IssueTracker

Component used: Androidx Lint

Not sure if this is the best component for this issue. This might be a bug somewhere else, and the correct thing to do is fix the bug, instead of issue a lint warning.

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I've just fixed an unexpected race-condition-y bug setting a view's layout params using Kotlin scope functions.

tl;dr

Setting a view's layout params using with and other scope functions like so:

with(view.layoutParams as ConstraintLayout.LayoutParams) {
    startToEnd = ...
    // other params here
}

can silently not set the params, resulting in visual bugs at run time. This might be worth a lint check.

I suspect this is an unintended interaction between Kotlin's scope functions and Kotlin's Java interoperability with class fields that have get and set methods (https://kotlinlang.org/docs/java-interop.html#getters-and-setters).

The problem

I have a custom view that displays a preview image and some metadata.

Depending on the aspect ratio of the image the overall layout looks like this (text in [...] is the view IDs) for landscape images:

.--------------------------.
| [preview_card_wrapper]   |
|                          |
| .----------------------. |
| | [card_image]         | |
| |                      | |
| |                      | |
| `----------------------' |
| .----------------------. |
| | [card_info]          | |
| |                      | |
| `----------------------' |
`--------------------------'

and this for portrait images:

.----------------------------------.
| [preview_card_wrapper]           |
|                                  |
| .-------------. .--------------. |
| | [card_image]| | [card_info]  | |
| |             | |              | |
| |             | |              | |
| `-------------' `--------------' |
`----------------------------------'

preview_card_wrapper is a ConstraintLayout. At run time I check the aspect ratio of the image and call this if the image is portrait.

private fun setStartEndLayout() = with(binding) {
    // Move image to start with fixed width to allow space for cardInfo.
    with(cardImage.layoutParams as ConstraintLayout.LayoutParams) {
        height = ConstraintLayout.LayoutParams.MATCH_CONSTRAINT
        width = cardImage.resources.getDimensionPixelSize(DR.dimen.card_image_horizontal_width)
        bottomToBottom = ConstraintLayout.LayoutParams.PARENT_ID
    }

    // Move cardInfo to end of image
    with(cardInfo.layoutParams as ConstraintLayout.LayoutParams) {
        startToEnd = cardImage.id
        topToTop = ConstraintLayout.LayoutParams.PARENT_ID

        startToStart = ConstraintLayout.LayoutParams.UNSET
        topToBottom = ConstraintLayout.LayoutParams.UNSET
    }
}

There's a similar function if the image's aspect ratio is landscape.

Sometimes, this doesn't work. Specifically, the card_info view behaves as though the startToEnd constraint is not updated. The card_info view's starting edge matches the parent, so the card_image and card_info end up overlapping.

In normal use of the app this happens rarely, but frequently enough for users to report the problem. I was eventually able to engineer a situation in my app where I can reproduce this 100%.

Adding the following to the end of that function resolves the issue 100% of the time:

    cardInfo.layoutParams = cardInfo.layoutParams

As does re-writing the second with block to the following that doesn't use scope functions:

    val lp = cardInfo.layoutParams as ConstraintLayout.LayoutParams
    lp.startToEnd = cardImage.id
    lp.topToTop = ConstraintLayout.LayoutParams.PARENT_ID

    lp.startToStart = ConstraintLayout.LayoutParams.UNSET
    lp.topToBottom = ConstraintLayout.LayoutParams.UNSET
    cardInfo.layoutParams = lp

Other attempts to fix this (e.g., calling cardInfo.requestLayout() after the with block in the original code) do not work.