将画布添加到相对布局

Question

Currently I am writing a program that is supposed to add an image to a canvas and then add that canvas to a relative layout. The problem that I am having is that when I call this the layout is displayed but the canvas is not drawn on the layout.

canvas = (RelativeLayout) findViewById(R.id.canvasLayout);                  
imageCanvas = new Canvas();
mainImage = BitmapFactory.decodeResource(getResources(), R.drawable.image);

imageCanvas.drawBitmap(mainImage, 50, 50, null);

imageCanvas.drawColor(Color.BLACK);
canvas.draw(imageCanvas);

That is the code that is attempting to add the image to the canvas and then the canvas to the layout.

I am completely lost as to what to do to attempt to fix this.

Answer 1

The canvas.draw(imageCanvas) call you are using actually draws canvas (the RelativeLayout) onto imageCanvas (the Canvas object) - likely not what you were intending. There are several ways I would suggest to go about getting the drawable onto the RelativeLayout:

The first would be to create an ImageView, set it's image to your drawable, then add it to the layout. Depending on if you want the images to be added dynamically during runtime or not, you could actually do this in the xml graphic designer. This is probably the best way to go, as it gives you control over how large the image is, and how it gets scaled/cropped.

If you want to set a background to the layout, I believe you can actually set the Background attribute of the RelativeLayout directly to a drawable - this does not allow as much control over the scaling of the image though.

Finally, for complete control over how the RelativeLayout is draw, you can override the OnDraw method in any view. This method is called by the android system whenever it needs to refresh the view. If you do end up using this, creating a custom view class, which you then place in the RelativeLayout will probably work better.

Answer 2

canvas = (RelativeLayout) findViewById(R.id.canvasLayout);                  
imageCanvas = new Canvas();

mainImage = BitmapFactory.decodeResource(getResources(), R.drawable.image);

imageCanvas.drawBitmap(mainImage, 50, 50, null);



canvas.addView(imageCanvas);

Answer 3

The following will programmatically add an image to your layout from a bitmap resource which sounds like what you want to do. If you need to draw on top of that image, say, you would need to make a custom component by extending ImageView and overriding onDraw().

canvas = (RelativeLayout) findViewById(R.id.canvasLayout);                  
imageView = new ImageView(getApplicationContext());
mainImage = BitmapFactory.decodeResource(getResources(), R.drawable.image);
imageView.setImageBitmap( mainImage );
canvas.addView( imageView );