Custom view not drawing beyond bounds of screen

Question

I have a custom view which is nested inside of a ScrollView. The idea is that the custom view can be longer than the height of the screen, so when you swipe through the ScrollView you can see all of its contents.

In order to do this I adjust the clip of the custom view's canvas like this:

@Override
public void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    canvas.clipRect(mCanvasArea, Region.Op.REPLACE);
}

Where mCanvasArea is a pre-generated RectF which represents the entire area the canvas can draw. I logged it's value @ 10308.

However when the app runs, the custom view is unscrollable, it acts as if the visible content represents all of the content in the canvas, but I can see more peeking up from the bottom of the screen.

Here is my XML declaration for the view:

<ScrollView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_below="@id/activity_toolbar"
    android:fillViewport="true">

    <com.myapp.CustomView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>

</ScrollView>

(I had to add fillViewport in order to get the content to show up in the first place)

Is there anything else that needs to be done in order yo make the view scroll?

Answer 1

You will need to specify the height of the custom view, not the canvas. The canvas passed to onDraw reflects the width and height of the view.

Setting the view to wrap_content doesn't mean it will grow whenever you draw outside of its bounds. It will only "wrap" the child views, not the canvas.

To set your view's height programmatically:

getLayoutParams().height = 500;
invalidate();

I'm not sure what you are going for with this exactly so I can't get too much more specific about where you should set the height, but I'd recommend not changing your view's dimensions in onDraw because when you invalidate it will cause another draw and you'll need some funky logic to handle this recursion.

Instead you could determine the height you need when the view is constructed, and then override onMeasure with something like this:

public void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  super.onMeasure(widthMeasureSpec, calculatedHeight);
}

This would force your custom view into the calculated height.