensions();
    }

    private void calculateDimensions() {
        if (numColumns < 1 || numRows < 1) {
            return;
        }

        cellWidth = getWidth() / numColumns;
        cellHeight = getHeight() / numRows;

        cellChecked = new boolean[numColumns][numRows];

        invalidate();
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawColor(Color.WHITE);

        if (numColumns == 0 || numRows == 0) {
            return;
        }

        int width = getWidth();
        int height = getHeight();

        for (int i = 0; i < numColumns; i++) {
            for (int j = 0; j < numRows; j++) {
                if (cellChecked[i][j]) {

                    canvas.drawRect(i * cellWidth, j * cellHeight,
                                    (i + 1) * cellWidth, (j + 1) * cellHeight,
                                    blackPaint);
                }
            }
        }

        for (int i = 1; i < numColumns; i++) {
            canvas.drawLine(i * cellWidth, 0, i * cellWidth, height, blackPaint);
        }

        for (int i = 1; i < numRows; i++) {
            canvas.drawLine(0, i * cellHeight, width, i * cellHeight, blackPaint);
        }
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        if (event.getAction() == MotionEvent.ACTION_DOWN) {
            int column = (int)(event.getX() / cellWidth);
            int row = (int)(event.getY() / cellHeight);

            cellChecked[column][row] = !cellChecked[column][row];
            invalidate();
        }

        return true;
    }
}

Here's a simple Activity for demonstration:

public class MainActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        PixelGridView pixelGrid = new PixelGridView(this);
        pixelGrid.setNumColumns(4);
        pixelGrid.setNumRows(6);

        setContentView(pixelGrid);
    }
}