Cannot draw on Chip when text alignment is center

Question

I am extending Chip class to perform some drawing over it for my lib , my use case is more complex but for simplicity let's say i am just drawing a diagonal line

my code

class MyChip (context: Context,attributeSet: AttributeSet) : Chip(context,attributeSet){

override fun onDraw(canvas: Canvas) {
    super.onDraw(canvas)
    //just want to draw a diagonal line
    canvas.drawLine(0f,0f,width/1f,height/1f,paint)
  }
 }

xml

when i don't have attribute android:textAlignment="center" everything works fine , but with that attribute we can not draw anything on chip. I tried everything but couldn't figure out why is it happening.

Please Help

Cheticamp · Accepted Answer

The Chip widget adheres strongly to the Material Design guidelines and is not (IMO) amenable to change. I suggest that you look at other widgets - maybe a MaterialButton to see if something else may suit your needs.

The attribute you reference, textAlignment, is available in TextView which is a class that Chip is based on. Chips expects wrap_content and also expect for text to be aligned to the start. Somewhere in the mix, your over-draw is lost. I doubt that something like that has ever been tested as it does not adhere to the Material guidelines.

However, if you must use a Chip, here is a way to do so with a custom view:

class MyChip @JvmOverloads constructor(
    context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0
) : Chip(context, attrs, defStyleAttr) {

    private val mLinePaint = Paint().apply {
        color = Color.BLUE
        strokeWidth = 10f
    }

    init {
        doOnNextLayout {
            // Turn off center alignment if specified. We will handle centering ourselves.
            if (isTextAlignmentCenter()) {
                textAlignment = View.TEXT_ALIGNMENT_GRAVITY
                // Get the length of all the stuff before the text.
                val lengthBeforeText =
                    if (isChipIconVisible) iconStartPadding + chipIconSize + iconEndPadding else 0f
                val chipCenter = width / 2
                // Chips have only one line, so we can get away with this.
                val textWidth = layout.getLineWidth(0)
                val newTextStartPadding = chipCenter - (textWidth / 2) - lengthBeforeText
                textStartPadding = max(0f, newTextStartPadding)
                textEndPadding = 0f
            }
        }
    }

    override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)
        //just want to draw a diagonal line
        canvas.drawLine(0f, 0f, width.toFloat(), height.toFloat(), mLinePaint)
    }

    private fun isTextAlignmentCenter() = textAlignment == View.TEXT_ALIGNMENT_CENTER
}

A sample layout:

activity_main.xml

And the result:

I looked into the issue a little more deeply. For some reason, canvas operations such a drawLine(), drawRect(), etc. do not function. However, I can draw text on the canvas and fill it with a paint or a color.

Update: So, I tracked the problem down to the bringTextIntoView() method of TextView. For a reason that is not clear to me, the view is scrolled quite a few places (large, like 10's of thousands) in the positive direction. It is in this scrolled position that the text is written. To draw on the Chip, we must also scroll to this position. This scroll explains why I could fill the canvas with a color but could not draw on it so it would be visible.

The following will capture the "bad" scroll position and scroll to that position before drawing on the Chip. The screen capture looks the same as the above image.

MyChip.kt

class MyChip @JvmOverloads constructor(
    context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0
) : Chip(context, attrs, defStyleAttr)/*, View.OnScrollChangeListener*/ {

    private val mLinePaint = Paint().apply {
        color = Color.BLUE
        strokeWidth = 10f
    }

    private var mBadScroll = 0f

    override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)
        //just want to draw a diagonal line
        canvas.withTranslation(x = mBadScroll) {
            canvas.drawLine(0f, 0f, this@MyChip.width.toFloat(), height.toFloat(), mLinePaint)
        }
    }

    private fun isTextAlignmentCenter() = textAlignment == View.TEXT_ALIGNMENT_CENTER

    override fun scrollTo(x: Int, y: Int) {
        super.scrollTo(x, y)
        if (isTextAlignmentCenter()) {
            mBadScroll = scrollX.toFloat()
        }
    }
}

Update: Horizontal scrolling is still the issue. In onMeasure() method of TextView, the wanted width is set to VERY_WIDE if horizontal scrolling is enabled for the view. (And it is for Chip.) VERY_WIDE is 1024*1024 or one megabyte. This is the source of the large scroll that we see later on. This problem with Chip can be replicated directly with TextView if we call setHorizontallyScrolling(true) in the TextView.

Chips have only one line and, probably, shouldn't scroll. Calling setHorizontallyScrolling(true) doesn't make the Chip or TextView scrollable anyway. The above code now just becomes:

MyChip.kt

class MyChip @JvmOverloads constructor(
    context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0
) : Chip(context, attrs, defStyleAttr) {

    private val mLinePaint = Paint().apply {
        color = Color.BLUE
        strokeWidth = 10f
    }

    init {
        setHorizontallyScrolling(false)
    }

    override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)
        //just want to draw a diagonal line
        canvas.drawLine(0f, 0f, this@MyChip.width.toFloat(), height.toFloat(), mLinePaint)
    }
}

Horizontal scrolling is set to "true" in the constructor for Chip.

Cannot draw on Chip when text alignment is center

Answers (1)

Related Questions