为什么将 ImageSpan 添加到 Snackbar 的操作文本在 Android 设备 SDK 级别 26 上有效，但在 SDK 级别 25 上无效？

Question

I want to show a Snackbar and use an image instead of text for the action.

I use the following code:

    val imageSpan = ImageSpan(this, R.drawable.star)
    val builder = SpannableStringBuilder(" ")
    builder.setSpan(
        imageSpan,
        0,
        1,
        SpannableString.SPAN_EXCLUSIVE_EXCLUSIVE
    )
    Snackbar.make(findViewById(R.id.container), "Hello Snackbar", Snackbar.LENGTH_INDEFINITE)
        .setAction(builder) {}.show()

drawable_star being a vector graphic asset, but the same happens with a png .

On an Android device lvl 26 and above this yields:

as expected, whereas on device lvl 25 the image is not visible:

Does someone know the reason for this and if there a workaround?

PS: You can check out my test project here: https://github.com/fmweigl/SpannableTest

Answer 1

That's due to the textAllCaps bug on versions prior to Oreo. That Button 's default style will have that attribute set to true , which simply causes the Button 's text to be converted to all uppercase. That conversion is done with the platform AllCapsTransformationMethod class, which, on Nougat 7.1 and below, would treat everything as flat String s, essentially stripping any formatting spans you've set.

The fix is to turn that attribute off, and handle any uppercase conversion you might need yourself, in code. Snackbar offers the snackbarButtonStyle attribute as a means to style the action Button , and we can create a simple style to modify that value. For example, from your styles.xml :

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <!-- Customize your theme here. -->
    <item name="colorPrimary">@color/colorPrimary</item>
    <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
    <item name="colorAccent">@color/colorAccent</item>

    <item name="snackbarButtonStyle">@style/NoCapsButton</item>>
</style>

<style name="NoCapsButton" parent="Widget.AppCompat.Button">
    <item name="textAllCaps">false</item>
</style>

_{(If you're using a Material Components theme, the parent for NoCapsButton should instead be Widget.MaterialComponents.Button.TextButton.Snackbar .)}

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In this specific case, that's all you need to do, since there's no text to convert.