DrawingView | android上的一个涂鸦控件。可以设置画笔的粗细，颜色，撤销上一笔涂鸦，提供保存图片的接口。 | Canvas library

You're calling findViewById on the Activity, not the dialog. So its searching for that view in the Activity's view tree. Use var smallbtn=dialog.findViewById(R.id.ib_small_brush) and it should work.

I found a solution to define an attribute in the MainActivity.kt through activity_main.xml to content_main.xml (included layout). The key word here is DataBinding. The project is completely reproducible and I provide first Kotlin and at the very end the JAVA files.

KOTLIN:

To enable DataBinding you need to go to your build.gradle(Module) and add following code:

Perhaps, you have to obtain correct translation values by re-calculating the transform matrix that is compatible to the View's transform.

Your main problem is that you set drawingView.b1.paint.color in your MainActivity, but in Balls.kt.draw() you set it back to paint.color = color Then, you should invalidate() the view when you make changes to it.

Meanwhile I refactored your code a little bit, I post it below. The only change in activity_main.xml was that I deleted the Buttons onClick attribute.

If you are using the same object in the list it will point to the same reference. Changing an object will affect all items in the list.

In your case, all paths inside mUserList will have the same mNativePath.

If you want to keep references to all Paths, then you should initiate a new Stroke object when adding to the mUserList.

Here is a simplified example to understand better why is this happening:

1. Create a custom ImageView which extends the androidx.appcompat.widget.AppCompatImageView class, which implements an OnTouchListener with an ArrayList which will keep track of the Dots.

2. You override onDraw(Canvas canvas) of the custom ImageView and iterate through the ArrayList of Dots and draw each Dot in the list using canvas.drawCircle(float cx, float cy, float radius, @NonNull Paint paint).

3. Whenever MotionEvent.ACTION_DOWN is fired you check if the touch was inside an existing dot.

   If it was you set that Dot to a global variable i.e. touchedDot, when the users moves OnTouchListener fires MotionEvent.ACTION_MOVE which you then check if touchedDot != null and if so simply change its x and y to match the events via touchedDot.x = event.getX() and touchedDot.y = event.getY() and then call invalidate() which will call the ImageViews onDraw method and the dot will be moved as the users finger moves. When the user lifts their finger either from a touch or a move, MotionEvent.ACTION_UP is fired, there you simply check if touchedDot == null and if so you then create a new Dot at the x and y they touched in, otherwise you set touchedDot = null to reset it for the next move or touch event.

Here is an example I created using Picasso to load the image into the custom ImageView:

build.gradle:

As you can see from the GitHub link in the comments:
https://github.com/xamarin/xamarin-macios/issues/10072
the problem is likely inherent in iOS.

So the source code and solution outlined in Addendum #2 above will solve this problem thoroughly.

So I think this one is closed.

Solved it!

This is kind of a complicated subject - if you want to draw a Path with a particular fill and colour, you need to create a Paint and use setColor (and setStyle to make it Paint.Style.FILL.

But Paths are for vector graphics, they're all defined lines and curves. Unless you're creating the shapes that the user can fill in as paths in the first place, you'll have trouble defining one to fit an arbitrary area.

I'm guessing you're actually using bitmaps, like a normal paint program, and you want to tap on a pixel and have that change colour, and also recursively change the colour of surrounding pixels if they meet a certain threshold. So you'll have to get the Canvas's Bitmap, and work out how to move through the pixels, changing them as you go (e.g. with Bitmap.setPixel())

There are a lot of algorithms for doing this, you'll just need to pick one and implement it. You probably don't want to use Paths to do it though!