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SavedStateHandle

public final class SavedStateHandle
extends Object

java.lang.Object
   ↳ androidx.lifecycle.SavedStateHandle

A handle to saved state passed down to ViewModel. You should use SavedStateViewModelFactory if you want to receive this object in ViewModel's constructor.

This is a key-value map that will let you write and retrieve objects to and from the saved state. These values will persist after the process is killed by the system and remain available via the same object.

You can read a value from it via get(String) or observe it via LiveData returned by getLiveData(String).

You can write a value to it via set(String, Object) or setting a value to MutableLiveData returned by getLiveData(String).

Summary

Public constructors

SavedStateHandle(Map<String, Object> initialState)

Creates a handle with the given initial arguments.

SavedStateHandle()

Creates a handle with the empty state.

Public methods

void clearSavedStateProvider(String key)

Clear any SavedStateRegistry.SavedStateProvider that was previously set via setSavedStateProvider(String, SavedStateProvider).

boolean contains(String key)
<T> T get(String key)

Returns a value associated with the given key.

<T> MutableLiveData<T> getLiveData(String key)

Returns a LiveData that access data associated with the given key.

<T> MutableLiveData<T> getLiveData(String key, T initialValue)

Returns a LiveData that access data associated with the given key.

Set<String> keys()

Returns all keys contained in this SavedStateHandle

Returned set contains all keys: keys used to get LiveData-s, to set SavedStateProviders and keys used in regular set(String, Object).

<T> T remove(String key)

Removes a value associated with the given key.

<T> void set(String key, T value)

Associate the given value with the key.

void setSavedStateProvider(String key, SavedStateRegistry.SavedStateProvider provider)

Set a SavedStateRegistry.SavedStateProvider that will have its state saved into this SavedStateHandle.

Inherited methods

Public constructors

SavedStateHandle

public SavedStateHandle (Map<String, Object> initialState)

Creates a handle with the given initial arguments.

Parameters
initialState Map

SavedStateHandle

public SavedStateHandle ()

Creates a handle with the empty state.

Public methods

clearSavedStateProvider

public void clearSavedStateProvider (String key)

Clear any SavedStateRegistry.SavedStateProvider that was previously set via setSavedStateProvider(String, SavedStateProvider). Note: calling this method within SavedStateRegistry.SavedStateProvider.saveState() is supported, but will only affect future state saving operations.

Parameters
key String: a key previously used with setSavedStateProvider(String, SavedStateRegistry.SavedStateProvider)

contains

public boolean contains (String key)

Parameters
key String

Returns
boolean true if there is value associated with the given key.

get

public T get (String key)

Returns a value associated with the given key.

Parameters
key String

Returns
T

getLiveData

public MutableLiveData<T> getLiveData (String key)

Returns a LiveData that access data associated with the given key.

Parameters
key String

Returns
MutableLiveData<T>

getLiveData

public MutableLiveData<T> getLiveData (String key, 
                T initialValue)

Returns a LiveData that access data associated with the given key. 

LiveData<String> liveData = savedStateHandle.get(KEY, "defaultValue");
LiveData can have null as a valid value. If the initialValue is null and the data does not already exist in the SavedStateHandle, the value of the returned LiveData will be set to null and observers will be notified. You can call getLiveData(String) if you want to avoid dispatching null to observers. 
String defaultValue = ...; // nullable
     LiveData<String> liveData;
     if (defaultValue != null) {
         liveData = savedStateHandle.getLiveData(KEY, defaultValue);
     } else {
         liveData = savedStateHandle.getLiveData(KEY);
     }

Parameters
key String: The identifier for the value
initialValue T: If no value exists with the given key, a new one is created with the given initialValue. Note that passing null will create a LiveData with null value.

Returns
MutableLiveData<T>

keys

public Set<String> keys ()

Returns all keys contained in this SavedStateHandle

Returned set contains all keys: keys used to get LiveData-s, to set SavedStateProviders and keys used in regular set(String, Object).

Returns
Set<String>

remove

public T remove (String key)

Removes a value associated with the given key. If there is a LiveData associated with the given key, it will be removed as well.

All changes to LiveData previously returned by getLiveData(String) won't be reflected in the saved state. Also that LiveData won't receive any updates about new values associated by the given key.

Parameters
key String: a key
Returns
T a value that was previously associated with the given key.

set

public void set (String key, 
                T value)

Associate the given value with the key. The value must have a type that could be stored in Bundle

Parameters
key String

value T

setSavedStateProvider

public void setSavedStateProvider (String key, 
                SavedStateRegistry.SavedStateProvider provider)

Set a SavedStateRegistry.SavedStateProvider that will have its state saved into this SavedStateHandle. This provides a mechanism to lazily provide the Bundle of saved state for the given key.

Calls to get(String) with this same key will return the previously saved state as a Bundle if it exists. 

     Bundle previousState = savedStateHandle.get("custom_object");
     if (previousState != null) {
         // Convert the previousState into your custom object
     }
     savedStateHandle.setSavedStateProvider("custom_object", () -> {
         Bundle savedState = new Bundle();
         // Put your custom object into the Bundle, doing any conversion required
         return savedState;
     });
Note: calling this method within SavedStateRegistry.SavedStateProvider.saveState() is supported, but will only affect future state saving operations.

Parameters
key String: a key which will populated with a Bundle produced by the provider
provider SavedStateRegistry.SavedStateProvider: a SavedStateProvider which will receive a callback to SavedStateRegistry.SavedStateProvider.saveState() when the state should be saved