Add an API for preloading resources.

library/src/androidTest/java/com/bumptech/glide/request/target/PreloadTargetTest.java

+package com.bumptech.glide.request.target;
+
+import com.bumptech.glide.request.Request;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.RobolectricTestRunner;
+
+import static org.mockito.Matchers.eq;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+
+@RunWith(RobolectricTestRunner.class)
+public class PreloadTargetTest {
+
+    @Test
+    public void testCallsSizeReadyWithGivenDimensions() {
+        int width = 1234;
+        int height = 456;
+        PreloadTarget<Object> target = PreloadTarget.obtain(width, height);
+        SizeReadyCallback cb = mock(SizeReadyCallback.class);
+        target.getSize(cb);
+
+        verify(cb).onSizeReady(eq(width), eq(height));
+    }
+
+    @Test
+    public void testClearsTargetInOnResourceReady() {
+        Request request = mock(Request.class);
+        PreloadTarget<Object> target = PreloadTarget.obtain(100, 100);
+        target.setRequest(request);
+        target.onResourceReady(new Object(), null);
+
+        verify(request).clear();
+    }
+}
\ No newline at end of file

library/src/main/java/com/bumptech/glide/GenericRequestBuilder.java

@@ -28,6 +28,7 @@ import com.bumptech.glide.request.animation.GlideAnimationFactory;
 import com.bumptech.glide.request.animation.NoAnimation;
 import com.bumptech.glide.request.animation.ViewAnimation;
 import com.bumptech.glide.request.animation.ViewPropertyAnimation;
+import com.bumptech.glide.request.target.PreloadTarget;
 import com.bumptech.glide.request.target.Target;
 import com.bumptech.glide.util.Util;
 
@@ -598,6 +599,21 @@ public class GenericRequestBuilder<ModelType, DataType, ResourceType, TranscodeT
         return target;
     }
 
+    /**
+     * Preloads the resource into the cache using the given width and height.
+     *
+     * <p>
+     *     Pre-loading is useful for making sure that resources you are going to to want in the near future are
+     *     available quickly.
+     * </p>
+     *
+     * @see com.bumptech.glide.ListPreloader
+     */
+    public Target<TranscodeType> preload(int width, int height) {
+        final PreloadTarget<TranscodeType> target = PreloadTarget.obtain(width, height);
+        return into(target);
+    }
+
     void applyCenterCrop() {
         // To be implemented by subclasses when possible.
     }

library/src/main/java/com/bumptech/glide/request/target/PreloadTarget.java

+package com.bumptech.glide.request.target;
+
+import com.bumptech.glide.Glide;
+import com.bumptech.glide.request.animation.GlideAnimation;
+
+/**
+ * A one time use {@link com.bumptech.glide.request.target.Target} class that loads a resource into memory and then
+ * clears itself.
+ *
+ * @param <Z> The type of resource that will be loaded into memory.
+ */
+public final class PreloadTarget<Z> extends SimpleTarget<Z> {
+
+    /**
+     * Returns a PreloadTarget.
+     *
+     * @param width The width of the desired resourece.
+     * @param height The height of the desired resource.
+     * @param <Z> The type of the desired resource.
+     */
+    public static <Z> PreloadTarget<Z> obtain(int width, int height) {
+        return new PreloadTarget<Z>(width, height);
+    }
+
+    private PreloadTarget(int width, int height) {
+        super(width, height);
+    }
+
+    @Override
+    public void onResourceReady(Z resource, GlideAnimation<? super Z> glideAnimation) {
+        Glide.clear(this);
+    }
+}