iOS端UnityWebRequest后台下载中断问题求助（Unity2019.2）

Hey there, let's figure out how to fix this iOS background download interruption issue with your Unity 2019.2 project. The logs you shared point to two key problems: iOS suspending your network requests when the app goes background, and mismanaged background tasks. Here's a step-by-step solution:

1. Enable iOS Background Modes Permissions #

First, you need to tell iOS your app needs to perform network tasks in the background.

• Open your Unity project, go to Player Settings > iOS > Other Settings > Configuration
• Find the Background Modes section, check the boxes for:
  • Background fetch
  • Remote notifications (this helps keep network sessions alive, even if you don't use push notifications)
• Alternatively, if you're building directly to Xcode, add the UIBackgroundModes key to your Info.plist with values fetch and remote-notification.

2. Manage iOS Background Tasks in Your Download Code #

iOS requires apps to explicitly request time to finish tasks when entering the background. Unity's default UnityWebRequest doesn't handle this automatically, so we'll add code to wrap your download in a background task.

Add this C# helper class to your project:

using UnityEngine;
using System.Runtime.InteropServices;

public class iOSBackgroundTaskHelper : MonoBehaviour
{
    [DllImport("__Internal")]
    private static extern int StartBackgroundTask(string taskName);

    [DllImport("__Internal")]
    private static extern void EndBackgroundTask(int taskId);

    private static int currentBackgroundTaskId = -1;

    public static void StartDownloadBackgroundTask()
    {
#if UNITY_IOS && !UNITY_EDITOR
        currentBackgroundTaskId = StartBackgroundTask("AssetBundleDownload");
#endif
    }

    public static void EndDownloadBackgroundTask()
    {
#if UNITY_IOS && !UNITY_EDITOR
        if (currentBackgroundTaskId != -1)
        {
            EndBackgroundTask(currentBackgroundTaskId);
            currentBackgroundTaskId = -1;
        }
#endif
    }
}

Then create a small iOS plugin file (name it BackgroundTask.m and place it in your Assets/Plugins/iOS folder):

#import <UIKit/UIKit.h>

int StartBackgroundTask(const char* taskName) {
    NSString* name = [NSString stringWithUTF8String:taskName];
    UIApplication* app = [UIApplication sharedApplication];
    UIBackgroundTaskIdentifier taskId = [app beginBackgroundTaskWithName:name expirationHandler:^{
        [app endBackgroundTask:taskId];
    }];
    return (int)taskId;
}

void EndBackgroundTask(int taskId) {
    UIApplication* app = [UIApplication sharedApplication];
    [app endBackgroundTask:(UIBackgroundTaskIdentifier)taskId];
}

Now update your download coroutine to use this helper:

private static IEnumerator DownloadBundleCoroutine (UnityWebRequest wr, bool forImmediateLoad, AssetBundleDownloadProgress progress, string bundleName, DownloadBundleFinishedDelegate finished, DownloadBundleErrorDelegate error) {
    // Start background task before sending request
    iOSBackgroundTaskHelper.StartDownloadBackgroundTask();

    yield return wr.SendWebRequest();

    // End background task once download completes/fails
    iOSBackgroundTaskHelper.EndDownloadBackgroundTask();

    if (wr.isNetworkError) {
        // Handle the "network connection lost" error specifically with retry logic
        if (wr.error.Contains("-1005") || wr.error.Contains("The network connection was lost")) {
            // Optionally add a retry limit to avoid infinite loops
            Debug.Log($"Download interrupted, retrying {bundleName}...");
            // Re-initiate the download
            yield return DownloadBundleCoroutine(wr, forImmediateLoad, progress, bundleName, finished, error);
            yield break;
        }
        error(wr.error);
    } else {
        AssetBundle bundle = ((DownloadHandlerAssetBundle)wr.downloadHandler).assetBundle;
        if (bundle == null) {
            error.Invoke($"Error loading bundle {bundleName}, probably another bundle with same files is already loaded.");
        } else {
            finished.Invoke(bundle, progress);
            if(!forImmediateLoad) bundle.Unload(true);
        }
    }
}

3. Adjust UnityWebRequest Settings for iOS #

A couple of small tweaks can help with iOS stability:

• Remove the Cache-Control, Pragma, and Expires headers if you don't absolutely need them—iOS's network stack handles caching more reliably on its own, and these headers might interfere with background session behavior.
• Ensure you're on the latest patch release of Unity 2019.2, as later patches include fixes for iOS network handling.

4. Test the Background Behavior #

When testing, use Xcode's Debug > Simulate Background option to properly trigger the background state. This will help you verify if the background task is working correctly and if the download resumes or completes as expected.

The core idea here is explicitly asking iOS for extra time to finish the download when the app goes background, plus adding retry logic for when the connection does get dropped. This should resolve the -1005 network error and the background task warning you're seeing.

内容的提问来源于stack exchange，提问作者Florian