Add transferInitializing to BaseDataSource.

This allows implementations to notify when the transfer is about to be started.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=203102117

extensions/cronet/src/main/java/com/google/android/exoplayer2/ext/cronet/CronetDataSource.java

@@ -301,6 +301,7 @@ public class CronetDataSource extends BaseDataSource implements HttpDataSource {
     }
     currentUrlRequest.start();
 
+    transferInitializing(dataSpec);
     try {
       boolean connectionOpened = blockUntilConnectTimeout();
       if (exception != null) {

extensions/okhttp/src/main/java/com/google/android/exoplayer2/ext/okhttp/OkHttpDataSource.java

@@ -165,6 +165,7 @@ public class OkHttpDataSource extends BaseDataSource implements HttpDataSource {
     this.dataSpec = dataSpec;
     this.bytesRead = 0;
     this.bytesSkipped = 0;
+    transferInitializing(dataSpec);
     Request request = makeRequest(dataSpec);
     try {
       response = callFactory.newCall(request).execute();

extensions/rtmp/src/main/java/com/google/android/exoplayer2/ext/rtmp/RtmpDataSource.java

@@ -51,6 +51,7 @@ public final class RtmpDataSource extends BaseDataSource {
 
   @Override
   public long open(DataSpec dataSpec) throws RtmpIOException {
+    transferInitializing(dataSpec);
     rtmpClient = new RtmpClient();
     rtmpClient.open(dataSpec.uri.toString(), false);
 

library/core/src/main/java/com/google/android/exoplayer2/upstream/AssetDataSource.java

@@ -74,6 +74,7 @@ public final class AssetDataSource extends BaseDataSource {
       } else if (path.startsWith("/")) {
         path = path.substring(1);
       }
+      transferInitializing(dataSpec);
       inputStream = assetManager.open(path, AssetManager.ACCESS_RANDOM);
       long skipped = inputStream.skip(dataSpec.position);
       if (skipped < dataSpec.position) {

library/core/src/main/java/com/google/android/exoplayer2/upstream/BaseDataSource.java

@@ -20,8 +20,9 @@ import java.util.ArrayList;
 /**
  * Base {@link DataSource} implementation to keep a list of {@link TransferListener}s.
  *
- * <p>Subclasses must call {@link #transferStarted(DataSpec)}, {@link #bytesTransferred(int)}, and
- * {@link #transferEnded()} to inform listeners of data transfers.
+ * <p>Subclasses must call {@link #transferInitializing(DataSpec)}, {@link
+ * #transferStarted(DataSpec)}, {@link #bytesTransferred(int)}, and {@link #transferEnded()} to
+ * inform listeners of data transfers.
  */
 public abstract class BaseDataSource implements DataSource {
 
@@ -44,6 +45,15 @@ public abstract class BaseDataSource implements DataSource {
   }
 
   /**
+   * Notifies listeners that data transfer for the specified {@link DataSpec} is being initialized.
+   *
+   * @param dataSpec {@link DataSpec} describing the data for initializing transfer.
+   */
+  protected final void transferInitializing(DataSpec dataSpec) {
+    // TODO: notify listeners.
+  }
+
+  /**
    * Notifies listeners that data transfer for the specified {@link DataSpec} started.
    *
    * @param dataSpec {@link DataSpec} describing the data being transferred.

library/core/src/main/java/com/google/android/exoplayer2/upstream/ByteArrayDataSource.java

@@ -44,6 +44,7 @@ public final class ByteArrayDataSource extends BaseDataSource {
   @Override
   public long open(DataSpec dataSpec) throws IOException {
     uri = dataSpec.uri;
+    transferInitializing(dataSpec);
     readPosition = (int) dataSpec.position;
     bytesRemaining = (int) ((dataSpec.length == C.LENGTH_UNSET)
         ? (data.length - dataSpec.position) : dataSpec.length);

library/core/src/main/java/com/google/android/exoplayer2/upstream/ContentDataSource.java

@@ -73,6 +73,7 @@ public final class ContentDataSource extends BaseDataSource {
   public long open(DataSpec dataSpec) throws ContentDataSourceException {
     try {
       uri = dataSpec.uri;
+      transferInitializing(dataSpec);
       assetFileDescriptor = resolver.openAssetFileDescriptor(uri, "r");
       if (assetFileDescriptor == null) {
         throw new FileNotFoundException("Could not open file descriptor for: " + uri);

library/core/src/main/java/com/google/android/exoplayer2/upstream/DataSchemeDataSource.java

@@ -39,6 +39,7 @@ public final class DataSchemeDataSource extends BaseDataSource {
 
   @Override
   public long open(DataSpec dataSpec) throws IOException {
+    transferInitializing(dataSpec);
     this.dataSpec = dataSpec;
     Uri uri = dataSpec.uri;
     String scheme = uri.getScheme();

library/core/src/main/java/com/google/android/exoplayer2/upstream/DefaultHttpDataSource.java

@@ -201,6 +201,7 @@ public class DefaultHttpDataSource extends BaseDataSource implements HttpDataSou
     this.dataSpec = dataSpec;
     this.bytesRead = 0;
     this.bytesSkipped = 0;
+    transferInitializing(dataSpec);
     try {
       connection = makeConnection(dataSpec);
     } catch (IOException e) {

library/core/src/main/java/com/google/android/exoplayer2/upstream/FileDataSource.java

@@ -57,6 +57,7 @@ public final class FileDataSource extends BaseDataSource {
   public long open(DataSpec dataSpec) throws FileDataSourceException {
     try {
       uri = dataSpec.uri;
+      transferInitializing(dataSpec);
       file = new RandomAccessFile(dataSpec.uri.getPath(), "r");
       file.seek(dataSpec.position);
       bytesRemaining = dataSpec.length == C.LENGTH_UNSET ? file.length() - dataSpec.position

library/core/src/main/java/com/google/android/exoplayer2/upstream/RawResourceDataSource.java

@@ -105,6 +105,7 @@ public final class RawResourceDataSource extends BaseDataSource {
         throw new RawResourceDataSourceException("Resource identifier must be an integer.");
       }
 
+      transferInitializing(dataSpec);
       assetFileDescriptor = resources.openRawResourceFd(resourceId);
       inputStream = new FileInputStream(assetFileDescriptor.getFileDescriptor());
       inputStream.skip(assetFileDescriptor.getStartOffset());

library/core/src/main/java/com/google/android/exoplayer2/upstream/UdpDataSource.java

@@ -100,7 +100,7 @@ public final class UdpDataSource extends BaseDataSource {
     uri = dataSpec.uri;
     String host = uri.getHost();
     int port = uri.getPort();
-
+    transferInitializing(dataSpec);
     try {
       address = InetAddress.getByName(host);
       socketAddress = new InetSocketAddress(address, port);

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeDataSource.java

@@ -104,10 +104,12 @@ public class FakeDataSource extends BaseDataSource {
   public final long open(DataSpec dataSpec) throws IOException {
     Assertions.checkState(!openCalled);
     openCalled = true;
+
     // DataSpec requires a matching close call even if open fails.
     uri = dataSpec.uri;
     openedDataSpecs.add(dataSpec);
 
+    transferInitializing(dataSpec);
     fakeData = fakeDataSet.getData(uri.toString());
     if (fakeData == null) {
       throw new IOException("Data not found: " + dataSpec.uri);