Add support for BindAttr annotation

butterknife-annotations/src/main/java/butterknife/BindAttr.java

@@ -0,0 +1,28 @@
+package butterknife;
+
+import androidx.annotation.AttrRes;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+/**
+ * Bind a field to the specified attribute ID.
+ * <p>
+ * It supports only colors presented as {@code int} (annotated with
+ * {@link androidx.annotation.ColorInt}) and {@link android.content.res.ColorStateList}.
+ * <pre><code>
+ * {@literal @}BindColor(R.attr.colorAccent)
+ * {@literal @}ColorInt
+ * int colorAccent;
+ * {@literal @}BindColor(R.attr.someTextColor)
+ * ColorStateList textColor;
+ * </code></pre>
+ */
+@Target(FIELD)
+@Retention(RUNTIME)
+public @interface BindAttr {
+  @AttrRes int value();
+}

butterknife-compiler/src/main/java/butterknife/compiler/ButterKnifeProcessor.java

@@ -1,7 +1,9 @@
 package butterknife.compiler;
 
+import androidx.annotation.ColorInt;
 import butterknife.BindAnim;
 import butterknife.BindArray;
+import butterknife.BindAttr;
 import butterknife.BindBitmap;
 import butterknife.BindBool;
 import butterknife.BindColor;
@@ -167,6 +169,7 @@ private Set<Class<? extends Annotation>> getSupportedAnnotations() {
 
     annotations.add(BindAnim.class);
     annotations.add(BindArray.class);
+    annotations.add(BindAttr.class);
     annotations.add(BindBitmap.class);
     annotations.add(BindBool.class);
     annotations.add(BindColor.class);
@@ -225,6 +228,17 @@ private Map<TypeElement, BindingSet> findAndParseTargets(RoundEnvironment env) {
       }
     }
 
+    // Process each @BindAttr element.
+    for (Element element : env.getElementsAnnotatedWith(BindAttr.class)) {
+      if (!SuperficialValidation.validateElement(element)) continue;
+      try {
+          parseBindAttribute(element, builderMap, erasedTargetNames);
+      } catch (Exception e) {
+        logParsingError(element, BindAttr.class, e);
+      }
+    }
+
+
     // Process each @BindBitmap element.
     for (Element element : env.getElementsAnnotatedWith(BindBitmap.class)) {
       if (!SuperficialValidation.validateElement(element)) continue;
@@ -430,6 +444,48 @@ private boolean isBindingInWrongPackage(Class<? extends Annotation> annotationCl
     return false;
   }
 
+  private void parseBindAttribute(Element element, Map<TypeElement, BindingSet.Builder> builderMap,
+      Set<TypeElement> erasedTargetNames) {
+    boolean hasError = false;
+    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();
+
+    // Verify that the target type is supported.
+    FieldAttrBinding.Type type = null;
+    TypeMirror elementType = element.asType();
+    if (COLOR_STATE_LIST_TYPE.equals(elementType.toString())) {
+      type = FieldAttrBinding.Type.COLOR_STATE_LIST;
+    } else if (elementType.getKind() == TypeKind.INT
+               && element.getAnnotation(ColorInt.class) != null) {
+      type = FieldAttrBinding.Type.COLOR_INT;
+    } else {
+      error(
+         element,
+         "@%s doesn't support this type. for more info see the docs of the annotation. (%s.%s)",
+         BindAttr.class.getSimpleName(),
+         enclosingElement.getQualifiedName(),
+         element.getSimpleName()
+      );
+      hasError = true;
+    }
+
+    // Verify common generated code restrictions.
+    hasError |= isInaccessibleViaGeneratedCode(BindAttr.class, "fields", element);
+    hasError |= isBindingInWrongPackage(BindAttr.class, element);
+
+    if (hasError) {
+      return;
+    }
+
+    // Assemble information on the field.
+    String name = element.getSimpleName().toString();
+    int id = element.getAnnotation(BindAttr.class).value();
+    Id resourceId = elementToId(element, BindAttr.class, id);
+    BindingSet.Builder builder = getOrCreateBindingBuilder(builderMap, enclosingElement);
+
+    builder.addResource(new FieldAttrBinding(resourceId, name, type));
+    erasedTargetNames.add(enclosingElement);
+  }
+
   private void parseBindView(Element element, Map<TypeElement, BindingSet.Builder> builderMap,
       Set<TypeElement> erasedTargetNames) {
     TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

