Add additional downloader implementations

RecordingDownloader relies on the real downloader and saves the request/response pair into a json file.
MockDownloader uses json files from above and mocks responses for specific requests.

Author: XiangRongLin

extractor/build.gradle

@@ -9,4 +9,6 @@ dependencies {
 
     testImplementation 'junit:junit:4.13.1'
     testImplementation "com.squareup.okhttp3:okhttp:3.12.11"
+    testImplementation 'com.google.code.gson:gson:2.8.6'
+    testImplementation 'commons-io:commons-io:2.8.0'
 }

extractor/src/test/java/org/schabi/newpipe/downloader/MockDownloader.java

@@ -0,0 +1,55 @@
+package org.schabi.newpipe.downloader;
+
+import com.google.gson.GsonBuilder;
+
+import org.schabi.newpipe.extractor.downloader.Downloader;
+import org.schabi.newpipe.extractor.downloader.Request;
+import org.schabi.newpipe.extractor.downloader.Response;
+
+import java.io.File;
+import java.io.FileReader;
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Map;
+
+import javax.annotation.Nonnull;
+
+class MockDownloader extends Downloader {
+
+    private final String path;
+    private final Map<Request, Response> mocks;
+
+    public MockDownloader(@Nonnull String path) throws IOException {
+        this.path = path;
+        this.mocks = new HashMap<>();
+        File folder = new File(path);
+        for (File file : folder.listFiles()) {
+            final FileReader reader = new FileReader(file);
+            final TestRequestResponse response = new GsonBuilder()
+                    .create()
+                    .fromJson(reader, TestRequestResponse.class);
+            reader.close();
+            mocks.put(response.getRequest(), response.getResponse());
+        }
+
+        //shared find proper solution
+        File clientVersion = new File("src/test/resources/org/schabi/newpipe/extractor/services/youtube/client_version.json");
+        final FileReader reader = new FileReader(clientVersion);
+        final TestRequestResponse response = new GsonBuilder()
+                .create()
+                .fromJson(reader, TestRequestResponse.class);
+        reader.close();
+        mocks.put(response.getRequest(), response.getResponse());
+    }
+
+    @Override
+    public Response execute(@Nonnull Request request) {
+        Response result = mocks.get(request);
+        if (result == null) {
+            throw new NullPointerException("No mock response for request with url '" + request.url()
+                    + "' exists in path '" + path + "'.\nPlease make sure to run the tests with " +
+                    "the RecordingDownloader first after changes.");
+        }
+        return result;
+    }
+}

extractor/src/test/java/org/schabi/newpipe/downloader/RecordingDownloader.java

@@ -0,0 +1,53 @@
+package org.schabi.newpipe.downloader;
+
+import com.google.gson.GsonBuilder;
+
+import org.apache.commons.io.FileUtils;
+import org.schabi.newpipe.extractor.downloader.Downloader;
+import org.schabi.newpipe.extractor.downloader.Request;
+import org.schabi.newpipe.extractor.downloader.Response;
+import org.schabi.newpipe.extractor.exceptions.ReCaptchaException;
+
+import java.io.File;
+import java.io.FileWriter;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
+import javax.annotation.Nonnull;
+
+class RecordingDownloader extends Downloader {
+
+    private int index = 0;
+    private final String path;
+
+    public RecordingDownloader(String stringPath) throws IOException {
+        this.path = stringPath;
+        Path path = Paths.get(stringPath);
+        File directory = path.toFile();
+        if (directory.exists()) {
+            FileUtils.cleanDirectory(directory);
+        }
+        Files.createDirectories(path);
+    }
+
+    @Override
+    public Response execute(@Nonnull Request request) throws IOException, ReCaptchaException {
+        Downloader downloader = DownloaderTestImpl.getInstance();
+        Response response = downloader.execute(request);
+
+        File outputFile = new File(path + File.separator + index + ".json");
+        index++;
+        outputFile.createNewFile();
+        FileWriter writer = new FileWriter(outputFile);
+        new GsonBuilder()
+                .setPrettyPrinting()
+                .create()
+                .toJson(new TestRequestResponse(request, response), writer);
+        writer.flush();
+        writer.close();
+
+        return response;
+    }
+}

extractor/src/test/java/org/schabi/newpipe/downloader/TestRequestResponse.java

@@ -0,0 +1,28 @@
+package org.schabi.newpipe.downloader;
+
+import org.schabi.newpipe.extractor.downloader.Request;
+import org.schabi.newpipe.extractor.downloader.Response;
+
+final class TestRequestResponse {
+    private final String comment;
+    private final Request request;
+    private final Response response;
+
+    public TestRequestResponse(Request request, Response response) {
+        this.comment = "Auto-generated for tests. See RecordingDownloader";
+        this.request = request;
+        this.response = response;
+    }
+
+    public String getComment() {
+        return comment;
+    }
+
+    public Request getRequest() {
+        return request;
+    }
+
+    public Response getResponse() {
+        return response;
+    }
+}