IssueTracker

Some more details in b/388353336

Start of trace:

 java.lang.NullPointerException	
        at androidx.compose.ui.node.LayoutNodeDrawScope.drawContent(LayoutNodeDrawScope.kt:46)
	at androidx.compose.foundation.draganddrop.CacheDrawScopeDragShadowCallback$cachePicture$1$1$1.invoke(AndroidDragAndDropSource.android.kt:55)
	at androidx.compose.foundation.draganddrop.CacheDrawScopeDragShadowCallback$cachePicture$1$1$1.invoke(AndroidDragAndDropSource.android.kt:55)
	at androidx.compose.ui.draw.CacheDrawScope$record$1.invoke(DrawModifier.kt:347)
	at androidx.compose.ui.draw.CacheDrawScope$record$1.invoke(DrawModifier.kt:338)
	at androidx.compose.ui.graphics.layer.GraphicsLayer.draw$ui_graphics_release(AndroidGraphicsLayer.android.kt:1078)

This happens because when we draw we use a shared ContentDrawScope implementation that we render all DrawModifierNodes in. We do this by mutating a property as we draw successive nodes. However, this means that if anyone captures a reference to this draw scope and tries to draw it later on, this will cause a NPE error as we don't know what the 'current node' that needs to be drawn in.

For the general case this is an error and scopes shouldn't be captured and stored outside the scope in this way, but for GraphicsLayer this is needed when recording drawContent to support some edge cases, such as when we need to draw a layer implementation that can't be drawn in software, within software rendering (such as drawing to a Bitmap). For this we just end up re-invoking the draw block we captured.

We considered some solutions where we would allocate a new draw scope instance per modifier, but this adds some overhead and 'storing' these draw scopes for future use is challenging. (we considered putting them on the node object, in a map on the node coordinator, or some other chain structure on the node coordinator, but none of these are really great)

Instead we can explicitly support the GraphicsLayer.record case by overriding the DrawScope#GraphicsLayer.record function and making it handle this case properly. This can still cause issues if using the non-DrawScope scoped record function, but developers should move to the DrawScope one instead.