/*
* Copyright (C) 2010 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package android.content;
import android.database.ContentObserver;
import android.os.Handler;
import android.util.DebugUtils;
import java.io.FileDescriptor;
import java.io.PrintWriter;
/**
* 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 {@link android.app.LoaderManager} for more detail.
*
* <p><b>Note on threading:</b> 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 {@link AsyncTaskLoader}) will often perform their work
* in a separate thread, but when delivering their results this too should
* be done on the main thread.</p>
*
* <p>Subclasses generally must implement at least {@link #onStartLoading()},
* {@link #onStopLoading()}, {@link #onForceLoad()}, and {@link #onReset()}.</p>
*
* <p>Most implementations should not derive directly from this class, but
* instead inherit from {@link AsyncTaskLoader}.</p>
*
* <div class="special reference">
* <h3>Developer Guides</h3>
* <p>For more information about using loaders, read the
* <a href="{@docRoot}guide/topics/fundamentals/loaders.html">Loaders</a> developer guide.</p>
* </div>
*
* @param <D> The result returned when the load is complete
*/
public class Loader<D> {
int mId;
OnLoadCompleteListener<D> mListener;
OnLoadCanceledListener<D> mOnLoadCanceledListener;
Context mContext;
boolean mStarted = false;
boolean mAbandoned = false;
boolean mReset = true;
boolean mContentChanged = false;
boolean mProcessingChange = false;
/**
* 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. You do not normally need to use this yourself;
* it is used for you by {@link CursorLoader} to take care of executing
* an update when the cursor's backing data changes.
*/
public final class ForceLoadContentObserver extends ContentObserver {
public ForceLoadContentObserver() {
super(new Handler());
}
@Override
public boolean deliverSelfNotifications() {
return true;
}
@Override
public void onChange(boolean selfChange) {
onContentChanged();
}
}
/**
* Interface that is implemented to discover when a Loader has finished
* loading its data. You do not normally need to implement this yourself;
* it is used in the implementation of {@link android.app.LoaderManager}
* to find out when a Loader it is managing has completed so that this can
* be reported to its client. This interface should only be used if a
* Loader is not being used in conjunction with LoaderManager.
*/
public interface OnLoadCompleteListener<D> {
/**
* Called on the thread that created the Loader when the load is complete.
*
* @param loader the loader that completed the load
* @param data the result of the load
*/
public void onLoadComplete(Loader<D> loader, D data);
}
/**
* Interface that is implemented to discover when a Loader has been canceled
* before it finished loading its data. You do not normally need to implement
* this yourself; it is used in the implementation of {@link android.app.LoaderManager}
* to find out when a Loader it is managing has been canceled so that it
* can schedule the next Loader. This interface should only be used if a
* Loader is not being used in conjunction with LoaderManager.
*/
public interface OnLoadCanceledListener<D> {
/**
* Called on the thread that created the Loader when the load is canceled.
*
* @param loader the loader that canceled the load
*/
public void onLoadCanceled(Loader<D> loader);
}
/**
* 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 {@link #getContext()} to retrieve
* the Loader's Context, don't use the constructor argument directly.
* The Context returned by {@link #getContext} is safe to use across
* Activity instances.
*
* @param context used to retrieve the application context.
*/
public Loader(Context context) {
mContext = context.getApplicationContext();
}
/**
* 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.
*
* @param data the result of the load
*/
public void deliverResult(D data) {
if (mListener != null) {
mListener.onLoadComplete(this, data);
}
}
/**
* Informs the registered {@link OnLoadCanceledListener} that the load has been canceled.
* Should only be called by subclasses.
*
* Must be called from the process's main thread.
*/
public void deliverCancellation() {
if (mOnLoadCanceledListener != null) {
mOnLoadCanceledListener.onLoadCanceled(this);
}
}
/**
* @return an application context retrieved from the Context passed to the constructor.
*/
public Context getContext() {
return mContext;
}
/**
* @return the ID of this loader
*/
public int getId() {
return mId;
}
/**
* 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.
*
* <p>Must be called from the process's main thread.
*/
public void registerListener(int id, OnLoadCompleteListener<D> listener) {
if (mListener != null) {
throw new IllegalStateException("There is already a listener registered");
}
mListener = listener;
mId = id;
}
/**
* Remove a listener that was previously added with {@link #registerListener}.
*
* Must be called from the process's main thread.
*/
public void unregisterListener(OnLoadCompleteListener<D> listener) {
if (mListener == null) {
throw new IllegalStateException("No listener register");
}
if (mListener != listener) {
throw new IllegalArgumentException("Attempting to unregister the wrong listener");
}
mListener = null;
}
/**
* 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.
