在ImageView /位图上绘制画布

Question

I've been trying to draw a Canvas on an ImageView but after creating a bitmap I see no Image on my screen. The only thing I get is the small circle I've drawn in the onDraw Method. I want to have the Canvas on the ImageView .

I've got the following code:

public class MainActivity extends AppCompatActivity {

  ImageView imageView;
  Bitmap myBitmap;
  Paint paint;
  Button button;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    button = (Button) findViewById(R.id.button);

  }

  public void onClickButton(View view) {

    View v = new DrawCanvas(getApplicationContext());
    Bitmap bitmap = Bitmap.createBitmap(48, 48, Bitmap.Config.ARGB_8888);
    Canvas canvas = new Canvas(bitmap);

    v.draw(canvas);
    imageView = (ImageView) findViewById(R.id.testpicture);
    imageView.setImageBitmap(bitmap);
  }
}

and

public class DrawCanvas extends View {

    Paint paint = new Paint();

    public DrawCanvas(Context context) {
      super(context);
    }

    public DrawCanvas(Context context, AttributeSet attrs) {
      super(context, attrs);
    }


    @Override
    public void onDraw(Canvas canvas) {
      super.onDraw(canvas);
      paint.setColor(Color.RED);

      canvas.drawCircle(25, 25, 5, paint);
    }
}

Thanks in advance!

Answer 1

First, you're overcomplicating things. You don't need, or want, a view to do what you're doing. If you want to draw a circle, do it directly to the canvas by using canvas.drawCircle in your onClick.

Second, alling setImageBitmap overwrites any previous bitmap you set. If you want this to show over the previous image, you have 2 options:

1)Use two image views on top of one another, and set the top image to the bitmap made on the canvas (to make this work you may need to make sure the background color on the bitmap is transparent).

2)Create a new view subclassed off ImageView that displays 2 bitmaps by overriding onDraw.

The first approach is probably easier.