Android APIs
public class

Loader

extends Object
java.lang.Object
   ↳ android.content.Loader<D>
Known Direct Subclasses
Known Indirect Subclasses

Class Overview

An abstract class that performs asynchronous loading of data. While Loaders are active they should monitor the source of their data and deliver new results when the contents change. See LoaderManager for more detail.

Note on threading: Clients of loaders should as a rule perform any calls on to a Loader from the main thread of their process (that is, the thread the Activity callbacks and other things occur on). Subclasses of Loader (such as AsyncTaskLoader) will often perform their work in a separate thread, but when delivering their results this too should be done on the main thread.

Subclasses generally must implement at least onStartLoading(), onStopLoading(), onForceLoad(), and onReset().

Most implementations should not derive directly from this class, but instead inherit from AsyncTaskLoader.

Developer Guides

For more information about using loaders, read the Loaders developer guide.

Summary

Nested Classes
class Loader.ForceLoadContentObserver An implementation of a ContentObserver that takes care of connecting it to the Loader to have the loader re-load its data when the observer is told it has changed. 
interface Loader.OnLoadCanceledListener<D> Interface that is implemented to discover when a Loader has been canceled before it finished loading its data. 
interface Loader.OnLoadCompleteListener<D> Interface that is implemented to discover when a Loader has finished loading its data. 
Public Constructors
Loader(Context context)
Stores away the application context associated with context.
Public Methods
void abandon()
This function will normally be called for you automatically by LoaderManager when restarting a Loader.
boolean cancelLoad()
Attempt to cancel the current load task.
void commitContentChanged()
Commit that you have actually fully processed a content change that was returned by takeContentChanged().
String dataToString(D data)
For debugging, converts an instance of the Loader's data class to a string that can be printed.
void deliverCancellation()
Informs the registered Loader.OnLoadCanceledListener that the load has been canceled.
void deliverResult(D data)
Sends the result of the load to the registered listener.
void dump(String prefix, FileDescriptor fd, PrintWriter writer, String[] args)
Print the Loader's state into the given stream.
void forceLoad()
Force an asynchronous load.
Context getContext()
int getId()
boolean isAbandoned()
Return whether this loader has been abandoned.
boolean isReset()
Return whether this load has been reset.
boolean isStarted()
Return whether this load has been started.
void onContentChanged()
Called when Loader.ForceLoadContentObserver detects a change.
void registerListener(int id, OnLoadCompleteListener<D> listener)
Registers a class that will receive callbacks when a load is complete.
void registerOnLoadCanceledListener(OnLoadCanceledListener<D> listener)
Registers a listener that will receive callbacks when a load is canceled.
void reset()
This function will normally be called for you automatically by LoaderManager when destroying a Loader.
void rollbackContentChanged()
Report that you have abandoned the processing of a content change that was returned by takeContentChanged() and would like to rollback to the state where there is again a pending content change.
final void startLoading()
This function will normally be called for you automatically by LoaderManager when the associated fragment/activity is being started.
void stopLoading()
This function will normally be called for you automatically by LoaderManager when the associated fragment/activity is being stopped.
boolean takeContentChanged()
Take the current flag indicating whether the loader's content had changed while it was stopped.
String toString()
Returns a string containing a concise, human-readable description of this object.
void unregisterListener(OnLoadCompleteListener<D> listener)
Remove a listener that was previously added with registerListener(int, Loader.OnLoadCompleteListener).
void unregisterOnLoadCanceledListener(OnLoadCanceledListener<D> listener)
Unregisters a listener that was previously added with registerOnLoadCanceledListener(Loader.OnLoadCanceledListener).
Protected Methods
void onAbandon()
Subclasses implement this to take care of being abandoned.
boolean onCancelLoad()
Subclasses must implement this to take care of requests to cancelLoad().
void onForceLoad()
Subclasses must implement this to take care of requests to forceLoad().
void onReset()
Subclasses must implement this to take care of resetting their loader, as per reset().
void onStartLoading()
Subclasses must implement this to take care of loading their data, as per startLoading().
void onStopLoading()
Subclasses must implement this to take care of stopping their loader, as per stopLoading().
[Expand]
Inherited Methods
From class java.lang.Object

Public Constructors

public Loader (Context context)

Added in API level 11

Stores away the application context associated with context. Since Loaders can be used across multiple activities it's dangerous to store the context directly; always use getContext() to retrieve the Loader's Context, don't use the constructor argument directly. The Context returned by getContext() is safe to use across Activity instances.

