Loader.java revision 9567a66a5e6f49dd8495fb5f6e2efb9f32e84b35
1/* 2 * Copyright (C) 2010 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17package android.content; 18 19import android.database.ContentObserver; 20import android.os.Handler; 21import android.util.DebugUtils; 22 23import java.io.FileDescriptor; 24import java.io.PrintWriter; 25 26/** 27 * An abstract class that performs asynchronous loading of data. While Loaders are active 28 * they should monitor the source of their data and deliver new results when the contents 29 * change. See {@link android.app.LoaderManager} for more detail. 30 * 31 * <p><b>Note on threading:</b> Clients of loaders should as a rule perform 32 * any calls on to a Loader from the main thread of their process (that is, 33 * the thread the Activity callbacks and other things occur on). Subclasses 34 * of Loader (such as {@link AsyncTaskLoader}) will often perform their work 35 * in a separate thread, but when delivering their results this too should 36 * be done on the main thread.</p> 37 * 38 * <p>Subclasses generally must implement at least {@link #onStartLoading()}, 39 * {@link #onStopLoading()}, {@link #onForceLoad()}, and {@link #onReset()}.</p> 40 * 41 * <p>Most implementations should not derive directly from this class, but 42 * instead inherit from {@link AsyncTaskLoader}.</p> 43 * 44 * @param <D> The result returned when the load is complete 45 */ 46public class Loader<D> { 47 int mId; 48 OnLoadCompleteListener<D> mListener; 49 Context mContext; 50 boolean mStarted = false; 51 boolean mAbandoned = false; 52 boolean mReset = true; 53 boolean mContentChanged = false; 54 55 public final class ForceLoadContentObserver extends ContentObserver { 56 public ForceLoadContentObserver() { 57 super(new Handler()); 58 } 59 60 @Override 61 public boolean deliverSelfNotifications() { 62 return true; 63 } 64 65 @Override 66 public void onChange(boolean selfChange) { 67 onContentChanged(); 68 } 69 } 70 71 public interface OnLoadCompleteListener<D> { 72 /** 73 * Called on the thread that created the Loader when the load is complete. 74 * 75 * @param loader the loader that completed the load 76 * @param data the result of the load 77 */ 78 public void onLoadComplete(Loader<D> loader, D data); 79 } 80 81 /** 82 * Stores away the application context associated with context. 83 * Since Loaders can be used across multiple activities it's dangerous to 84 * store the context directly; always use {@link #getContext()} to retrieve 85 * the Loader's Context, don't use the constructor argument directly. 86 * The Context returned by {@link #getContext} is safe to use across 87 * Activity instances. 88 * 89 * @param context used to retrieve the application context. 90 */ 91 public Loader(Context context) { 92 mContext = context.getApplicationContext(); 93 } 94 95 /** 96 * Sends the result of the load to the registered listener. Should only be called by subclasses. 97 * 98 * Must be called from the process's main thread. 99 * 100 * @param data the result of the load 101 */ 102 public void deliverResult(D data) { 103 if (mListener != null) { 104 mListener.onLoadComplete(this, data); 105 } 106 } 107 108 /** 109 * @return an application context retrieved from the Context passed to the constructor. 110 */ 111 public Context getContext() { 112 return mContext; 113 } 114 115 /** 116 * @return the ID of this loader 117 */ 118 public int getId() { 119 return mId; 120 } 121 122 /** 123 * Registers a class that will receive callbacks when a load is complete. 124 * The callback will be called on the process's main thread so it's safe to 125 * pass the results to widgets. 126 * 127 * <p>Must be called from the process's main thread. 128 */ 129 public void registerListener(int id, OnLoadCompleteListener<D> listener) { 130 if (mListener != null) { 131 throw new IllegalStateException("There is already a listener registered"); 132 } 133 mListener = listener; 134 mId = id; 135 } 136 137 /** 138 * Remove a listener that was previously added with {@link #registerListener}. 139 * 140 * Must be called from the process's main thread. 