feat(non linear font scaling): add FontScaleConverterFactory.isNonLinearFontScalingActive()

This helps us determine if we can skip certain calculations as a
performance optimization, for instance when calculating line height
proportionally.

Bug: 273326061
Test: atest frameworks/base/core/java/android/content/res/FontScaleConverterFactory.java

Change-Id: I364a978b457c9b7a7073b34fa8cc1f3abaeabbac

core/java/android/content/res/FontScaleConverterFactory.java

@@ -34,6 +34,8 @@ public class FontScaleConverterFactory {
     @VisibleForTesting
     static final SparseArray<FontScaleConverter> LOOKUP_TABLES = new SparseArray<>();
 
+    private static float sMinScaleBeforeCurvesApplied = 1.05f;
+
     static {
         // These were generated by frameworks/base/tools/fonts/font-scaling-array-generator.js and
         // manually tweaked for optimum readability.
@@ -82,10 +84,29 @@ public class FontScaleConverterFactory {
                         new float[] {  16f,   20f,   24f,   26f,   30f,   34f,   36f,   38f,  100})
         );
 
+        sMinScaleBeforeCurvesApplied = getScaleFromKey(LOOKUP_TABLES.keyAt(0)) - 0.02f;
+        if (sMinScaleBeforeCurvesApplied <= 1.0f) {
+            throw new IllegalStateException(
+                    "You should only apply non-linear scaling to font scales > 1"
+            );
+        }
     }
 
     private FontScaleConverterFactory() {}
 
+    /**
+     * Returns true if non-linear font scaling curves would be in effect for the given scale, false
+     * if the scaling would follow a linear curve or for no scaling.
+     *
+     * <p>Example usage:
+     * <code>isNonLinearFontScalingActive(getResources().getConfiguration().fontScale)</code>
+     *
+     * @hide
+     */
+    public static boolean isNonLinearFontScalingActive(float fontScale) {
+        return fontScale >= sMinScaleBeforeCurvesApplied;
+    }
+
     /**
      * Finds a matching FontScaleConverter for the given fontScale factor.
      *
@@ -97,10 +118,7 @@ public class FontScaleConverterFactory {
      */
     @Nullable
     public static FontScaleConverter forScale(float fontScale) {
-        if (fontScale <= 1) {
-            // We don't need non-linear curves for shrinking text or for 100%.
-            // Also, fontScale==0 should not have a curve either.
-            // And ignore negative font scales; that's just silly.
+        if (!isNonLinearFontScalingActive(fontScale)) {
             return null;
         }
 

core/tests/coretests/src/android/content/res/FontScaleConverterFactoryTest.kt

@@ -122,6 +122,22 @@ class FontScaleConverterFactoryTest {
         }
     }
 
+    @SmallTest
+    fun testIsNonLinearFontScalingActive() {
+        assertThat(FontScaleConverterFactory.isNonLinearFontScalingActive(1f)).isFalse()
+        assertThat(FontScaleConverterFactory.isNonLinearFontScalingActive(0f)).isFalse()
+        assertThat(FontScaleConverterFactory.isNonLinearFontScalingActive(-1f)).isFalse()
+        assertThat(FontScaleConverterFactory.isNonLinearFontScalingActive(0.85f)).isFalse()
+        assertThat(FontScaleConverterFactory.isNonLinearFontScalingActive(1.02f)).isFalse()
+        assertThat(FontScaleConverterFactory.isNonLinearFontScalingActive(1.10f)).isFalse()
+        assertThat(FontScaleConverterFactory.isNonLinearFontScalingActive(1.15f)).isTrue()
+        assertThat(FontScaleConverterFactory.isNonLinearFontScalingActive(1.1499999f))
+                .isTrue()
+        assertThat(FontScaleConverterFactory.isNonLinearFontScalingActive(1.5f)).isTrue()
+        assertThat(FontScaleConverterFactory.isNonLinearFontScalingActive(2f)).isTrue()
+        assertThat(FontScaleConverterFactory.isNonLinearFontScalingActive(3f)).isTrue()
+    }
+
     @LargeTest
     @Test
     fun allFeasibleScalesAndConversionsDoNotCrash() {