Parameters
context used to retrieve the application context.

Public Methods

public void abandon ()

Added in API level 11

This function will normally be called for you automatically by LoaderManager when restarting a Loader. When using a Loader with LoaderManager, you must not call this method yourself, or you will conflict with its management of the Loader. Tell the Loader that it is being abandoned. This is called prior to reset() to have it retain its current data but not report any new data.

public boolean cancelLoad ()

Added in API level 11

Attempt to cancel the current load task. Must be called on the main thread of the process.

Cancellation is not an immediate operation, since the load is performed in a background thread. If there is currently a load in progress, this method requests that the load be canceled, and notes this is the case; once the background thread has completed its work its remaining state will be cleared. If another load request comes in during this time, it will be held until the canceled load is complete.

Returns
  • Returns false if the task could not be canceled, typically because it has already completed normally, or because startLoading() hasn't been called; returns true otherwise. When true is returned, the task is still running and the Loader.OnLoadCanceledListener will be called when the task completes.

public void commitContentChanged ()

Added in API level 18

Commit that you have actually fully processed a content change that was returned by takeContentChanged(). This is for use with rollbackContentChanged() to handle situations where a load is cancelled. Call this when you have completely processed a load without it being cancelled.

public String dataToString (D data)

Added in API level 11

For debugging, converts an instance of the Loader's data class to a string that can be printed. Must handle a null data.

public void deliverCancellation ()

Added in API level 16

Informs the registered Loader.OnLoadCanceledListener that the load has been canceled. Should only be called by subclasses. Must be called from the process's main thread.

public void deliverResult (D data)

Added in API level 11

Sends the result of the load to the registered listener. Should only be called by subclasses. Must be called from the process's main thread.

Parameters
data the result of the load

public void dump (String prefix, FileDescriptor fd, PrintWriter writer, String[] args)

Added in API level 11

Print the Loader's state into the given stream.

Parameters
prefix Text to print at the front of each line.
fd The raw file descriptor that the dump is being sent to.
writer A PrintWriter to which the dump is to be set.
args Additional arguments to the dump request.

public void forceLoad ()

Added in API level 11

Force an asynchronous load. Unlike startLoading() this will ignore a previously loaded data set and load a new one. This simply calls through to the implementation's onForceLoad(). You generally should only call this when the loader is started -- that is, isStarted() returns true.

Must be called from the process's main thread.

public Context getContext ()

Added in API level 11

Returns
  • an application context retrieved from the Context passed to the constructor.

public int getId ()

Added in API level 11

Returns
  • the ID of this loader

public boolean isAbandoned ()

Added in API level 11

Return whether this loader has been abandoned. In this state, the loader must not report any new data, and must keep its last reported data valid until it is finally reset.

public boolean isReset ()

Added in API level 11

Return whether this load has been reset. That is, either the loader has not yet been started for the first time, or its reset() has been called.

public boolean isStarted ()

Added in API level 11

Return whether this load has been started. That is, its startLoading() has been called and no calls to stopLoading() or reset() have yet been made.

public void onContentChanged ()

Added in API level 11

Called when Loader.ForceLoadContentObserver detects a change. The default implementation checks to see if the loader is currently started; if so, it simply calls forceLoad(); otherwise, it sets a flag so that takeContentChanged() returns true.

Must be called from the process's main thread.

public void registerListener (int id, OnLoadCompleteListener<D> listener)

Added in API level 11

Registers a class that will receive callbacks when a load is complete. The callback will be called on the process's main thread so it's safe to pass the results to widgets.

Must be called from the process's main thread.

public void registerOnLoadCanceledListener (OnLoadCanceledListener<D> listener)

Added in API level 16

Registers a listener that will receive callbacks when a load is canceled. The callback will be called on the process's main thread so it's safe to pass the results to widgets. Must be called from the process's main thread.

Parameters
listener The listener to register.

public void reset ()

Added in API level 11

This function will normally be called for you automatically by LoaderManager when destroying a Loader. When using a Loader with LoaderManager, you must not call this method yourself, or you will conflict with its management of the Loader. Resets the state of the Loader. The Loader should at this point free all of its resources, since it may never be called again; however, its startLoading() may later be called at which point it must be able to start running again.

