java画布和位图变量设置为null

Question

我正在为安卓开发一个简单的绘图应用程序。这个应用程序的结构很简单——它有一个包含文本的MainActivity类，还有一个按钮，用于为第二个类充气，这是一个包含图形的自定义视图

主类的代码如下：

public class MainActivity extends AppCompatActivity {

DrawingView dv;
ViewGroup parent;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    parent = (ViewGroup) findViewById(R.id.activity_main);
}

public void crossClass(View v) {
    //setContentView(R.layout.sample_drawing_view);
    dv = new DrawingView(getApplicationContext());
    View viw = getLayoutInflater().inflate(R.layout.sample_drawing_view, parent);
    System.out.println("Attempted to Inflate DV");

}


public void refreshCallback(View v){
    dv.refresh();
}

}

该类最重要的部分是创建DrawingView的实例，以便在以后按下按钮时调用该类中的方法

方法（dv.refresh（））很简单

public void refresh(){
    System.out.println("Refresh Command Sent");
    drawCanvas.drawColor(Color.WHITE);
    invalidate();
}

它所做的只是在画布上画白色

这给了我一个错误：

java.lang.NullPointerException: Attempt to invoke virtual method void 安卓.graphics.Canvas.drawColor(int) on a null object reference

我使用调试器进行了一些检查，发现在调用该方法时，Canvas和Bitmap变量为null，Picture Here.

然而，当我将断点设置为类创建之后，变量就不是空的Picture Here

我将把整个工程视图类代码放在下面，但我不知道是什么原因导致它们变为空

public class DrawingView extends View
{
//Constant
public int Colour = 0xFF000000;
public int width;
public int height;

//Drawing Var
public Canvas drawCanvas;
public Bitmap canvasBitmap;
public Path drawPath;
public Paint drawPaint;
public Paint canvasPaint;


public DrawingView(Context context){
    super(context);
    get_dim(context);
    setupDrawing();
    setupUI();
}

public DrawingView(Context context, AttributeSet attributeSet){
    super(context, attributeSet);
    setupDrawing();
    setupUI();
}

public void get_dim(Context context){
    WindowManager wm = (WindowManager)   context.getSystemService(Context.WINDOW_SERVICE);
    Display display = wm.getDefaultDisplay();
    Point size = new Point();
    display.getSize(size);
    width = size.x;
    height = size.y;

}

public void setupUI() {

}

public void setupDrawing() {
    drawPath = new Path();
    drawPaint = new Paint();

    drawPaint.setColor(Colour);

    drawPaint.setAntiAlias(true);
    drawPaint.setStrokeWidth(20);
    drawPaint.setStyle(Paint.Style.STROKE);
    drawPaint.setStrokeJoin(Paint.Join.ROUND);
    drawPaint.setStrokeCap(Paint.Cap.ROUND);

    canvasPaint = new Paint(Paint.DITHER_FLAG);
}

@Override
protected void onDraw(Canvas canvas){
    canvas.drawBitmap(canvasBitmap, 0, 0, canvasPaint);
    canvas.drawPath(drawPath, drawPaint);
}

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh){
    super.onSizeChanged(w, h, oldw, oldh);
    System.out.println("ON SIZE CHANGED CALLED");
    canvasBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
    drawCanvas = new Canvas(canvasBitmap);
}

public void handelAction(int action, int x, int y){
    switch(action) {
        case 0:
            drawPath.moveTo(x, y);
            break;
        case 2:
            drawPath.lineTo(x, y);
            break;
        case 1:
            drawCanvas.drawPath(drawPath, drawPaint);
            break;
    }
    {
        invalidate();
    }
}

@Override
public boolean onTouchEvent(MotionEvent event){
    float x = event.getX();
    float y = event.getY();

    System.out.println(event.getAction());
    handelAction(event.getAction(), (int) x, (int) y);
    return true;
}

public void refresh(){
    System.out.println("Refresh Command Sent");
    drawCanvas.drawColor(Color.WHITE);
    invalidate();
}

}