Add TimeToFirstByteEstimator

PiperOrigin-RevId: 363188096

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

@@ -17,6 +17,7 @@ package com.google.android.exoplayer2.upstream;
 
 import android.os.Handler;
 import androidx.annotation.Nullable;
+import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.util.Assertions;
 import java.util.concurrent.CopyOnWriteArrayList;
 
@@ -107,6 +108,14 @@ public interface BandwidthMeter {
   long getBitrateEstimate();
 
   /**
+   * Returns the estimated time to first byte, in microseconds, or {@link C#TIME_UNSET} if no
+   * estimate is available.
+   */
+  default long getTimeToFirstByteEstimateUs() {
+    return C.TIME_UNSET;
+  }
+
+  /**
    * Returns the {@link TransferListener} that this instance uses to gather bandwidth information
    * from data transfers. May be null if the implementation does not listen to data transfers.
    */

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

+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.android.exoplayer2.upstream;
+
+import com.google.android.exoplayer2.C;
+
+/** Provides an estimate of the time to first byte of a transfer. */
+public interface TimeToFirstByteEstimator {
+  /**
+   * Returns the estimated time to first byte of the response body, in microseconds, or {@link
+   * C#TIME_UNSET} if no estimate is available.
+   */
+  long getTimeToFirstByteEstimateUs();
+
+  /** Resets the estimator. The estimator should drop all samples and reset to its initial state. */
+  void reset();
+
+  /**
+   * Called when a transfer is being initialized.
+   *
+   * @param dataSpec Describes the data for which the transfer is initialized.
+   */
+  void onTransferInitializing(DataSpec dataSpec);
+
+  /**
+   * Called when a transfer starts.
+   *
+   * @param dataSpec Describes the data being transferred.
+   */
+  void onTransferStart(DataSpec dataSpec);
+}