Implement a new optional model interface Model. (bumptech#2353)

This allows models to handle equity for internal states that can be different from the target image result that is handled with equals()/hashCode()
For example to allow restart of image download from a new source but still using the same cache in Glide.

Author: Tolriq

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

@@ -359,15 +359,11 @@ public <Y extends Target<TranscodeType>> Y into(@NonNull Y target) {
     Request request = buildRequest(target);
 
     Request previous = target.getRequest();
-    // When request was failed or cancelled, be sure to use the updated model as it can contains
-    // unexposed data that could help the request to succeed on restart.
-    // See https://github.com/bumptech/glide/issues/2270
-    if (request.isEquivalentTo(previous)
-      && (Preconditions.checkNotNull(previous).isComplete()
-         || Preconditions.checkNotNull(previous).isRunning())) {
+    if (request.isEquivalentTo(previous)) {
       request.recycle();
       // If the request is completed, beginning again will ensure the result is re-delivered,
-      // triggering RequestListeners and Targets. If the request is already
+      // triggering RequestListeners and Targets. If the request is failed, beginning again will
+      // restart the request, giving it another chance to complete. If the request is already
       // running, we can let it continue running without interruption.
       if (!Preconditions.checkNotNull(previous).isRunning()) {
         previous.begin();

library/src/main/java/com/bumptech/glide/load/model/Model.java

@@ -0,0 +1,28 @@
+package com.bumptech.glide.load.model;
+
+/**
+ * An optional interface that models can implement to enhance control over Glide behaviors.
+ */
+public interface Model {
+
+    /**
+     * Returns {@code true} if this model produces the same image using the same mechanism
+     * (server, authentication, source etc) as the given model.
+     * <p>
+     * Models must also implement {@link #equals(Object other)} and {@link #hashCode()}
+     * to ensure that caching functions correctly.
+     * If this object returns {@code true} from this method for a given Model,
+     * it must also be equal to and have the same hash code as the given model.
+     * <p>
+     * However, this model may be equal to and have the same hash code as a given model
+     * but still return {@code false} from this method.
+     * This method optionally allows you to differentiate between Models that load the same image
+     * via multiple different means.
+     * For example one Model might load the image from server A and another model might load
+     * the same image from server B.
+     * The models must be equal to each other with the same hash code because they load
+     * the same image. However two requests made with the different models are not exactly the
+     * same because the way the image is loaded will differ.
+     */
+    boolean isEquivalentTo(Object other);
+}

library/src/main/java/com/bumptech/glide/request/SingleRequest.java

@@ -578,7 +578,7 @@ public boolean isEquivalentTo(Request o) {
       SingleRequest that = (SingleRequest) o;
       return overrideWidth == that.overrideWidth
           && overrideHeight == that.overrideHeight
-          && Util.bothNullOrEqual(model, that.model)
+          && Util.bothModelsNullEquivalentOrEquals(model, that.model)
           && transcodeClass.equals(that.transcodeClass)
           && requestOptions.equals(that.requestOptions)
           && priority == that.priority;

library/src/main/java/com/bumptech/glide/util/Util.java

@@ -4,6 +4,8 @@
 import android.graphics.Bitmap;
 import android.os.Build;
 import android.os.Looper;
+
+import com.bumptech.glide.load.model.Model;
 import com.bumptech.glide.request.target.Target;
 import java.util.ArrayDeque;
 import java.util.ArrayList;
@@ -190,6 +192,16 @@ public static boolean bothNullOrEqual(Object a, Object b) {
     return a == null ? b == null : a.equals(b);
   }
 
+  public static boolean bothModelsNullEquivalentOrEquals(Object a, Object b) {
+    if (a == null) {
+      return b == null;
+    }
+    if (a instanceof Model) {
+      return ((Model) a).isEquivalentTo(b);
+    }
+    return a.equals(b);
+  }
+
   public static int hashCode(int value) {
     return hashCode(value, HASH_ACCUMULATOR);
   }