Technical August 18, 2020
Android Activity Result Contracts
Before Activity Result Contracts
Previously, passing data from an exiting activity to the launching activity or handling permission request results would require creating Intents and overriding onActivityResult() and using some if / else or switch statements (or when statements if you prefer Kotlin). While this is easy enough to use, if your activity may need to handle the result of a variety of exiting intents / permission results. The onActivityResult() method can quickly become a long and complex mess that isn’t the easiest to read.
Suppose our MainActivity needs to handle requesting microphone, camera, and location permissions in addition to a string message received from another activity. That might look something like this.
Built-In Contracts
Take a Picture / Video
Easily capture images / videos from the Camera without having to deal with Media Intent. For photos use TakePicture() and for videos use TakeVideo().
Request Permission(s)
Ask the user to grant Runtime Permission(s). To request one permission use RequestPermission() which will return a singleBoolean indicating if the permission was granted.
To request multiple permissions simultaneously use RequestMultiplePermissions() which would return a map of Booleans indicating which permissions were granted.
And More!
Custom Contracts
Getting Started
Activity Result APIs were introduced in AndroidX Activity 1.2.0-alpha02 and Fragment 1.3.0-alpha02 and they provide components for registering for a result, launching the result, and handling the result once it is dispatched by the system.
Add the following dependencies to the app’s build.gradle file. (At time of writing these were the latest versions of activity-ktx and fragment-ktx. Feel free to use any version later than this.)
Example Contract
Note: We will be using View Binding for code simplicity.
For this example, we want a simple MessageContract that will return a String from one activity to another.
Let’s start by defining a MessageContract which will create an Intent launching MessageActivity.
And a simple MessageActivity layout could contain an EditText and Button defining an onClick method called onSubmitMessage() . Here’s the MessageActivity layout XML from our code.
Let’s define how MessageActivity should handle the button click in onSubmitMessage() — by packaging the String inputted by the user in the EditText in an Intent, setting the Activity result to RESULT_OK , and finishing the activity withfinish().
When the MessageActivity is closed, the resulting String can be parsed from the Intent and returned in the MessageContract's parseResult() .
Note that in the above example we pass in Unit as the input and a String as the output. If you wanted to provide arguments to the next activity, you would do so in the input portion. We just use Unit when you want to specify the input as nothing.
In our receiving activity, we need to specify a variable for the ActivityResultLauncher returned by the registerForActivityResult() where we pass in our custom MessageContract() .
Here we will define a lambda that is to be executed when the contract parses the result. In this example, we are just displaying the String message in a Toast to the user. The result it is expected to be a String as was specified to be the output type by our contract — class MessageContract : ActivityResultContract<Unit, String>() .
To launch the messageLauncher in our receiving activity, we simply need to call messageLauncher.launch(). This will call the createIntent() method we overrode in our custom MessageContract and create and launch the MessageActivity Intent.