butterknife-compiler/src/main/java/butterknife/compiler/FieldAttrBinding.java

@@ -0,0 +1,51 @@
+package butterknife.compiler;
+
+import com.squareup.javapoet.CodeBlock;
+
+import static butterknife.compiler.BindingSet.UTILS;
+
+final class FieldAttrBinding implements ResourceBinding {
+
+  enum Type {
+    COLOR_INT {
+      @Override
+      CodeBlock render(String name, Id id) {
+        return CodeBlock.of("target.$L = $T.getThemeColor(context, $L)", name, UTILS, id.code);
+      }
+    },
+    COLOR_STATE_LIST {
+      @Override
+      CodeBlock render(String name, Id id) {
+        return CodeBlock.of("target.$L = $T.getThemeColorStateList(context, $L)", name, UTILS,
+           id.code);
+      }
+    };
+
+    abstract CodeBlock render(String name, Id id);
+  }
+
+  private final Id id;
+  private final String name;
+  private final Type type;
+
+  FieldAttrBinding(Id id, String name, Type type) {
+    this.id = id;
+    this.name = name;
+    this.type = type;
+  }
+
+  @Override
+  public Id id() {
+    return id;
+  }
+
+  @Override
+  public boolean requiresResources(int sdk) {
+    return false;
+  }
+
+  @Override
+  public CodeBlock render(int sdk) {
+    return type.render(name, id);
+  }
+}

butterknife-integration-test/src/androidTest/java/com/example/butterknife/functional/BindAttrTest.java

@@ -0,0 +1,54 @@
+package com.example.butterknife.functional;
+
+import android.content.Context;
+import android.content.res.ColorStateList;
+import android.view.ContextThemeWrapper;
+import android.view.View;
+import androidx.annotation.ColorInt;
+import androidx.test.InstrumentationRegistry;
+import butterknife.BindAttr;
+import butterknife.ButterKnife;
+import butterknife.Unbinder;
+import butterknife.internal.Utils;
+import com.example.butterknife.test.R;
+import org.junit.Test;
+
+import static com.google.common.truth.Truth.assertThat;
+
+public final class BindAttrTest {
+
+  private final Context context = new ContextThemeWrapper(InstrumentationRegistry.getContext(), R.style.Theme_App);
+  private final View tree = ViewTree.create(1);
+
+  static class ColorIntTarget {
+    @BindAttr(R.attr.colorAccent)
+    @ColorInt
+    int actual;
+  }
+
+  @Test public void asInt() {
+    ColorIntTarget target = new ColorIntTarget();
+    int expected = Utils.getThemeColor(context, R.attr.colorAccent);
+
+    Unbinder unbinder = ButterKnife.bind(target, tree);
+    assertThat(target.actual).isEqualTo(expected);
+
+    unbinder.unbind();
+    assertThat(target.actual).isEqualTo(expected);
+  }
+
+  static class ColorStateListTarget {
+    @BindAttr(R.attr.textColorStateList) ColorStateList actual;
+  }
+
+  @Test public void asColorStateList() {
+    ColorStateListTarget target = new ColorStateListTarget();
+    ColorStateList expected = Utils.getThemeColorStateList(context, R.attr.textColorStateList);
+
+    Unbinder unbinder = ButterKnife.bind(target, tree);
+    assertThat(target.actual.toString()).isEqualTo(expected.toString());
+
+    unbinder.unbind();
+    assertThat(target.actual.toString()).isEqualTo(expected.toString());
+  }
+}

butterknife-integration-test/src/androidTest/res/values/values.xml

@@ -15,4 +15,12 @@
     <item>2</item>
     <item>3</item>
   </integer-array>
+  <attr name="colorAccent" format="reference|color"/>
+  <attr name="textColorStateList" format="reference"/>
+
+  <style name="Theme.App" parent="android:Theme.DeviceDefault">
+    <item name="colorAccent">#f00</item>
+    <item name="textColorStateList">@color/colors</item>
+  </style>
+
 </resources>

butterknife-runtime/src/main/java/butterknife/internal/Utils.java

@@ -1,17 +1,20 @@
 package butterknife.internal;
 
 import android.content.Context;
+import android.content.res.ColorStateList;
 import android.content.res.Resources;
 import android.graphics.drawable.Drawable;
 import android.util.TypedValue;
 import android.view.View;
 import androidx.annotation.AttrRes;
+import androidx.annotation.ColorInt;
 import androidx.annotation.DimenRes;
 import androidx.annotation.DrawableRes;
 import androidx.annotation.IdRes;
 import androidx.annotation.UiThread;
 import androidx.core.content.ContextCompat;
 import androidx.core.graphics.drawable.DrawableCompat;
+
 import java.lang.reflect.Array;
 import java.util.List;
 
@@ -133,6 +136,19 @@ public static <T> T castParam(Object value, String from, int fromPos, String to,
     }
   }
 
+  @ColorInt
+  @UiThread
+  public static int getThemeColor(Context context, @AttrRes int colorAttr) {
+    context.getTheme().resolveAttribute(colorAttr, VALUE, true);
+    return VALUE.data;
+  }
+
+  @UiThread
+  public static ColorStateList getThemeColorStateList(Context context, @AttrRes int colorAttr) {
+    context.getTheme().resolveAttribute(colorAttr, VALUE, true);
+    return ContextCompat.getColorStateList(context, VALUE.resourceId);
+  }
+
   private static String getResourceEntryName(View view, @IdRes int id) {
     if (view.isInEditMode()) {
       return "<unavailable while editing>";