*
* @param listener The listener to register.
*/
public void registerOnLoadCanceledListener(OnLoadCanceledListener<D> listener) {
if (mOnLoadCanceledListener != null) {
throw new IllegalStateException("There is already a listener registered");
}
mOnLoadCanceledListener = listener;
}
/**
* Unregisters a listener that was previously added with
* {@link #registerOnLoadCanceledListener}.
*
* Must be called from the process's main thread.
*
* @param listener The listener to unregister.
*/
public void unregisterOnLoadCanceledListener(OnLoadCanceledListener<D> listener) {
if (mOnLoadCanceledListener == null) {
throw new IllegalStateException("No listener register");
}
if (mOnLoadCanceledListener != listener) {
throw new IllegalArgumentException("Attempting to unregister the wrong listener");
}
mOnLoadCanceledListener = null;
}
/**
* Return whether this load has been started. That is, its {@link #startLoading()}
* has been called and no calls to {@link #stopLoading()} or
* {@link #reset()} have yet been made.
*/
public boolean isStarted() {
return mStarted;
}
/**
* Return whether this loader has been abandoned. In this state, the
* loader <em>must not</em> report any new data, and <em>must</em> keep
* its last reported data valid until it is finally reset.
*/
public boolean isAbandoned() {
return mAbandoned;
}
/**
* Return whether this load has been reset. That is, either the loader
* has not yet been started for the first time, or its {@link #reset()}
* has been called.
*/
public boolean isReset() {
return mReset;
}
/**
* This function will normally be called for you automatically by
* {@link android.app.LoaderManager} when the associated fragment/activity
* is being started. When using a Loader with {@link android.app.LoaderManager},
* you <em>must not</em> 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 {@link #stopLoading} will stop the delivery of callbacks.
*
* <p>This updates the Loader's internal state so that
* {@link #isStarted()} and {@link #isReset()} will return the correct
* values, and then calls the implementation's {@link #onStartLoading()}.
*
* <p>Must be called from the process's main thread.
*/
public final void startLoading() {
mStarted = true;
mReset = false;
mAbandoned = false;
onStartLoading();
}
/**
* Subclasses must implement this to take care of loading their data,
* as per {@link #startLoading()}. This is not called by clients directly,
* but as a result of a call to {@link #startLoading()}.
*/
protected void onStartLoading() {
}
/**
* Attempt to cancel the current load task.
* Must be called on the main thread of the process.
*
* <p>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.
*
* @return Returns <tt>false</tt> if the task could not be canceled,
* typically because it has already completed normally, or
* because {@link #startLoading()} hasn't been called; returns
* <tt>true</tt> otherwise. When <tt>true</tt> is returned, the task
* is still running and the {@link OnLoadCanceledListener} will be called
* when the task completes.
*/
public boolean cancelLoad() {
return onCancelLoad();
}
/**
* Subclasses must implement this to take care of requests to {@link #cancelLoad()}.
* This will always be called from the process's main thread.
*
* @return Returns <tt>false</tt> if the task could not be canceled,
* typically because it has already completed normally, or
* because {@link #startLoading()} hasn't been called; returns
* <tt>true</tt> otherwise. When <tt>true</tt> is returned, the task
* is still running and the {@link OnLoadCanceledListener} will be called
* when the task completes.
*/
protected boolean onCancelLoad() {
return false;
}
/**
* Force an asynchronous load. Unlike {@link #startLoading()} this will ignore a previously
* loaded data set and load a new one. This simply calls through to the
* implementation's {@link #onForceLoad()}. You generally should only call this
* when the loader is started -- that is, {@link #isStarted()} returns true.
*
* <p>Must be called from the process's main thread.
*/
public void forceLoad() {
onForceLoad();
}
/**
* Subclasses must implement this to take care of requests to {@link #forceLoad()}.
* This will always be called from the process's main thread.
*/
protected void onForceLoad() {
}
/**
* This function will normally be called for you automatically by
* {@link android.app.LoaderManager} when the associated fragment/activity
* is being stopped. When using a Loader with {@link android.app.LoaderManager},
* you <em>must not</em> call this method yourself, or you will conflict
* with its management of the Loader.
*
* <p>Stops delivery of updates until the next time {@link #startLoading()} is called.
* Implementations should <em>not</em> 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 {@link #startLoading()} is later called.
*
* <p>This updates the Loader's internal state so that
* {@link #isStarted()} will return the correct
* value, and then calls the implementation's {@link #onStopLoading()}.
*
* <p>Must be called from the process's main thread.
