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Android Reference

Loader


android.content

Class Loader<D>

  • Type Parameters:
    D - The result returned when the load is complete
    Direct Known Subclasses:
    AsyncTaskLoader


    public class Loader<D>
    extends Object
    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.

    • Constructor Detail

      • 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 - used to retrieve the application context.
    • Method Detail

      • 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 - the result of the load
      • deliverCancellation

        public void deliverCancellation()
        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.
      • getContext

        public Context getContext()
        Returns:
        an application context retrieved from the Context passed to the constructor.
      • getId

        public int getId()
        Returns:
        the ID of this loader
      • registerListener

        public void registerListener(int id,
                            Loader.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.

      • registerOnLoadCanceledListener

        public void registerOnLoadCanceledListener(Loader.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 - The listener to register.
      • 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.
      • 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.
      • 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.
      • 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.

      • 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().
      • 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:
        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.
      • 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:
        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.
      • 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.

      • 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.
      • 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.

      • 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.
      • 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.
      • 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().
      • 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.

      • 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.
      • 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.
      • onContentChanged

        public void onContentChanged()
        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.

      • 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.
      • toString

        public String toString()
        Description copied from class: Object
        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.

        Overrides:
        toString in class Object
        Returns:
        a printable representation of this object.
      • dump

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


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