Android Cheatsheet

This is a collection of visual guides that I find myself coming back to repeatedly while doing Android development. Open to contributions!

Table of contents

Activities and Fragments

User interface

Kotlin

Jetpack Compose

Architecture

Build process

General software engineering

Activities and Fragments

Lifecycle flowchart

Callback methods

Callback	When	What	Is Activity visible
onCreate()	Activity is first created	All the necessary UI elements should be initialized	no
onStart()	Activity becomes visible to user	Code that maintains the UI, start async tasks (get data from API or database), register listeners	yes
onResume()	Activity becomes interactable to user	Starts animations	yes
onPause()	User is leaving the activity	Stops animations	partially visible
onStop()	Activity is no longer visible to user	Unregisters listeners and resources allocated in onStart()	no
onRestart()	Activity in the stopped state is about to start again (on back click)	Cursor objects should be re-queried	no
onDestroy()	Activity is destroyed from memory		no

Launch modes

Mode	Default	Instantiation	New Task on Launch	Allow other activities within Task
standard	Yes	Everytime an intent is created, a new instance is created. Also instances can be member of multiple tasks and more than one instance in a Task.	No. Open in the same Task that originated the intent	Yes
singleTop	No	Exactly like standard but if the activity is at the top of the Task stack then it uses the existing instance.	No. Open in the same Task that originated the intent	Yes
singleTask	No	Single instance	Yes. Always a Root Task.	Yes
singleInstance	No	Single instance	Yes. Always a Root Task.	Never. Always the only activity in the task

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User interface

RecyclerView pieces

Piece	What it does
RecyclerView	The ViewGroup that contains the views corresponding to your data.
item	One data item of the list to display. Can be of any type; often a data class you define.
Adapter	Takes data and prepares it for RecyclerView to display.
ViewHolders	A pool of views for RecyclerView to use and reuse to display items. Each individual ViewHolder is a wrapper around a View.
View	A layout that can display one data item.
LayoutManager	Measures and positions individual item views within a RecyclerView and determines the policy for when to recycle item views that are no longer visible. The ones provided in the RecyclerView library are LinearLayoutManager, GridLayoutManager, and StaggeredGridLayoutManager

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ImageView ScaleTypes

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PorterDuff modes for tinting

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Kotlin

Kotlin Standard Library functions

Type system

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Kotlin Flow Cheatsheet

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Jetpack Compose

Three phases of a frame

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Centering items in Composables

Choosing an API to implement an animation

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Architecture

Uncle Bob's clean architecture

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Build process

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General software engineering

Code review checklist

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Gang of Four design patterns

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