*/
public void stopLoading() {
mStarted = false;
onStopLoading();
}
/**
* Subclasses must implement this to take care of stopping their loader,
* as per {@link #stopLoading()}. This is not called by clients directly,
* but as a result of a call to {@link #stopLoading()}.
* This will always be called from the process's main thread.
*/
protected void onStopLoading() {
}
/**
* This function will normally be called for you automatically by
* {@link android.app.LoaderManager} when restarting a Loader. When using
* a Loader with {@link android.app.LoaderManager},
* you <em>must not</em> 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 {@link #reset} to have it retain its current data but not report
* any new data.
*/
public void abandon() {
mAbandoned = true;
onAbandon();
}
/**
* Subclasses implement this to take care of being abandoned. This is
* an optional intermediate state prior to {@link #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 <em>must</em> keep its last reported data valid until the final
* {@link #onReset()} happens. You can retrieve the current abandoned
* state with {@link #isAbandoned}.
*/
protected void onAbandon() {
}
/**
* This function will normally be called for you automatically by
* {@link android.app.LoaderManager} when destroying a Loader. When using
* a Loader with {@link android.app.LoaderManager},
* you <em>must not</em> 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
* {@link #startLoading()} may later be called at which point it must be
* able to start running again.
*
* <p>This updates the Loader's internal state so that
* {@link #isStarted()} and {@link #isReset()} will return the correct
* values, and then calls the implementation's {@link #onReset()}.
*
* <p>Must be called from the process's main thread.
*/
public void reset() {
onReset();
mReset = true;
mStarted = false;
mAbandoned = false;
mContentChanged = false;
mProcessingChange = false;
}
/**
* Subclasses must implement this to take care of resetting their loader,
* as per {@link #reset()}. This is not called by clients directly,
* but as a result of a call to {@link #reset()}.
* This will always be called from the process's main thread.
*/
protected void onReset() {
}
/**
* 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 boolean takeContentChanged() {
boolean res = mContentChanged;
mContentChanged = false;
mProcessingChange |= res;
return res;
}
/**
* Commit that you have actually fully processed a content change that
* was returned by {@link #takeContentChanged}. This is for use with
* {@link #rollbackContentChanged()} to handle situations where a load
* is cancelled. Call this when you have completely processed a load
* without it being cancelled.
*/
public void commitContentChanged() {
mProcessingChange = false;
}
/**
* Report that you have abandoned the processing of a content change that
* was returned by {@link #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 void rollbackContentChanged() {
if (mProcessingChange) {
mContentChanged = true;
}
}
/**
* Called when {@link ForceLoadContentObserver} detects a change. The
* default implementation checks to see if the loader is currently started;
* if so, it simply calls {@link #forceLoad()}; otherwise, it sets a flag
* so that {@link #takeContentChanged()} returns true.
*
* <p>Must be called from the process's main thread.
*/
public void onContentChanged() {
if (mStarted) {
forceLoad();
} else {
// This loader has been stopped, so we don't want to load
// new data right now... but keep track of it changing to
// refresh later if we start again.
mContentChanged = true;
}
}
/**
* For debugging, converts an instance of the Loader's data class to
* a string that can be printed. Must handle a null data.
*/
public String dataToString(D data) {
StringBuilder sb = new StringBuilder(64);
DebugUtils.buildShortClassTag(data, sb);
sb.append("}");
return sb.toString();
}
@Override
public String toString() {
StringBuilder sb = new StringBuilder(64);
DebugUtils.buildShortClassTag(this, sb);
sb.append(" id=");
sb.append(mId);
sb.append("}");
return sb.toString();
}
/**
* Print the Loader's state into the given stream.
*
* @param prefix Text to print at the front of each line.
* @param fd The raw file descriptor that the dump is being sent to.
* @param writer A PrintWriter to which the dump is to be set.
* @param args Additional arguments to the dump request.
*/
public void dump(String prefix, FileDescriptor fd, PrintWriter writer, String[] args) {
writer.print(prefix); writer.print("mId="); writer.print(mId);
writer.print(" mListener="); writer.println(mListener);
if (mStarted || mContentChanged || mProcessingChange) {
writer.print(prefix); writer.print("mStarted="); writer.print(mStarted);
writer.print(" mContentChanged="); writer.print(mContentChanged);
writer.print(" mProcessingChange="); writer.println(mProcessingChange);
}
if (mAbandoned || mReset) {
writer.print(prefix); writer.print("mAbandoned="); writer.print(mAbandoned);
writer.print(" mReset="); writer.println(mReset);
}
}
} |