141 */ 142 public void unregisterListener(OnLoadCompleteListener<D> listener) { 143 if (mListener == null) { 144 throw new IllegalStateException("No listener register"); 145 } 146 if (mListener != listener) { 147 throw new IllegalArgumentException("Attempting to unregister the wrong listener"); 148 } 149 mListener = null; 150 } 151 152 /** 153 * Return whether this load has been started. That is, its {@link #startLoading()} 154 * has been called and no calls to {@link #stopLoading()} or 155 * {@link #reset()} have yet been made. 156 */ 157 public boolean isStarted() { 158 return mStarted; 159 } 160 161 /** 162 * Return whether this loader has been abandoned. In this state, the 163 * loader <em>must not</em> report any new data, and <em>must</em> keep 164 * its last reported data valid until it is finally reset. 165 */ 166 public boolean isAbandoned() { 167 return mAbandoned; 168 } 169 170 /** 171 * Return whether this load has been reset. That is, either the loader 172 * has not yet been started for the first time, or its {@link #reset()} 173 * has been called. 174 */ 175 public boolean isReset() { 176 return mReset; 177 } 178 179 /** 180 * Starts an asynchronous load of the Loader's data. When the result 181 * is ready the callbacks will be called on the process's main thread. 182 * If a previous load has been completed and is still valid 183 * the result may be passed to the callbacks immediately. 184 * The loader will monitor the source of 185 * the data set and may deliver future callbacks if the source changes. 186 * Calling {@link #stopLoading} will stop the delivery of callbacks. 187 * 188 * <p>This updates the Loader's internal state so that 189 * {@link #isStarted()} and {@link #isReset()} will return the correct 190 * values, and then calls the implementation's {@link #onStartLoading()}. 191 * 192 * <p>Must be called from the process's main thread. 193 */ 194 public final void startLoading() { 195 mStarted = true; 196 mReset = false; 197 mAbandoned = false; 198 onStartLoading(); 199 } 200 201 /** 202 * Subclasses must implement this to take care of loading their data, 203 * as per {@link #startLoading()}. This is not called by clients directly, 204 * but as a result of a call to {@link #startLoading()}. 205 */ 206 protected void onStartLoading() { 207 } 208 209 /** 210 * Force an asynchronous load. Unlike {@link #startLoading()} this will ignore a previously 211 * loaded data set and load a new one. This simply calls through to the 212 * implementation's {@link #onForceLoad()}. You generally should only call this 213 * when the loader is started -- that is, {@link #isStarted()} returns true. 214 * 215 * <p>Must be called from the process's main thread. 216 */ 217 public void forceLoad() { 218 onForceLoad(); 219 } 220 221 /** 222 * Subclasses must implement this to take care of requests to {@link #forceLoad()}. 223 * This will always be called from the process's main thread. 224 */ 225 protected void onForceLoad() { 226 } 227 228 /** 229 * Stops delivery of updates until the next time {@link #startLoading()} is called. 230 * Implementations should <em>not</em> invalidate their data at this point -- 231 * clients are still free to use the last data the loader reported. They will, 232 * however, typically stop reporting new data if the data changes; they can 233 * still monitor for changes, but must not report them to the client until and 234 * if {@link #startLoading()} is later called. 235 * 236 * <p>This updates the Loader's internal state so that 237 * {@link #isStarted()} will return the correct 238 * value, and then calls the implementation's {@link #onStopLoading()}. 239 * 240 * <p>Must be called from the process's main thread. 241 */ 242 public void stopLoading() { 243 mStarted = false; 244 onStopLoading(); 245 } 246 247 /** 248 * Subclasses must implement this to take care of stopping their loader, 249 * as per {@link #stopLoading()}. This is not called by clients directly, 250 * but as a result of a call to {@link #stopLoading()}. 251 * This will always be called from the process's main thread. 