1 /* 2 * Copyright (C) 2006 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 17 package android.os; 18 19 import android.annotation.NonNull; 20 import android.annotation.Nullable; 21 import android.util.Log; 22 import android.util.Printer; 23 24 /** 25 * Class used to run a message loop for a thread. Threads by default do 26 * not have a message loop associated with them; to create one, call 27 * {@link #prepare} in the thread that is to run the loop, and then 28 * {@link #loop} to have it process messages until the loop is stopped. 29 * 30 * <p>Most interaction with a message loop is through the 31 * {@link Handler} class. 32 * 33 * <p>This is a typical example of the implementation of a Looper thread, 34 * using the separation of {@link #prepare} and {@link #loop} to create an 35 * initial Handler to communicate with the Looper. 36 * 37 * <pre> 38 * class LooperThread extends Thread { 39 * public Handler mHandler; 40 * 41 * public void run() { 42 * Looper.prepare(); 43 * 44 * mHandler = new Handler() { 45 * public void handleMessage(Message msg) { 46 * // process incoming messages here 47 * } 48 * }; 49 * 50 * Looper.loop(); 51 * } 52 * }</pre> 53 */ 54 public final class Looper { 55 /* 56 * API Implementation Note: 57 * 58 * This class contains the code required to set up and manage an event loop 59 * based on MessageQueue. APIs that affect the state of the queue should be 60 * defined on MessageQueue or Handler rather than on Looper itself. For example, 61 * idle handlers and sync barriers are defined on the queue whereas preparing the 62 * thread, looping, and quitting are defined on the looper. 63 */ 64 65 private static final String TAG = "Looper"; 66 67 // sThreadLocal.get() will return null unless you've called prepare(). 68 static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>(); 69 private static Looper sMainLooper; // guarded by Looper.class 70 71 final MessageQueue mQueue; 72 final Thread mThread; 73 74 private Printer mLogging; 75 76 /** Initialize the current thread as a looper. 77 * This gives you a chance to create handlers that then reference 78 * this looper, before actually starting the loop. Be sure to call 79 * {@link #loop()} after calling this method, and end it by calling 80 * {@link #quit()}. 81 */ prepare()82 public static void prepare() { 83 prepare(true); 84 } 85 prepare(boolean quitAllowed)86 private static void prepare(boolean quitAllowed) { 87 if (sThreadLocal.get() != null) { 88 throw new RuntimeException("Only one Looper may be created per thread"); 89 } 90 sThreadLocal.set(new Looper(quitAllowed)); 91 } 92 93 /** 94 * Initialize the current thread as a looper, marking it as an 95 * application's main looper. The main looper for your application 96 * is created by the Android environment, so you should never need 97 * to call this function yourself. See also: {@link #prepare()} 98 */ prepareMainLooper()99 public static void prepareMainLooper() { 100 prepare(false); 101 synchronized (Looper.class) { 102 if (sMainLooper != null) { 103 throw new IllegalStateException("The main Looper has already been prepared."); 104 } 105 sMainLooper = myLooper(); 106 } 107 } 108 109 /** 110 * Returns the application's main looper, which lives in the main thread of the application. 111 */ getMainLooper()112 public static Looper getMainLooper() { 113 synchronized (Looper.class) { 114 return sMainLooper; 115 } 116 } 117 118 /** 119 * Run the message queue in this thread. Be sure to call 120 * {@link #quit()} to end the loop. 121 */ loop()122 public static void loop() { 123 final Looper me = myLooper(); 124 if (me == null) { 125 throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); 126 } 127 final MessageQueue queue = me.mQueue; 128 129 // Make sure the identity of this thread is that of the local process, 130 // and keep track of what that identity token actually is. 131 Binder.clearCallingIdentity(); 132 final long ident = Binder.clearCallingIdentity(); 133 134 for (;;) { 135 Message msg = queue.next(); // might block 136 if (msg == null) { 137 // No message indicates that the message queue is quitting. 138 return; 139 } 140 141 // This must be in a local variable, in case a UI event sets the logger 142 Printer logging = me.mLogging; 143 if (logging != null) { 144 logging.println(">>>>> Dispatching to " + msg.target + " " + 145 msg.callback + ": " + msg.what); 146 } 147 148 msg.target.dispatchMessage(msg); 149 150 if (logging != null) { 151 logging.println("<<<<< Finished to " + msg.target + " " + msg.callback); 152 } 153 154 // Make sure that during the course of dispatching the 155 // identity of the thread wasn't corrupted. 