Smart cast to 'Bitmap!' is impossible, because 'textBitmap' is a local variable that is captured by a changing closure - android-studio

Question

when ever I build my project I got this error
here is the kotlin class code
var textBitmap: Bitmap? = null
dynamicItem.dynamicText[imageKey]?.let { drawingText ->
dynamicItem.dynamicTextPaint[imageKey]?.let { drawingTextPaint ->
drawTextCache[imageKey]?.let {
textBitmap = it
} ?: kotlin.run {
textBitmap = Bitmap.createBitmap(drawingBitmap.width, drawingBitmap.height, Bitmap.Config.ARGB_8888)
val drawRect = Rect(0, 0, drawingBitmap.width, drawingBitmap.height)
val textCanvas = Canvas(textBitmap)
drawingTextPaint.isAntiAlias = true
val fontMetrics = drawingTextPaint.getFontMetrics();
val top = fontMetrics.top
val bottom = fontMetrics.bottom
val baseLineY = drawRect.centerY() - top / 2 - bottom / 2
textCanvas.drawText(drawingText, drawRect.centerX().toFloat(), baseLineY, drawingTextPaint);
drawTextCache.put(imageKey, textBitmap as Bitmap)
}
I couldn't figure out how to fix it

Answer 1

Instead of doing nested let like that, i would prefer to do some guard clause
val drawingText = dynamicItem.dynamicText[imageKey] ?: return // or you could assign an empty string `?: "" `
val drawingTextPaint = dynamicItem.dynamicTextPaint[imageKey] ?: return
val textBitmap: Bitmap = drawTextCache[imageKey] ?: Bitmap.createBitmap(drawingBitmap.width, drawingBitmap.height, Bitmap.Config.ARGB_8888).applyCanvas {
val drawRect = Rect(0, 0, drawingBitmap.width, drawingBitmap.height)
val fontMetrics = drawingTextPaint.getFontMetrics()
val top = fontMetrics.top
val bottom = fontMetrics.bottom
val baseLineY = drawRect.centerY() - top / 2 - bottom / 2
drawingTextPaint.isAntiAlias = true
drawText(drawingText, drawRect.centerX().toFloat(), baseLineY, drawingTextPaint);
}
drawTextCache.put(imageKey, textBitmap)

Answer 2

Basically Kotlin can't smart cast textBitmap to a non-null Bitmap inside that lambda. You're probably getting the error on the Canvas(textBitmap) call, which can't take a null parameter, and the compiler can't guarantee textBitmap isn't null at that moment.
It's a limitation of lambdas referencing external vars which can be changed - I think it's because a lambda could potentially be run at some other time, so no guarantees can be made about what's happening to that external variable and whether something else could have modified it. I don't know the details, there's some chat here if you like.
The fix is pretty easy though, if all you're doing is creating a textBitmap variable and assigning something to it:
// Assign it as a result of the expression - no need to create a var first and keep
// changing the value, no need for a temporary null value, it can just be a val
val textBitmap: Bitmap? =
dynamicItem.dynamicText[imageKey]?.let { drawingText ->
dynamicItem.dynamicTextPaint[imageKey]?.let { drawingTextPaint ->
drawTextCache[imageKey]
?: Bitmap.createBitmap(drawingBitmap.width, drawingBitmap.height, Bitmap.Config.ARGB_8888).apply {
val drawRect = Rect(0, 0, drawingBitmap.width, drawingBitmap.height)
val textCanvas = Canvas(this)
drawingTextPaint.isAntiAlias = true
val fontMetrics = drawingTextPaint.getFontMetrics();
val top = fontMetrics.top
val bottom = fontMetrics.bottom
val baseLineY = drawRect.centerY() - top / 2 - bottom / 2
textCanvas.drawText(drawingText, drawRect.centerX().toFloat(), baseLineY, drawingTextPaint);
drawTextCache.put(imageKey, this)
}
}
}
I'd recommend breaking the bitmap creation part out into its own function for readability, and personally I'd avoid the nested lets (because it's not immediately obvious what you get in what situation) but that's a style choice