java - 我添加到自定义 Android View 的所有方法都会导致 NullPointerException

Question

我已经尝试解决这个问题大约一周了，但我在这里查看了很多问题和答案，但我无法弄清楚:

我有一个自定义 View 类(下面的 DoodleView)，允许用户用手指在屏幕上绘图。只要我不调用添加到 View 中的任何方法来允许更改参数(绘画颜色、画笔大小等)， View 就可以正常工作。我相信我正确调用了方法，并且对象已初始化。我在这里缺少什么？

DoodleView代码(自定义 View 类):

package fibrs.coloringbook;

import android.R.bool;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;

public class DoodleView extends View implements DoodleInterface {

    private Bitmap  mBitmap;
    private Canvas  mCanvas;
    private Path    mPath;
    private Paint   mBitmapPaint;
    public Paint mPaint;
    Context context;
    //private boolean first;

    public DoodleView(Context c) {
        super(c);
        context=c;
        mPath = new Path();
        mBitmapPaint = new Paint(Paint.DITHER_FLAG);
        mPaint = new Paint();

        mPaint.setAntiAlias(true);
        mPaint.setDither(true);
        mPaint.setColor(0xFFFF0000);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeJoin(Paint.Join.ROUND);
        mPaint.setStrokeCap(Paint.Cap.ROUND);
        mPaint.setStrokeWidth(20); 
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
        canvas.drawPath(mPath, mPaint);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
            mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
            mCanvas = new Canvas(mBitmap);
    }

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;


    // This is one of the methods that throws nullpointerexception everytime
        public void setColor(int color){
        mPaint.setColor(color);
    }

    private void touch_start(float x, float y) {
        //showDialog(); 
        mPath.reset();
        mPath.moveTo(x, y);
        mX = x;
        mY = y;
    }
    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            mPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2);
            mX = x;
            mY = y;
        }
    }
    private void touch_up() {
        mPath.lineTo(mX, mY);
        // commit the path to our offscreen
        mCanvas.drawPath(mPath, mPaint);
        // kill this so we don't double draw
        mPath.reset();
        mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SCREEN));
        //mPaint.setMaskFilter(null);
    }

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

    switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            touch_start(x, y);
            invalidate();
            break;
        case MotionEvent.ACTION_MOVE:

            touch_move(x, y);
            invalidate();
            break;
        case MotionEvent.ACTION_UP:
            touch_up();
            invalidate();
            break;
    }
    return true;
    } 
}

Activity :

package fibrs.coloringbook;

//Imports

public class ColorTimeActivity extends Activity {

    private ImageView mImageView3;
    String filePath;
    Bitmap bitmapImage = null;
    DoodleView mDV;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,WindowManager.LayoutParams.FLAG_FULLSCREEN);
        setContentView(R.layout.activity_color_time);

        Log.v(getString(R.string.app_name), "Layout set (and inflated?).");

        mDV = new DoodleView(this);
        mDV = (DoodleView) findViewById(R.id.canvas);

        mImageView3 = (ImageView) findViewById(R.id.imageView3);
        getPictureIntent(1);

    };

        // Other items omitted

    public void colorChoser(int which){
        Toast toast = Toast.makeText(this, Integer.toString(which), Toast.LENGTH_SHORT);
        toast.show();
        int color = 0xFFFFFF;
        switch (which){
        case 1: {
            color = 0xFFFF0000; //red
        } break;
        case 2: {
            color = 0xFFFFA500; //orange
        } break;
        case 3: {
            color = 0xFFFFFF00; //yellow
        } break;
        case 4: {
            color = 0xFF00FF00; //green
        } break;
        case 5: {
            color = 0xFF0000FF; //blue
        } break;
        case 6: {
            color = 0xFF800080; //purple
        } break;
        case 7: {
            color = 0xFF000000; //black
        } break;
        case 8: {
            color = 0xFFFFFFFF; //white
        } break;
        }

        TextView textElement = (TextView) findViewById(R.id.color_button);
        textElement.setTextColor(color);
        mDV.setColor(color);
        //mDV.mPaint.setColor(color); //This also causes a nullpointerexception
    }

预先感谢您的所有帮助。如果格式不正确，我深表歉意。

Answer 1

请发布异常堆栈跟踪。

但是，在这里:

mDV = new DoodleView(this);
mDV = (DoodleView) findViewById(R.id.canvas);

您正在创建 DoodleView，将引用放入 mDV 中，然后将引用替换为 findViewById() 返回的内容。这可能是null。尝试在 null 上调用方法将导致 NPE。

相反，可以将 DoodleView 添加到 XML 中的 activity_color_time 布局，或者在代码中调用 addView() 将其作为子项附加到布局中的一些 View 组。