156 final long newIdent = Binder.clearCallingIdentity(); 157 if (ident != newIdent) { 158 Log.wtf(TAG, "Thread identity changed from 0x" 159 + Long.toHexString(ident) + " to 0x" 160 + Long.toHexString(newIdent) + " while dispatching to " 161 + msg.target.getClass().getName() + " " 162 + msg.callback + " what=" + msg.what); 163 } 164 165 msg.recycleUnchecked(); 166 } 167 } 168 169 /** 170 * Return the Looper object associated with the current thread. Returns 171 * null if the calling thread is not associated with a Looper. 172 */ myLooper()173 public static @Nullable Looper myLooper() { 174 return sThreadLocal.get(); 175 } 176 177 /** 178 * Return the {@link MessageQueue} object associated with the current 179 * thread. This must be called from a thread running a Looper, or a 180 * NullPointerException will be thrown. 181 */ myQueue()182 public static @NonNull MessageQueue myQueue() { 183 return myLooper().mQueue; 184 } 185 Looper(boolean quitAllowed)186 private Looper(boolean quitAllowed) { 187 mQueue = new MessageQueue(quitAllowed); 188 mThread = Thread.currentThread(); 189 } 190 191 /** 192 * Returns true if the current thread is this looper's thread. 193 */ isCurrentThread()194 public boolean isCurrentThread() { 195 return Thread.currentThread() == mThread; 196 } 197 198 /** 199 * Control logging of messages as they are processed by this Looper. If 200 * enabled, a log message will be written to <var>printer</var> 201 * at the beginning and ending of each message dispatch, identifying the 202 * target Handler and message contents. 203 * 204 * @param printer A Printer object that will receive log messages, or 205 * null to disable message logging. 206 */ setMessageLogging(@ullable Printer printer)207 public void setMessageLogging(@Nullable Printer printer) { 208 mLogging = printer; 209 } 210 211 /** 212 * Quits the looper. 213 * <p> 214 * Causes the {@link #loop} method to terminate without processing any 215 * more messages in the message queue. 216 * </p><p> 217 * Any attempt to post messages to the queue after the looper is asked to quit will fail. 218 * For example, the {@link Handler#sendMessage(Message)} method will return false. 219 * </p><p class="note"> 220 * Using this method may be unsafe because some messages may not be delivered 221 * before the looper terminates. Consider using {@link #quitSafely} instead to ensure 222 * that all pending work is completed in an orderly manner. 223 * </p> 224 * 225 * @see #quitSafely 226 */ quit()227 public void quit() { 228 mQueue.quit(false); 229 } 230 231 /** 232 * Quits the looper safely. 233 * <p> 234 * Causes the {@link #loop} method to terminate as soon as all remaining messages 235 * in the message queue that are already due to be delivered have been handled. 236 * However pending delayed messages with due times in the future will not be 237 * delivered before the loop terminates. 238 * </p><p> 239 * Any attempt to post messages to the queue after the looper is asked to quit will fail. 240 * For example, the {@link Handler#sendMessage(Message)} method will return false. 241 * </p> 242 */ quitSafely()243 public void quitSafely() { 244 mQueue.quit(true); 245 } 246 247 /** 248 * Gets the Thread associated with this Looper. 249 * 250 * @return The looper's thread. 251 */ getThread()252 public @NonNull Thread getThread() { 253 return mThread; 254 } 255 256 /** 257 * Gets this looper's message queue. 258 * 259 * @return The looper's message queue. 260 */ getQueue()261 public @NonNull MessageQueue getQueue() { 262 return mQueue; 263 } 264 265 /** 266 * Dumps the state of the looper for debugging purposes. 267 * 268 * @param pw A printer to receive the contents of the dump. 269 * @param prefix A prefix to prepend to each line which is printed. 270 */ dump(@onNull Printer pw, @NonNull String prefix)271 public void dump(@NonNull Printer pw, @NonNull String prefix) { 272 pw.println(prefix + toString()); 273 mQueue.dump(pw, prefix + " "); 274 } 275 276 @Override toString()277 public String toString() { 278 return "Looper (" + mThread.getName() + ", tid " + mThread.getId() 279 + ") {" + Integer.toHexString(System.identityHashCode(this)) + "}"; 280 } 281 } 282