Fix: Unity Addressables Download Progress Callback Not Firing

Here is how to fix Unity Addressables download progress that never updates between 0 and 100. Your loading screen freezes at 0%, then jumps directly to complete. Or you call handle.PercentComplete and get values that bear no resemblance to actual bytes downloaded. The Addressables API exposes two different progress sources, and only one of them is what you actually want for a download UI.

The Symptom

You start a content download with Addressables.DownloadDependenciesAsync(label). Your UI subscribes to handle.Completed hoping for incremental progress, but only gets a single callback at the end. You try handle.PercentComplete and observe values like 0, 0, 0, 1 with nothing between. Players see a stuck progress bar even though the network is clearly active.

What Causes This

Completed is a one-shot event. AsyncOperationHandle exposes a Completed callback, not a progress callback. It fires exactly once when the entire operation finishes. There is no built-in per-frame event for progress.

PercentComplete averages internal stages. The PercentComplete property tracks the operation through several phases (initialization, dependency resolution, download, load). For a content-only download, the value can spike from 0 to 1 because most stages are instant.

Asset is already cached. If the bundle is already on disk, no download happens and the progress is meaningless. Always check GetDownloadSizeAsync first to determine if a download is even necessary.

autoReleaseHandle dropping the handle early. If you call DownloadDependenciesAsync with autoReleaseHandle: true and store the handle, the handle becomes invalid as soon as the operation completes. Polling it for progress after that throws or returns nonsense.

The Fix

Step 1: Use GetDownloadStatus and poll. Spin a coroutine that updates the UI every frame.

using System.Collections;
using UnityEngine;
using UnityEngine.AddressableAssets;
using UnityEngine.ResourceManagement.AsyncOperations;

public class ContentDownloader : MonoBehaviour
{
    [SerializeField] private string downloadLabel = "dlc_pack_1";
    [SerializeField] private UnityEngine.UI.Slider progressBar;

    public IEnumerator DownloadAndTrack()
    {
        AsyncOperationHandle sizeHandle = Addressables.GetDownloadSizeAsync(downloadLabel);
        yield return sizeHandle;

        long totalBytes = (long)sizeHandle.Result;
        if (totalBytes == 0) yield break;  // Cached, nothing to download

        AsyncOperationHandle handle = Addressables.DownloadDependenciesAsync(
            downloadLabel, autoReleaseHandle: false);

        while (!handle.IsDone)
        {
            DownloadStatus status = handle.GetDownloadStatus();
            progressBar.value = status.Percent;
            yield return null;
        }

        Addressables.Release(handle);
    }
}

Step 2: Use bytes for accurate UI text. Showing “42 MB / 120 MB” reads more clearly than “35%”.

DownloadStatus s = handle.GetDownloadStatus();
label.text = $"{s.DownloadedBytes / 1048576}MB / {s.TotalBytes / 1048576}MB";

Step 3: Disable autoReleaseHandle when polling. If you want to read the handle after completion, pass autoReleaseHandle: false and release manually.

Step 4: Handle errors gracefully. Network drops are common on mobile. Watch for handle.Status == AsyncOperationStatus.Failed and surface a retry option.

if (handle.Status == AsyncOperationStatus.Failed)
{
    Debug.LogError($"Download failed: {handle.OperationException}");
    Addressables.Release(handle);
    ShowRetryButton();
}

Why GetDownloadStatus and Not PercentComplete

GetDownloadStatus walks the dependency tree and sums up only the download bytes for assets that are actually being fetched. PercentComplete divides total operations into stages with implementation-defined weights. The two values can disagree by 30% or more during a real download. For a player-facing UI, always use GetDownloadStatus.

Understanding the issue

Addressables decouple asset references from filesystem paths. The flexibility comes with configuration cost; bugs in the configuration manifest as 'this thing should load but doesn't'.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

The triage path for this kind of bug is long. The symptom appears in gameplay, but the cause is in a different system. The reporter describes the gameplay effect; the engineer has to translate that into a hypothesis about the underlying cause. Misdirection is common.

At the engine level, the behavior comes from a deliberate design decision in Unity. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

After applying the fix, the verification step has three parts: confirm the original repro is resolved, confirm no obvious regressions in adjacent functionality, and (for shipping titles) deploy to a small player cohort first and watch the crash and report rates. Each step catches something the others miss.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

Live games surface this bug class at scale. What's a rare edge case in development becomes a daily occurrence once you have a few thousand concurrent players. The class isn't 'this player has a unique setup'; it's 'one in N thousand sessions will trigger this exact combination'.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

Diagnostic approach

Before applying any fix, gather enough context to be confident you're addressing the actual cause and not a similar-looking symptom. The cheapest diagnostic step is reproducing the bug deterministically - if you can't get the same failure twice in a row, your fix attempts will be hard to evaluate. Lock down the reproduction first.

For Unity-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

The tooling around this bug class matters as much as the fix itself. Good logging, accessible profilers, and clear error messages turn 30-minute investigations into 5-minute ones. If your project doesn't have visibility into this code path, the first fix should add the visibility - the second fix uses it.

Within Unity, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Edge cases for this class of issue often involve specific timing: the first frame after a state change, the last frame before a transition, frames where multiple subsystems update simultaneously. Reproducing these reliably is part of what makes the bug class hard to test.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“Poll the handle. Use bytes. Release manually. Network failures need a retry path.”

Related Issues

For other Addressables download problems, see Addressables Failed To Load and Remote Catalog Update Failing.

PercentComplete lies. GetDownloadStatus does not.