Add new glue code for writing native apps.

This factors out the boiler-plate code from the sample
app to a common glue code that can be used for everyone
writing this style of app: a dedicated app thread that
takes care of waiting for events and processing them.

As part of doing this, ALooper has a new facility to allow
registration of fds that cause ALooper_pollOnce() to return
the fd that has data, allowing the app to drive the loop
without callbacks.  Hopefully this makes some people feel better. :)

Also do some other cleanup of the ALooper API, plus some
actual documentation.

Change-Id: Ic53bd56bdf627e3ba28a3c093faa06a92be522b8

1 files changed, 130 insertions, 4 deletions

diff --git a/native/include/android/looper.h b/native/include/android/looper.h
index 90a8983..2917216 100644
--- a/native/include/android/looper.h
+++ b/native/include/android/looper.h
@@ -24,25 +24,151 @@
 extern "C" {
 #endif
 
+/**
+ * ALooper
+ *
+ * A looper is the state tracking an event loop for a thread.
+ * Loopers do not define event structures or other such things; rather
+ * they are a lower-level facility to attach one or more discrete objects
+ * listening for an event.  An "event" here is simply data available on
+ * a file descriptor: each attached object has an associated file descriptor,
+ * and waiting for "events" means (internally) polling on all of these file
+ * descriptors until one or more of them have data available.
+ *
+ * A thread can have only one ALooper associated with it.
+ */
 struct ALooper;
 typedef struct ALooper ALooper;
 
+/**
+ * For callback-based event loops, this is the prototype of the function
+ * that is called.  It is given the file descriptor it is associated with,
+ * a bitmask of the poll events that were triggered (typically POLLIN), and
+ * the data pointer that was originally supplied.
+ *
+ * Implementations should return 1 to continue receiving callbacks, or 0
+ * to have this file descriptor and callback unregistered from the looper.
+ */
 typedef int ALooper_callbackFunc(int fd, int events, void* data);
 
+/**
+ * Return the ALooper associated with the calling thread, or NULL if
+ * there is not one.
+ */
 ALooper* ALooper_forThread();
 
-ALooper* ALooper_prepare();
+enum {
+    /**
+     * Option for ALooper_prepare: this ALooper will accept calls to
+     * ALooper_addFd() that do not have a callback (that is provide NULL
+     * for the callback).  In this case the caller of ALooper_pollOnce()
+     * or ALooper_pollAll() MUST check the return from these functions to
+     * discover when data is available on such fds and process it.
+     */
+    ALOOPER_PREPARE_ALLOW_NON_CALLBACKS = 1<<0
+};
 
-int32_t ALooper_pollOnce(int timeoutMillis);
+/**
+ * Prepare an ALooper associated with the calling thread, and return it.
+ * If the thread already has an ALooper, it is returned.  Otherwise, a new
+ * one is created, associated with the thread, and returned.
+ *
+ * The opts may be ALOOPER_PREPARE_ALLOW_NON_CALLBACKS or 0.
+ */
+ALooper* ALooper_prepare(int32_t opts);
+
+enum {
+    /**
+     * Result from ALooper_pollOnce() and ALooper_pollAll(): one or
+     * more callbacks were executed.
+     */
+    ALOOPER_POLL_CALLBACK = -1,
+    
+    /**
+     * Result from ALooper_pollOnce() and ALooper_pollAll(): the
+     * timeout expired.
+     */
+    ALOOPER_POLL_TIMEOUT = -2,
+    
+    /**
+     * Result from ALooper_pollOnce() and ALooper_pollAll(): an error
+     * occurred.
+     */
+    ALOOPER_POLL_ERROR = -3,
+};
 
+/**
+ * Wait for events to be available, with optional timeout in milliseconds.
+ * Invokes callbacks for all file descriptors on which an event occurred.
+ *
+ * If the timeout is zero, returns immediately without blocking.
+ * If the timeout is negative, waits indefinitely until an event appears.
+ *
+ * Returns ALOOPER_POLL_CALLBACK if a callback was invoked.
+ *
+ * Returns ALOOPER_POLL_TIMEOUT if there was no data before the given
+ * timeout expired.
+ *
+ * Returns ALOPER_POLL_ERROR if an error occurred.
+ *
+ * Returns a value >= 0 containing a file descriptor if it has data
+ * and it has no callback function (requiring the caller here to handle it).
+ * In this (and only this) case outEvents and outData will contain the poll
+ * events and data associated with the fd.
+ *
+ * This method does not return until it has finished invoking the appropriate callbacks
+ * for all file descriptors that were signalled.
+ */
+int32_t ALooper_pollOnce(int timeoutMillis, int* outEvents, void** outData);
+
+/**
+ * Like ALooper_pollOnce(), but performs all pending callbacks until all
+ * data has been consumed or a file descriptor is available with no callback.
+ * This function will never return ALOOPER_POLL_CALLBACK.
+ */
+int32_t ALooper_pollAll(int timeoutMillis, int* outEvents, void** outData);
+
+/**
+ * Acquire a reference on the given ALooper object.  This prevents the object
+ * from being deleted until the reference is removed.  This is only needed
+ * to safely hand an ALooper from one thread to another.
+ */
 void ALooper_acquire(ALooper* looper);
 
+/**
+ * Remove a reference that was previously acquired with ALooper_acquire().
+ */
 void ALooper_release(ALooper* looper);
 
-void ALooper_setCallback(ALooper* looper, int fd, int events,
+/**
+ * Add a new file descriptor to be polled by the looper.  If the same file
+ * descriptor was previously added, it is replaced.
+ *
+ * "fd" is the file descriptor to be added.
+ * "events" are the poll events to wake up on.  Typically this is POLLIN.
+ * "callback" is the function to call when there is an event on the file
+ * descriptor.
+ * "id" is an identifier to associated with this file descriptor, or 0.
+ * "data" is a private data pointer to supply to the callback.
+ *
+ * There are two main uses of this function:
+ *
+ * (1) If "callback" is non-NULL, then
+ * this function will be called when there is data on the file descriptor.  It
+ * should execute any events it has pending, appropriately reading from the
+ * file descriptor.
+ *
+ * (2) If "callback" is NULL, the fd will be returned by ALooper_pollOnce
+ * when it has data available, requiring the caller to take care of processing
+ * it.
+ */
+void ALooper_addFd(ALooper* looper, int fd, int events,
         ALooper_callbackFunc* callback, void* data);
 
-int32_t ALooper_removeCallback(ALooper* looper, int fd);
+/**
+ * Remove a previously added file descriptor from the looper.
+ */
+int32_t ALooper_removeFd(ALooper* looper, int fd);
 
 #ifdef __cplusplus
 };