Great question! Let’s break this down clearly: you can show images in Apple Watch push notifications without pre-saving them on the watch, but you can’t send the image file directly in the APNs push payload. Here’s how it works:

• Why you can’t send images directly in the payload
  Apple Push Notification Service (APNs) limits payload sizes to roughly 4KB—way too small for image files. Instead, you’ll include a URL pointing to your image in the push payload.

• Use an iOS Notification Service Extension to handle the image
  This is the core solution. When the push arrives on the user’s iPhone, this extension runs in the background to:

  1. Fetch the image from the URL you included in the payload.
  2. Save the image as a temporary local file on the iPhone.
  3. Attach the image to the notification before it’s displayed and synced to the Apple Watch.

• Sync the enhanced notification to the Apple Watch
  Once the iPhone has processed the notification and added the image attachment, it automatically syncs this ready-to-display notification to the Apple Watch. The watch doesn’t need to download the image itself—it receives the fully prepared notification from the iPhone, so no pre-storage is needed on the watch side.

Key Implementation Notes #

• Payload setup: Add your image URL in a custom field (like media-url) and include required flags to trigger the extension. Example payload snippet:

  {
    "aps": {
      "alert": {
        "title": "New Image Alert",
        "body": "Check out this photo!"
      },
      "category": "IMAGE_NOTIFICATION",
      "mutable-content": 1
    },
    "media-url": "https://your-server.com/your-image.jpg"
  }
  

  The mutable-content: 1 flag tells iOS to route the notification through your Notification Service Extension, and the category links it to your extension’s configured settings.

• Extension configuration: In your Notification Service Extension’s Info.plist, set UNNotificationExtensionCategory to match the category in your payload (e.g., IMAGE_NOTIFICATION). This ensures the extension only processes the notifications you intend.

• Image best practices: Use watchOS-supported formats (JPEG, PNG, GIF) and optimize images for watch displays (aim for ~320x320 pixels for most models) to ensure fast loading and proper sizing.

• Network considerations: The extension uses system background networking to fetch the image, so you don’t need extra app permissions—just ensure your image URL uses HTTPS (required by APNs).

Once everything is set up, when the user gets the notification, their iPhone handles the image download and attachment, and the Apple Watch will display the image seamlessly without any pre-stored files on the watch.

内容的提问来源于stack exchange，提问作者Oskar Gusgård