Restart any non-running identical requests instead of ignoring them. (#2261)

Running requests are left to complete to avoid cancelling and restarting
expensive operations (network or I/O). Completed requests are asked to
begin again, but short cut the normal request pipeline and redeliver the previously loaded resource to ensure that Targets and RequestListeners 
are notified as expected. Failed, cancelled, paused etc requests (all 
other states) are simply restarted.

Author: sjudd

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

@@ -359,14 +359,19 @@ public <Y extends Target<TranscodeType>> Y into(@NonNull Y target) {
     Request request = buildRequest(target);
 
     Request previous = target.getRequest();
-    if (previous != null) {
-      if (request.isEquivalentTo(previous)) {
-        request.recycle();
-        return target;
+    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 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();
       }
-      requestManager.clear(target);
+      return target;
     }
 
+    requestManager.clear(target);
     target.setRequest(request);
     requestManager.track(target, request);
 

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

@@ -217,6 +217,24 @@ public void begin() {
       return;
     }
 
+    if (status == Status.RUNNING) {
+      throw new IllegalArgumentException("Cannot restart a running request");
+    }
+
+    // If we're restarted after we're complete (usually via something like a notifyDataSetChanged
+    // that starts an identical request into the same Target or View), we can simply use the
+    // resource and size we retrieved the last time around and skip obtaining a new size, starting a
+    // new load etc. This does mean that users who want to restart a load because they expect that
+    // the view size has changed will need to explicitly clear the View or Target before starting
+    // the new load.
+    if (status == Status.COMPLETE) {
+      onResourceReady(resource, DataSource.MEMORY_CACHE);
+      return;
+    }
+
+    // Restarts for requests that are neither complete nor running can be treated as new requests
+    // and can run again from the beginning.
+
     status = Status.WAITING_FOR_SIZE;
     if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
       onSizeReady(overrideWidth, overrideHeight);