Created Property class and related work.

api/src/main/groovy/com/jessebrault/ssg/property/Properties.groovy

@@ -0,0 +1,11 @@
+package com.jessebrault.ssg.property
+
+final class Properties {
+
+    static <T> Property<T> get() {
+        new SimpleProperty<>()
+    }
+
+    private Properties() {}
+
+}

api/src/main/groovy/com/jessebrault/ssg/property/Property.groovy

@@ -0,0 +1,38 @@
+package com.jessebrault.ssg.property
+
+import com.jessebrault.ssg.provider.Provider
+
+import java.util.function.Function
+import java.util.function.UnaryOperator
+
+interface Property<T> extends Provider<T> {
+
+    void set(T t)
+    void set(Provider<T> provider)
+    void unset()
+
+    default void leftShift(T t) {
+        this.set(t)
+    }
+
+    default void leftShift(Provider<T> provider) {
+        this.set(provider)
+    }
+
+    T getConvention()
+    void setConvention(T t)
+    void setConvention(Provider<T> provider)
+    void unsetConvention()
+
+    void map(UnaryOperator<T> operator)
+    void flatMap(Function<T, Provider<T>> function)
+
+    default void rightShift(UnaryOperator<T> operator) {
+        this.map(operator)
+    }
+
+    default void rightShift(Function<T, Provider<T>> function) {
+        this.flatMap(function)
+    }
+
+}

api/src/main/groovy/com/jessebrault/ssg/property/SimpleProperty.groovy

@@ -0,0 +1,82 @@
+package com.jessebrault.ssg.property
+
+import com.jessebrault.ssg.provider.AbstractProvider
+import com.jessebrault.ssg.provider.Provider
+import com.jessebrault.ssg.provider.Providers
+import groovy.transform.EqualsAndHashCode
+import groovy.transform.NullCheck
+import groovy.transform.PackageScope
+
+import java.util.function.Function
+import java.util.function.UnaryOperator
+
+@PackageScope
+@NullCheck
+@EqualsAndHashCode(includeFields = true)
+final class SimpleProperty<T> extends AbstractProvider<T> implements Property<T> {
+
+    private Provider<T> convention = Providers.getEmpty()
+    private Provider<T> tProvider = Providers.getEmpty()
+
+    @Override
+    T provide() {
+        this.tProvider.present ? this.tProvider.provide() : this.convention.provide()
+    }
+
+    @Override
+    void set(T t) {
+        this.tProvider = Providers.of(t)
+    }
+
+    @Override
+    void set(Provider<T> provider) {
+        this.tProvider = provider
+    }
+
+    @Override
+    void unset() {
+        this.tProvider = Providers.getEmpty()
+    }
+
+    @Override
+    T getConvention() {
+        this.convention.provide()
+    }
+
+    @Override
+    void setConvention(T t) {
+        this.convention = Providers.of(t)
+    }
+
+    @Override
+    void setConvention(Provider<T> provider) {
+        this.convention = provider
+    }
+
+    @Override
+    void unsetConvention() {
+        this.convention = Providers.getEmpty()
+    }
+
+    @Override
+    void map(UnaryOperator<T> operator) {
+        def oldTProvider = this.tProvider
+        this.tProvider = Providers.fromSupplier {
+            operator.apply(oldTProvider.provide())
+        }
+    }
+
+    @Override
+    void flatMap(Function<T, Provider<T>> function) {
+        def oldTProvider = this.tProvider
+        this.tProvider = Providers.fromSupplier {
+            function.apply(oldTProvider.provide()).provide()
+        }
+    }
+
+    @Override
+    String toString() {
+        "SimpleProperty(convention: ${ this.convention }, tProvider: ${ this.tProvider })"
+    }
+
+}

api/src/main/groovy/com/jessebrault/ssg/provider/AbstractProvider.groovy

@@ -23,4 +23,9 @@ abstract class AbstractProvider<T> implements Provider<T> {
         })
     }
 
+    @Override
+    boolean isEmpty() {
+        false
+    }
+
 }

api/src/main/groovy/com/jessebrault/ssg/provider/EmptyProvider.groovy

@@ -0,0 +1,39 @@
+package com.jessebrault.ssg.provider
+
+import groovy.transform.EqualsAndHashCode
+import groovy.transform.NullCheck
+import groovy.transform.PackageScope
+
+@PackageScope
+@NullCheck
+@EqualsAndHashCode
+final class EmptyProvider<T> implements Provider<T> {
+
+    @Override
+    T provide() {
+        throw new NullPointerException('EmptyProvider cannot provide a value.')
+    }
+
+    @Override
+    CollectionProvider<T> plus(Provider<T> other) {
+        CollectionProviders.fromSupplier {
+            [other.provide()] as Collection<T>
+        }
+    }
+
+    @Override
+    CollectionProvider<T> asType(Class<CollectionProvider> collectionProviderClass) {
+        CollectionProviders.fromCollection([]) as CollectionProvider<T>
+    }
+
+    @Override
+    boolean isEmpty() {
+        true
+    }
+
+    @Override
+    String toString() {
+        "EmptyProvider()"
+    }
+
+}

api/src/main/groovy/com/jessebrault/ssg/provider/Provider.groovy

@@ -1,7 +1,18 @@
 package com.jessebrault.ssg.provider
 
 interface Provider<T> {
+
     T provide()
+
+    @Deprecated
     CollectionProvider<T> plus(Provider<T> other)
+
     CollectionProvider<T> asType(Class<CollectionProvider> collectionProviderClass)
+
+    boolean isEmpty()
+
+    default boolean isPresent() {
+        !this.empty
+    }
+
 }

api/src/main/groovy/com/jessebrault/ssg/provider/Providers.groovy

@@ -1,9 +1,16 @@
 package com.jessebrault.ssg.provider
 
+import groovy.transform.NullCheck
+
 import java.util.function.Supplier
 
+@NullCheck
 final class Providers {
 
+    static <T> Provider<T> getEmpty() {
+        new EmptyProvider<>()
+    }
+
     static <T> Provider<T> of(T t) {
         new SimpleProvider<>(t)
     }

api/src/main/groovy/com/jessebrault/ssg/provider/SimpleProvider.groovy

@@ -18,4 +18,9 @@ final class SimpleProvider<T> extends AbstractProvider<T> {
         this.t
     }
 
+    @Override
+    String toString() {
+        "SimpleProvider(t: ${ this.t })"
+    }
+
 }

api/src/main/groovy/com/jessebrault/ssg/provider/SupplierBasedProvider.groovy

@@ -20,4 +20,9 @@ final class SupplierBasedProvider<T> extends AbstractProvider<T> {
         this.supplier.get()
     }
 
+    @Override
+    String toString() {
+        "SupplierBasedProvider(supplier: ${ this.supplier })"
+    }
+
 }