Flutter InAppWebView下载PDF：权限请求失败及下载失败求助

问题概述 #

使用 Flutter flutter_inappwebview 展示网页内容，在三星Galaxy S24（Android 13+）设备上无法成功请求下载权限；即使手动授予权限，PDF下载流程仍会失败。

现有权限检查与下载代码 #

权限检查函数 #

Future<void> _checkPermissions() async {
    if (Platform.isAndroid) {
      if (Platform.version.startsWith("13") || Platform.version.startsWith("14")) {
        await [
          Permission.photos,
          Permission.videos,
          Permission.audio,
        ].request();
      } else if (Platform.version.compareTo("30") >= 0) {
        await Permission.manageExternalStorage.request();
      } else {
        await Permission.storage.request();
      }
    } else if (Platform.isIOS) {
      await Permission.photos.request();
    }
  }

文件下载函数 #

Future<void> _downloadFile(String url) async {
    try {
      Directory? directory;

      if (Platform.isAndroid) {
        directory = await getExternalStorageDirectory();
        String downloadsPath = "${directory!.path}/Download";
        directory = Directory(downloadsPath);

        if (!await directory.exists()) {
          await directory.create(recursive: true);
        }
      } else if (Platform.isIOS) {
        directory = await getApplicationDocumentsDirectory();
      }

      if (directory == null) {
        throw Exception("Klasör bulunamadı!");
      }

      print("📥 Dosya şu dizine indirilecek: ${directory.path}");

      final taskId = await FlutterDownloader.enqueue(
        url: url,
        savedDir: directory.path,
        showNotification: true,
        openFileFromNotification: true,
      );

      if (taskId != null) {
        print("✅ İndirme başladı, Task ID: $taskId");
        ScaffoldMessenger.of(context).showSnackBar(
          const SnackBar(content: Text("Dosya indiriliyor...")),
        );
      } else {
        throw Exception("Flutter Downloader Task ID alınamadı!");
      }
    } catch (e) {
      print("❌ HATA: Dosya indirilemedi: $e");
      ScaffoldMessenger.of(context).showSnackBar(
        SnackBar(content: Text("İndirme başarısız: $e")),
      );
    }
  }

错误信息 #

下载时出现权限相关错误，提示无法访问存储路径：

现有配置文件 #

AndroidManifest.xml #

<uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.READ_MEDIA_IMAGES"/>
    <uses-permission android:name="android.permission.READ_MEDIA_VIDEO"/>
    <uses-permission android:name="android.permission.READ_MEDIA_AUDIO"/>
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
    <uses-permission android:name="android.permission.FOREGROUND_SERVICE"/>
    <uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES"/>
    <uses-permission android:name="android.permission.MANAGE_EXTERNAL_STORAGE"
        tools:ignore="ScopedStorage" />
    <uses-permission android:name="android.permission.ACCESS_MEDIA_LOCATION"/>
    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
    <queries>
        <package android:name="com.android.providers.downloads" />
    </queries>

    <application
        android:label="AppName"
        android:requestLegacyExternalStorage="true"
        android:usesCleartextTraffic="true"
        android:allowBackup="true"
        android:name="${applicationName}"
        android:icon="@mipmap/launcher_icon">
        ...

build.gradle.kts #

import java.util.Properties
import java.io.FileInputStream

plugins {
    id("com.android.application")
    id("kotlin-android")
    // The Flutter Gradle Plugin must be applied after the Android and Kotlin Gradle plugins.
    id("dev.flutter.flutter-gradle-plugin")
}

val keystoreProperties = Properties()
val keystorePropertiesFile = rootProject.file("key.properties")
if (keystorePropertiesFile.exists()) {
    keystoreProperties.load(FileInputStream(keystorePropertiesFile))
}

android {
    namespace = "com.biledem.sinavraporlarim"
    compileSdk = flutter.compileSdkVersion
    ndkVersion = "27.0.12077973"

    compileOptions {
        sourceCompatibility = JavaVersion.VERSION_11
        targetCompatibility = JavaVersion.VERSION_11
    }

    kotlinOptions {
        jvmTarget = JavaVersion.VERSION_11.toString()
    }

    defaultConfig {
        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
        applicationId = "com.biledem.sinavraporlarim"
        // You can update the following values to match your application needs.
        // For more information, see: https://flutter.dev/to/review-gradle-config.
        minSdk = 21
        targetSdk = 33
        versionCode = flutter.versionCode
        versionName = flutter.versionName
    }

    signingConfigs {
        create("release") {
            keyAlias = keystoreProperties["keyAlias"] as String
            keyPassword = keystoreProperties["keyPassword"] as String
            storeFile = keystoreProperties["storeFile"]?.let { file(it) }
            storePassword = keystoreProperties["storePassword"] as String
        }
    }
    buildTypes {
        release {
            // TODO: Add your own signing config for the release build.
            // Signing with the debug keys for now, so `flutter run --release` works.
            signingConfig = signingConfigs.getByName("release")
        }
    }
}

flutter {
    source = "../.."
}

修复方案 #

1. 修正权限请求逻辑（Android 13+） #

Android 13+ 下载文件无需请求媒体权限，需专门请求下载权限。修改_checkPermissions函数：

Future<void> _checkPermissions() async {
  if (Platform.isAndroid) {
    if (Platform.version.startsWith("13") || Platform.version.startsWith("14")) {
      // Android 13+ 请求下载权限
      await Permission.downloads.request();
    } else if (Platform.version.compareTo("30") >= 0) {
      await Permission.manageExternalStorage.request();
    } else {
      await Permission.storage.request();
    }
  } else if (Platform.isIOS) {
    await Permission.photos.request();
  }
}

2. 更换为公共Download目录路径 #

getExternalStorageDirectory()返回应用私有存储路径，Android 13+ 写入公共Download目录需使用系统标准路径。修改_downloadFile中的目录获取逻辑：

if (Platform.isAndroid) {
  // 获取系统公共Download目录
  directory = await getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);
}

需导入相关依赖：

import 'package:path_provider/path_provider.dart';

3. 完善AndroidManifest配置 #

• 给WRITE_EXTERNAL_STORAGE添加版本限制，Android 13+不再需要该权限：

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" android:maxSdkVersion="32"/>

• 移除不必要的权限（如ACCESS_COARSE_LOCATION、ACCESS_MEDIA_LOCATION，若无需相关功能）

4. 升级Flutter Downloader版本 #

确保使用最新稳定版的flutter_downloader，旧版本可能存在Android 13+适配问题。在pubspec.yaml中更新：

dependencies:
  flutter_downloader: ^1.11.6 # 替换为最新版本

5. 检查URL与权限验证 #

• 确认PDF URL可直接访问，无防盗链或跨域限制
• 若URL需要Cookie验证，在下载时携带请求头：

final taskId = await FlutterDownloader.enqueue(
  url: url,
  savedDir: directory.path,
  showNotification: true,
  openFileFromNotification: true,
  headers: {"Cookie": "your_session_cookie"}, // 根据实际情况添加
);

6. 校验权限授予状态 #

在执行下载前，检查权限是否已授予：

await _checkPermissions();
if (Platform.isAndroid && await Permission.downloads.isGranted) {
  _downloadFile(url);
} else {
  ScaffoldMessenger.of(context).showSnackBar(
    const SnackBar(content: Text("下载权限未授予，无法进行下载")),
  );
}

内容的提问来源于stack exchange，提问作者Mehmet Mutlu