diff --git a/pom.xml b/pom.xml
index 2de2f3a..7c6afa5 100644
--- a/pom.xml
+++ b/pom.xml
@@ -27,5 +27,11 @@
 			<artifactId>httpclient5-fluent</artifactId>
 			<version>5.2.1</version>
 		</dependency>
+		<dependency>
+			<groupId>junit</groupId>
+			<artifactId>junit</artifactId>
+			<version>4.13.2</version>
+			<scope>test</scope>
+		</dependency>
 	</dependencies>
 </project>
diff --git a/src/main/java/com/imyeyu/network/ProgressiveRequest.java b/src/main/java/com/imyeyu/network/ProgressiveRequest.java
index 8552330..80070f7 100644
--- a/src/main/java/com/imyeyu/network/ProgressiveRequest.java
+++ b/src/main/java/com/imyeyu/network/ProgressiveRequest.java
@@ -1,9 +1,13 @@
 package com.imyeyu.network;
 
 import com.imyeyu.io.IO;
+import com.imyeyu.java.TimiJava;
 import org.apache.hc.client5.http.fluent.Request;
 import org.apache.hc.client5.http.fluent.Response;
+import org.apache.hc.core5.http.Header;
 import org.apache.hc.core5.http.HttpEntity;
+import org.apache.hc.core5.http.HttpResponse;
+import org.apache.hc.core5.http.ProtocolException;
 import org.apache.hc.core5.http.io.HttpClientResponseHandler;
 import org.apache.hc.core5.http.io.entity.EntityUtils;
 
@@ -29,6 +33,10 @@ public class ProgressiveRequest {
 		this.callback = callback;
 	}
 
+	public static ProgressiveRequest wrap(Request request) {
+		return new ProgressiveRequest(request, null);
+	}
+
 	public static ProgressiveRequest wrap(Request request, ProgressiveCallback callback) {
 		return new ProgressiveRequest(request, callback);
 	}
@@ -43,6 +51,19 @@ public class ProgressiveRequest {
 		return os.toByteArray();
 	}
 
+	public long length() throws IOException, ProtocolException {
+		HttpResponse response = request.execute().returnResponse();
+		final int status = response.getCode();
+		if (status < 200 || 300 <= status) {
+			throw new IOException("HTTP error status: " + status);
+		}
+		final Header contentLengthHeader = response.getHeader("Content-Length");
+		if (contentLengthHeader == null) {
+			throw new IOException("Content-Length header missing");
+		}
+		return Long.parseLong(contentLengthHeader.getValue());
+	}
+
 	private void processResponse(Response response, OutputStream os) throws IOException {
 		response.handleResponse((HttpClientResponseHandler<Void>) resp -> {
 			HttpEntity entity = resp.getEntity();
@@ -55,14 +76,18 @@ public class ProgressiveRequest {
 			}
 			long length = entity.getContentLength();
 
-			IO.toOutputStream(entity.getContent(), os, new IO.OnWriteCallback() {
+			if (TimiJava.isEmpty(callback)) {
+				IO.toOutputStream(entity.getContent(), os);
+			} else {
+				IO.toOutputStream(entity.getContent(), os, new IO.OnWriteCallback() {
 
-				@Override
-				public boolean handler(long total, long now) {
+					@Override
+					public boolean handler(long total, long now) {
 
-					return callback.handler(length, total, now);
-				}
-			});
+						return callback.handler(length, total, now);
+					}
+				});
+			}
 			EntityUtils.consume(entity);
 			return null;
 		});