Question

我目前正在关注此answer。从这个答案可以看出，如果触摸了像素/网格，它将被检查。但是，我想检查一个网格作为起点，并在主要活动中设置控制/按钮以移动起点。例如，如果用户单击“前进”按钮，则起点将向前移动，检查新网格并将前一点设置为白色。现在我在moveForward（）方法中的checkedCell [column] [row]上得到nullpointerexception

所以，这是我的代码：

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 == 0 || numRows == 0)
        return;

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

    cellChecked = new boolean[numColumns][numRows];

    invalidate();
}

@Override
protected void onDraw(Canvas canvas)
{
    super.onDraw(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)
        return true;

    int column = (int)(event.getX() / cellWidth);
    int row = (int)(event.getY() / cellHeight);

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

    return true;
}
public void moveForward() {
//set column and row to be checked at [1][1] upon button click on mainactivity
    int column = 1;
    int row = 1;
    cellChecked[column][row] = !cellChecked[column][row];
    invalidate();
}
}

这是我调用moveForward（）方法的活动

public class MainActivity extends Activity {
PixelGridView pgv;
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_main);

    pgv = new PixelGridView(this);
    pgv.setNumColumns(20);
    pgv.setNumRows(15);

}

public void onBtnForwardPressed(View view) {
        pgv.moveForward();
    }
}

以及日志：

05-10 00:36:33.388: E/AndroidRuntime(8414): Caused by: java.lang.NullPointerException
05-10 00:36:33.388: E/AndroidRuntime(8414):     at com.test.PixelGridView.moveForward(PixelGridView.java:131)
05-10 00:36:33.388: E/AndroidRuntime(8414):     at com.test.MainActivity.onBtnForwardPressed(MainActivity.java:281)

moveForward（）方法移动机器人：

if (currentAngle == 0) {
        for (int i = 0; i < numColumns; i++) {
            for (int j = 0; j < numRows; j++) {
                if (cellChecked[i][j]) {
                    column1 = i;
                    row1 = j;

                }
            }
        } 
        cellChecked[column1][row1] = !cellChecked[column1][row1];           
        cellChecked[column1+1][row1] = !cellChecked[column1+1][row1];
        invalidate();
    }

Answer 1

您使用PixelGridView创建的new对象不在活动的布局中，因为您尚未添加它。如果您已在PixelGridView布局中定义activity_main，则应使用pgv方法将其分配给findViewById()。如果您打算以编程方式创建它，那么您应该使用addContentView()方法将其添加到Activity的布局中。

Android PixelGrid OnDraw检查/跟踪当前位置

1 个答案: