Is there any way to dynamically change an app icon like Calendar app does?
Whatever your home screen is has special hooks for whatever your calendar app is and whatever your alarm clock app is. In general, apps cannot update their icons.
What do you mean by hooks?
For example, Samsung can ship a Samsung calendar app on Samsung devices. Samsung's home screen on those same Samsung devices can have special rules for rendering an icon for Samsung's calendar app, rules that involve showing the day of the month. This is because Samsung wrote the home screen. If you install a third-party home screen, it may not do the same thing. After all, I can write a home screen in an hour or so, and I feel quite confident that I don't have to do anything special for Samsung's calendar app.
There's nothing stopping Samsung from exposing some sort of API to allow developers to hook into Samsung's home screen and notify it about this sort of thing. Whether Samsung intends for third parties to use that API, or whether it is somebody hacking into how Samsung does it for their own apps, I can't say.
(BTW, I am citing Samsung here as a possible example -- I don't know that they actually have this sort of feature, and if so on which devices they have it)
I seem to recall that somebody has a GitHub project that tries to wrap the proprietary APIs of various home screens. IIRC, some supported capabilities included either replacing the app icon or adding a badge (e.g., unread message count). However:
Only a small percentage of devices will support those proprietary APIs
Undocumented and unsupported APIs, discovered through reverse-engineering apps, are subject to change and may break in unexpected ways
I am quite certain that there is nothing in the Android SDK that supports dynamic app icons. The only thing that I know of, that people have tried, is using <activity-alias> to have N different "activities", all pointing to the same implementation, but having different icons. Using PackageManager and setComponentEnabledSetting(), the app disables the old launcher alias and enables a different one, in hopes that home screens will pick up on this and show the new icon. A few do. Others only would find out about the change on a reboot.
To flip the problem around, I can write a home screen. Perhaps I want to offer some way for apps to change their icons on the fly, even though there are no standards for it. Perhaps I don't. Perhaps I do not intend to use icons at all, as my home screen is optimized for the visually impaired, and so it is using text-to-speech and hardware key input. It's my home screen implementation, and I can do what I want.
Indeed even in the app drawer in Android O, the Clock app icon shows the current time and the Calendar app icon shows the current day-of-month.
This article explains:
Chris Lacy, the developer behind the well-known Action Launcher, has uncovered something in the APK file of the Clock app that comes in Android O: the manifest.xml mentions hours, minutes, and seconds as layers with default values, and the icon has separate images for the background and different elements.
That logically lead him to deduce that starting with Android O, the Clock app's icon will be animated to show the current time. That will happen both when the Clock is placed as a shortcut on the homescreen and when it's inside the app drawer.
It sounds like the Pixel launcher selects different icon image layer resources depending on time and date.
I have the same question but the answer provided here does not aswer it.
I have Google Pixel 2 with raw Android Ore 8.1 on the board and there are two apps that are changing over time: Calendar and Clock. The hands on the clock of Clock app show current time and the number on the Calendar icon correspond to today date of month.
So taking into account that it is default launcher probably there is some API to do similar thing.
For instance it might be the planetary app that reflects planets' positions on the icon. Or memory cleaner that shows percentage of used memory...
I'm a launcher developer.
I did nothing related to dynamic icon hooks. I just get the activities' icons by the standard way provided by Android API.
And I found that my launcher do show the current time in the Clock app icon and the current date in the Calendar icon.
The device is a Nubia X which runs Android 8.
The operating system should have done something with the two apps and changed their icons dynamically without the launcher beings aware of it.
These two apps are developed by the device manufacturer. You should not be able to do the same thing in a normal way as a third party developer.