252 */ 253 protected void onStopLoading() { 254 } 255 256 /** 257 * Tell the Loader that it is being abandoned. This is called prior 258 * to {@link #reset} to have it retain its current data but not report 259 * any new data. 260 */ 261 public void abandon() { 262 mAbandoned = true; 263 onAbandon(); 264 } 265 266 /** 267 * Subclasses implement this to take care of being abandoned. This is 268 * an optional intermediate state prior to {@link #onReset()} -- it means that 269 * the client is no longer interested in any new data from the loader, 270 * so the loader must not report any further updates. However, the 271 * loader <em>must</em> keep its last reported data valid until the final 272 * {@link #onReset()} happens. You can retrieve the current abandoned 273 * state with {@link #isAbandoned}. 274 */ 275 protected void onAbandon() { 276 } 277 278 /** 279 * Resets the state of the Loader. The Loader should at this point free 280 * all of its resources, since it may never be called again; however, its 281 * {@link #startLoading()} may later be called at which point it must be 282 * able to start running again. 283 * 284 * <p>This updates the Loader's internal state so that 285 * {@link #isStarted()} and {@link #isReset()} will return the correct 286 * values, and then calls the implementation's {@link #onReset()}. 287 * 288 * <p>Must be called from the process's main thread. 289 */ 290 public void reset() { 291 onReset(); 292 mReset = true; 293 mStarted = false; 294 mAbandoned = false; 295 mContentChanged = false; 296 } 297 298 /** 299 * Subclasses must implement this to take care of resetting their loader, 300 * as per {@link #reset()}. This is not called by clients directly, 301 * but as a result of a call to {@link #reset()}. 302 * This will always be called from the process's main thread. 303 */ 304 protected void onReset() { 305 } 306 307 /** 308 * Take the current flag indicating whether the loader's content had 309 * changed while it was stopped. If it had, true is returned and the 310 * flag is cleared. 311 */ 312 public boolean takeContentChanged() { 313 boolean res = mContentChanged; 314 mContentChanged = false; 315 return res; 316 } 317 318 /** 319 * Called when {@link ForceLoadContentObserver} detects a change. The 320 * default implementation checks to see if the loader is currently started; 321 * if so, it simply calls {@link #forceLoad()}; otherwise, it sets a flag 322 * so that {@link #takeContentChanged()} returns true. 323 * 324 * <p>Must be called from the process's main thread. 325 */ 326 public void onContentChanged() { 327 if (mStarted) { 328 forceLoad(); 329 } else { 330 // This loader has been stopped, so we don't want to load 331 // new data right now... but keep track of it changing to 332 // refresh later if we start again. 333 mContentChanged = true; 334 } 335 } 336 337 /** 338 * For debugging, converts an instance of the Loader's data class to 339 * a string that can be printed. Must handle a null data. 340 */ 341 public String dataToString(D data) { 342 StringBuilder sb = new StringBuilder(64); 343 DebugUtils.buildShortClassTag(data, sb); 344 sb.append("}"); 345 return sb.toString(); 346 } 347 348 @Override 349 public String toString() { 350 StringBuilder sb = new StringBuilder(64); 351 DebugUtils.buildShortClassTag(this, sb); 352 sb.append(" id="); 353 sb.append(mId); 354 sb.append("}"); 355 return sb.toString(); 356 } 357 358 /** 359 * Print the Loader's state into the given stream. 360 * 361 * @param prefix Text to print at the front of each line. 362 * @param fd The raw file descriptor that the dump is being sent to. 363 * @param writer A PrintWriter to which the dump is to be set. 364 * @param args Additional arguments to the dump request. 365 */ 366 public void dump(String prefix, FileDescriptor fd, PrintWriter writer, String[] args) { 367 writer.print(prefix); writer.print("mId="); writer.print(mId); 368 writer.print(" mListener="); writer.println(mListener); 369 writer.print(prefix); writer.print("mStarted="); writer.print(mStarted); 370 writer.print(" mContentChanged="); writer.print(mContentChanged); 371 writer.print(" mAbandoned="); writer.print(mAbandoned); 372 writer.print(" mReset="); writer.println(mReset); 373 } 374}