How to make any view to draw to canvas?

Question

I have a short question:

Suppose I have a (mutable) bitmap that I need to modify (add images, texts, etc...) .

Instead of messing around with many special classes for drawing to the canvas (paint, canvas, matrices and so on), I was thinking why not use the built in classes of Android for this task and only if i need really customized operations i could still use the canvas ?

So, for example, in order to show any kind of view (that has no parent, of course) on the bitmap, I could call the next function :

public void drawViewToBitmap(Bitmap b, View v, Rect rect) {
    Canvas c = new Canvas(b);
    // <= use rect to let the view to draw only into this boundary inside the bitmap
    view.draw(c);
}

Is such a thing possible? maybe that's even the way that it works behind the scenes?

what should i write in the part between the drawing and the creation of the canvas?

---

EDIT: i've tried the next code , but it didn't work:

public void drawFromViewToCanvas(final View view, final Rect rect, final Canvas canvas) {
    final int widthSpec = View.MeasureSpec.makeMeasureSpec(rect.width(), View.MeasureSpec.EXACTLY);
    final int heightSpec = View.MeasureSpec.makeMeasureSpec(rect.height(), View.MeasureSpec.EXACTLY);
    view.measure(widthSpec, heightSpec);
    // Lay the view out with the known dimensions
    view.layout(0, 0, rect.width(), rect.height());
    // Translate the canvas so the view is drawn at the proper coordinates
    canvas.save();
    canvas.translate(rect.left, rect.top);
    // Draw the View and clear the translation
    view.draw(canvas);
    canvas.restore();
}

example of usage:

final int imageSize = 50;
rect = new Rect(35, 344 , 35 + imageSize, 344  + imageSize);
final ImageView imageView = new ImageView(mContext);
imageView.setImageBitmap(bitmap);
imageView.setScaleType(ScaleType.CENTER_CROP);
drawFromViewToCanvas(imageView, getRect(), canvas);

---

EDIT: there is a sample on sony's website:

int measureWidth = View.MeasureSpec.makeMeasureSpec(bitmapWidth, View.MeasureSpec.EXACTLY);
int measuredHeight = View.MeasureSpec.makeMeasureSpec(bitmapHeight, View.MeasureSpec.EXACTLY);
view.measure(measureWidth, measuredHeight);
view.layout(0, 0, bitmapWidth, bitmapHeight);
view.draw(canvas);

wonder if it works.

Answer 1

Yeah, you can do this. Keep in mind, since you're not attaching it to a layout, you'll need to lay it out manually before drawing it:

view.layout(0, 0, viewWidth, viewHeight);

And unless you just know exactly what you want for those width and height parameters, you may also want to measure it first:

int widthSpec = MeasureSpec.makeMeasureSpec (ViewGroup.LayoutParams.WRAP_CONTENT, MeasureSpec.UNSPECIFIED;
int heightSpec = MeasureSpec.makeMeasureSpec (400, MeasureSpec.UNSPECIFIED;
view.measure(widthSpec, heightSpec);
view.layout(0, 0, view.getMeasuredWidth(), view.getMeasuredHeight());

EDIT:

If you already know the width and height you need:

//Lay the view out with the known dimensions
view.layout (0, 0, rect.width(), rect.height());

//Translate the canvas so the view is drawn at the proper coordinates
canvas.save();
canvas.translate(rect.left, rect.top);

//Draw the View and clear the translation
view.draw(canvas);
canvas.restore();

EDIT again:

Yes, tested. You can try this yourself:

public class DrawingActivity extends Activity {
    public void onCreate (Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        //Set a Rect for the 200 x 200 px center of a 400 x 400 px area
        Rect rect = new Rect();
        rect.set(100, 100, 300, 300);

        //Allocate a new Bitmap at 400 x 400 px
        Bitmap bitmap = Bitmap.createBitmap(400, 400, Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bitmap);

        //Make a new view and lay it out at the desired Rect dimensions
        TextView view = new TextView(this);
        view.setText("This is a custom drawn textview");
        view.setBackgroundColor(Color.RED);
        view.setGravity(Gravity.CENTER);

        //Measure the view at the exact dimensions (otherwise the text won't center correctly)
        int widthSpec = View.MeasureSpec.makeMeasureSpec(rect.width(), View.MeasureSpec.EXACTLY);
        int heightSpec = View.MeasureSpec.makeMeasureSpec(rect.height(), View.MeasureSpec.EXACTLY);
        view.measure(widthSpec, heightSpec);

        //Lay the view out at the rect width and height
        view.layout(0, 0, rect.width(), rect.height());

        //Translate the Canvas into position and draw it
        canvas.save();
        canvas.translate(rect.left, rect.top);
        view.draw(canvas);
        canvas.restore();

        //To make sure it works, set the bitmap to an ImageView
        ImageView imageView = new ImageView(this);
        imageView.setLayoutParams(new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
        setContentView(imageView);
        imageView.setScaleType(ImageView.ScaleType.CENTER);
        imageView.setImageBitmap(bitmap);
    }
}