This updates the Loader's internal state so that isStarted() and isReset() will return the correct values, and then calls the implementation's onReset().

Must be called from the process's main thread.

public void rollbackContentChanged ()

Added in API level 18

Report that you have abandoned the processing of a content change that was returned by takeContentChanged() and would like to rollback to the state where there is again a pending content change. This is to handle the case where a data load due to a content change has been canceled before its data was delivered back to the loader.

public final void startLoading ()

Added in API level 11

This function will normally be called for you automatically by LoaderManager when the associated fragment/activity is being started. When using a Loader with LoaderManager, you must not call this method yourself, or you will conflict with its management of the Loader. Starts an asynchronous load of the Loader's data. When the result is ready the callbacks will be called on the process's main thread. If a previous load has been completed and is still valid the result may be passed to the callbacks immediately. The loader will monitor the source of the data set and may deliver future callbacks if the source changes. Calling stopLoading() will stop the delivery of callbacks.

This updates the Loader's internal state so that isStarted() and isReset() will return the correct values, and then calls the implementation's onStartLoading().

Must be called from the process's main thread.

public void stopLoading ()

Added in API level 11

This function will normally be called for you automatically by LoaderManager when the associated fragment/activity is being stopped. When using a Loader with LoaderManager, you must not call this method yourself, or you will conflict with its management of the Loader.

Stops delivery of updates until the next time startLoading() is called. Implementations should not invalidate their data at this point -- clients are still free to use the last data the loader reported. They will, however, typically stop reporting new data if the data changes; they can still monitor for changes, but must not report them to the client until and if startLoading() is later called.

This updates the Loader's internal state so that isStarted() will return the correct value, and then calls the implementation's onStopLoading().

Must be called from the process's main thread.

public boolean takeContentChanged ()

Added in API level 11

Take the current flag indicating whether the loader's content had changed while it was stopped. If it had, true is returned and the flag is cleared.

public String toString ()

Added in API level 11

Returns a string containing a concise, human-readable description of this object. Subclasses are encouraged to override this method and provide an implementation that takes into account the object's type and data. The default implementation is equivalent to the following expression:

   getClass().getName() + '@' + Integer.toHexString(hashCode())

See Writing a useful toString method if you intend implementing your own toString method.

Returns
  • a printable representation of this object.

public void unregisterListener (OnLoadCompleteListener<D> listener)

Added in API level 11

Remove a listener that was previously added with registerListener(int, Loader.OnLoadCompleteListener). Must be called from the process's main thread.

public void unregisterOnLoadCanceledListener (OnLoadCanceledListener<D> listener)

Added in API level 16

Unregisters a listener that was previously added with registerOnLoadCanceledListener(Loader.OnLoadCanceledListener). Must be called from the process's main thread.

Parameters
listener The listener to unregister.

Protected Methods

protected void onAbandon ()

Added in API level 11

Subclasses implement this to take care of being abandoned. This is an optional intermediate state prior to onReset() -- it means that the client is no longer interested in any new data from the loader, so the loader must not report any further updates. However, the loader must keep its last reported data valid until the final onReset() happens. You can retrieve the current abandoned state with isAbandoned().

protected boolean onCancelLoad ()

Added in API level 16

Subclasses must implement this to take care of requests to cancelLoad(). This will always be called from the process's main thread.

Returns
  • Returns false if the task could not be canceled, typically because it has already completed normally, or because startLoading() hasn't been called; returns true otherwise. When true is returned, the task is still running and the Loader.OnLoadCanceledListener will be called when the task completes.

protected void onForceLoad ()

Added in API level 11

Subclasses must implement this to take care of requests to forceLoad(). This will always be called from the process's main thread.

protected void onReset ()

Added in API level 11

Subclasses must implement this to take care of resetting their loader, as per reset(). This is not called by clients directly, but as a result of a call to reset(). This will always be called from the process's main thread.

protected void onStartLoading ()

Added in API level 11

Subclasses must implement this to take care of loading their data, as per startLoading(). This is not called by clients directly, but as a result of a call to startLoading().

protected void onStopLoading ()

Added in API level 11

Subclasses must implement this to take care of stopping their loader, as per stopLoading(). This is not called by clients directly, but as a result of a call to stopLoading(). This will always be called from the process's main thread.