The exact method to backport is Typeface.create(Typeface base, int weight, boolean italic)

Motivation:

With android:textFontWeight backported (backed by the above-mentioned method) we could use text appearances with different weights - by referencing font family and weight - since API 16 (?) in AppCompatTextView.
This is currently only possible since API 28.

Another potential use is a future TypeAppearanceSpanCompat.

Example:

res/font/raleway.xml

<font-family xmlns:app="http://schemas.android.com/apk/res-auto">
<font
app:font="@font/raleway_thin"
app:fontStyle="normal"
app:fontWeight="100" />
<font
app:font="@font/raleway_thin_italic"
app:fontStyle="italic"
app:fontWeight="100" />
<font
app:font="@font/raleway_regular"
app:fontStyle="normal"
app:fontWeight="400" />
</font-family>

res/values/styles.xml

<resources>
<style name="TextAppearance.Raleway.Regular" parent="TextAppearance.AppCompat">
<item name="android:fontFamily">@font/raleway</item>
<item name="android:textFontWeight">400</item>
<item name="textFontWeight">400</item>
</style>

<style name="TextAppearance.Raleway.Thin" parent="TextAppearance.AppCompat">
<item name="android:fontFamily">@font/raleway</item>
<item name="android:textFontWeight">100</item>
<item name="textFontWeight">100</item>
</style>

<style name="TextAppearance.Raleway.Thin.Italic" parent="TextAppearance.AppCompat">
<item name="android:fontFamily">@font/raleway</item>
<item name="android:textFontWeight">100</item>
<item name="textFontWeight">100</item>
<item name="android:textStyle">italic</item>
</style>
</resources>

Example implementation:

On API 26-27 we could re-implement Typeface.create mentioned above as the backing JNI method already exists.
See https://github.com/consp1racy/android-commons/blob/3a90d3341aa05c6e57d2986d6d48d5446f2d95f6/text-span/src/main/java/net/xpece/android/graphics/WeightTypefaceOreo.kt

On API 21-25 we'd need to first create a Typeface with matching italic (using the Typeface.create with style API) and then use the JNI method to create weight alias.
See https://github.com/consp1racy/android-commons/blob/3a90d3341aa05c6e57d2986d6d48d5446f2d95f6/text-span/src/main/java/net/xpece/android/graphics/WeightTypefaceLollipop.kt

On API 20 and below we could leverage the fact that the only place weight typefaces could come from are from ResourcesCompat.getFont so we use TypefaceCompat internals to find the font family and look for best match in Java.
See https://github.com/consp1racy/android-commons/blob/3a90d3341aa05c6e57d2986d6d48d5446f2d95f6/text-span/src/main/java/net/xpece/android/graphics/WeightTypefaceLegacy.kt