java - Android: Customizing recent apps thumbnail (screenshot by default)

Question

The app I'm working on shows some sensitive information that must not be shown on the "Recent Tasks" screen when stopping the app by pressing the home button.

I'd like to blur the sensitive data in the screenshot or show the app logo instead.

I am aware of the following approaches but they don't fit my requirements:

• Setting the actvitie's android:excludeFromRecents to true in the manifiest prevents the app from being shown at all in the recent tasks. This would disrupt the user experience.
• Using FLAG_SECURE results in a blank card on the recents tasks screen. (How do I prevent Android taking a screenshot when my app goes to the background?) I don't like the blank screen. However, I'll stick to this solution if there is no workaround.
• Overriding onCreateThumbnail seems like the ideal solution but, unfortunately, doesn't work as it's currently not invoked by the OS :( (https://code.google.com/p/android/issues/detail?id=29370)

And then there are some workarounds that I tried out but that didn't work as hoped:

• Start a new activity that shows the app logo in onPause so that it's screenshot is shown instead of the actual activitie's one. But the new activity takes too long to open and it disrupts the user experience.
• Set the activitie's content view to an image of the app logo in onPause. That seemed like a great solution to me. Unfortunately, the screenshot for the recent tasks screen is taken at an unspecified time. During testing the app logo quickly appears before the app is closed when pressing 'Home' but the resulting screenshot shows the activity a short time before that.
• Removing the sensitive data from the widgets (e.g. textView.setText("")) has the same problem of screenshot timing just mentioned.

Any alternative ideas or solutions to the listed workarounds?

Answer 1

I looked into this a couple of months ago for the same purpose as you.

Unfortunately, I had to conclude that it is simply not possible. I dug through the android source code and confirmed it.

• There is no callbacks or methods from android that allows you to customize it (that works anyway). Besides FLAG_SECURE, this part of the code does not accept any input or change.
• OnPause and similar lifecycle methods are called too late (the screenshot is taken already). All lifecycle methods that would hint that you're about to go into the background runs too late.
• The image you see in the recent tasks is an actual screenshot - and thus isn't affected by changes you do (too late) to your view. That means you can't modify your view just-in-time (like making it invisible, replacing with something else, adding SECURE_FLAG, or any other obstruction of the view). As an aside, these images can be found on an emulator at /data/system_ce/0/recent_images.
• The only exception is using FLAG_SECURE, which will prevent the screenshot from being taken of your application. I experimented with setting this FLAG in onPause and removing it in onResume, however as mentioned already these lifecycle methods runs after the screenshot is taken already, and thus had absolutely no effect.

As discussed in How to change the snapshot shown by recent apps list? there used to be a callback that you could use to customize the thumbnail: onCreateThumbnail. However, this does not work and it is never called. To be clear, the callback is still there, it is simply never called by the OS. The fact that it stopped working is poorly documented, but apparently was silently deprecated/removed in 4.0.3

As for the thumbnail itself, it is a screenshot taken serverside. It is taken before onPause is called (or in fact before any callbacks indicating that your activity is about to go into the background is called).

When your app does go into the background, your actual view is animated (to get that zoom-out transition). That animation can be affected through changes you do in onPause (if you're fast enough that is) (I experimented with setting opacity to 0 on the window among other things). This will however only affect the animation. When the animation is finished, the view is replaced by the screenshot taken earlier.

Also see these questions that discuss this:

• When does Android take its recent apps switcher screenshot?
• Show custom application image in task manager on ICS or JB
• Android never call method onCreateThumbnail