Loader
public
class
Loader
extends Object
| java.lang.Object | |
| ↳ | android.content.Loader<D> |
This class was deprecated
in API level 28.
Use the Support Library
Loader
A 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
This class was deprecated
in API level 28.
Use |
interface |
Loader.OnLoadCanceledListener<D>
This interface was deprecated
in API level 28.
Use |
interface |
Loader.OnLoadCompleteListener<D>
This interface was deprecated
in API level 28.
Use |
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
|
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 |
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 |
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 |
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
|
void
|
rollbackContentChanged()
Report that you have abandoned the processing of a content change that
was returned by |
final
void
|
startLoading()
This function will normally be called for you automatically by
|
void
|
stopLoading()
This function will normally be called for you automatically by
|
boolean
|
takeContentChanged()
Take the current flag indicating whether the loader's content had changed while it was stopped. |
String
|
toString()
Returns a string representation of the object. |
void
|
unregisterListener(OnLoadCompleteListener<D> listener)
Remove a listener that was previously added with |
void
|
unregisterOnLoadCanceledListener(OnLoadCanceledListener<D> listener)
Unregisters a listener that was previously added with
|
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 |
void
|
onForceLoad()
Subclasses must implement this to take care of requests to |
void
|
onReset()
Subclasses must implement this to take care of resetting their loader,
as per |
void
|
onStartLoading()
Subclasses must implement this to take care of loading their data,
as per |
void
|
onStopLoading()
Subclasses must implement this to take care of stopping their loader,
as per |
Inherited methods | |
|---|---|
Public constructors
Loader
public Loader (Context context)
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 |
Context: used to retrieve the application context. |
Public methods
abandon
public void abandon ()
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.
cancelLoad
public boolean cancelLoad ()
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 | |
|---|---|
boolean |
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 OnLoadCanceledListener will be called
when the task completes. |
commitContentChanged
public void commitContentChanged ()
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.
dataToString
public String dataToString (D data)
For debugging, converts an instance of the Loader's data class to a string that can be printed. Must handle a null data.
| Parameters | |
|---|---|
data |
D |
| Returns | |
|---|---|
String |
|
deliverCancellation
public void deliverCancellation ()
Informs the registered OnLoadCanceledListener that the load has been canceled.
Should only be called by subclasses.
Must be called from the process's main thread.
deliverResult
public void deliverResult (D data)
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 |
D: the result of the load |
dump
public void dump (String prefix, FileDescriptor fd, PrintWriter writer, String[] args)
Print the Loader's state into the given stream.
| Parameters | |
|---|---|
prefix |
String: Text to print at the front of each line. |
fd |
FileDescriptor: The raw file descriptor that the dump is being sent to. |
writer |
PrintWriter: A PrintWriter to which the dump is to be set. |
args |
String: Additional arguments to the dump request. |
forceLoad
public void forceLoad ()
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.
getContext
public Context getContext ()
| Returns | |
|---|---|
Context |
an application context retrieved from the Context passed to the constructor. |
isAbandoned
public boolean isAbandoned ()
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.
| Returns | |
|---|---|
boolean |
|
isReset
public boolean isReset ()
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.
| Returns | |
|---|---|
boolean |
|
isStarted
public boolean isStarted ()
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.
| Returns | |
|---|---|
boolean |
|
onContentChanged
public void onContentChanged ()
Called when 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.
registerListener
public void registerListener (int id,
OnLoadCompleteListener<D> listener)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.
| Parameters | |
|---|---|
id |
int |
listener |
OnLoadCompleteListener |
registerOnLoadCanceledListener
public void registerOnLoadCanceledListener (OnLoadCanceledListener<D> listener)
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 |
OnLoadCanceledListener: The listener to register. |
reset
public void reset ()
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.
rollbackContentChanged
public 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. 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.
startLoading
public final void startLoading ()
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.
stopLoading
public void stopLoading ()
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.
takeContentChanged
public boolean takeContentChanged ()
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.
| Returns | |
|---|---|
boolean |
|
toString
public String toString ()
Returns a string representation of the object.
| Returns | |
|---|---|
String |
a string representation of the object. |
unregisterListener
public void unregisterListener (OnLoadCompleteListener<D> listener)
Remove a listener that was previously added with registerListener(int, OnLoadCompleteListener).
Must be called from the process's main thread.
| Parameters | |
|---|---|
listener |
OnLoadCompleteListener |
unregisterOnLoadCanceledListener
public void unregisterOnLoadCanceledListener (OnLoadCanceledListener<D> listener)
Unregisters a listener that was previously added with
registerOnLoadCanceledListener(OnLoadCanceledListener).
Must be called from the process's main thread.
| Parameters | |
|---|---|
listener |
OnLoadCanceledListener: The listener to unregister. |
Protected methods
onAbandon
protected void onAbandon ()
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().
onCancelLoad
protected boolean onCancelLoad ()
Subclasses must implement this to take care of requests to cancelLoad().
This will always be called from the process's main thread.
| Returns | |
|---|---|
boolean |
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 OnLoadCanceledListener will be called
when the task completes. |
onForceLoad
protected void onForceLoad ()
Subclasses must implement this to take care of requests to forceLoad().
This will always be called from the process's main thread.
onReset
protected void onReset ()
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.
onStartLoading
protected void onStartLoading ()
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().
onStopLoading
protected void onStopLoading ()
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.
Content and code samples on this page are subject to the licenses described in the Content License. Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.
Last updated 2024-04-04 UTC.