Controlling How NSThread and NSRunLoop Exit

原文地址:http://shaheengandhi.com/controlling-thread-exit/

While most concurrency can be dealt with by using GCD dispatch queues, there are some things for which you will want to use a separate

NSThread

.
For instance, concurrent network programming remains mostly in this area. You may consider using the popular

CocoaAsyncSocket

library,
which abstracts away much of the complexity of socket programming on iOS. But, for the sake of this post, assume some work requires a separate thread, and we desire to cleanly start and stop the thread. That is, when we decide to start or stop the thread,
we want to ensure it has been initialized or destroyed before moving on.

You can get the final version of the code example below at github.

Thread Setup

Remember that threads are an artefact of the operating system and not the objective-c runtime. That means all the nice stuff like an

NSAutoreleasePool

and
an

NSRunLoop

need to be managed by the thread's top level code. Here is
a code snippet of a thread's main method that sets up an autorelease pool as well as starts pumping

NSRunLoop

.

- (void)start
{
  if (_thread) {
    return;
  }

  _thread = [[NSThread alloc] initWithTarget:self selector:@selector(threadProc:) object:nil];
}

- (void)threadProc:(id)ignored
{
  @autoreleasepool {
    // Startup code here

    // Just spin in a tight loop running the runloop.
    do {
      [[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:1]]
    } while (TRUE);
  }
}

This code has many problems. Note that each run of the runloop has a timeout of 1 second. When the runloop has nothing to do for extended periods of time, the thread will still wake up every second and go through the

while

loop's
code, only to end up sitting in the runloop's wait condition. That burns CPU and battery. Also, this thread has no exit condition. Even if we want the thread to live until the end of the application's life, we probably want a way to shut it down cleanly. Here's
an enumeration of the problems we want to fix:

Unsure when the thread is ready for work
Never goes to sleep
No way to exit the thread cleanly

Solving all three problems at once is tricky. Fixing #2 means keeping the runloop asleep, preferrably waking only when there is work to be done (being notified by a runloop source). But, that makes it difficult to exit the thread in a timely manner.

Wait for Infinity

How can we make

NSRunLoop

wait for infinity? The goal is to make sure that
the thread goes to sleep. Looking at the documentation for

runUntilDate:

,
this is not the case:

If no input sources or timers are attached to the run loop, this method exits immediately; otherwise, it runs the receiver in the NSDefaultRunLoopMode by repeatedly invoking runMode:beforeDate: until the specified expiration date.

The second sentence means this code keeps spinning on the CPU, which is the exact opposite of our desired behavior. A better alternative is

runMode:beforeDate:

,
for which the documentation reads:

If no input sources or timers are attached to the run loop, this method exits immediately and returns NO; otherwise, it returns after either the first input source is processed or limitDate is reached.

At least there's a chance the thread will sleep when using this method. However, the thread still runs in a continuous loop when there are no sources or timers. If you are only using a run loop to dispatch

performSelector:

calls,
you need to add a runloop source of your own to make the thread sleep. As a side effect, it would be effective to use this source to send work to the thread, but that is an exercise left to the reader.

One last thing: what value of

NSDate

should be used? Any large value is
fine, as waking the thread once a day is a large enough to keep the thread silent.

+[NSDate
 distantFuture]

is a convenient factory for such a date.

static void DoNothingRunLoopCallback(void *info)
{
}

- (void)threadProc:(id)ignored
{
  @autoreleasepool {
    CFRunLoopSourceContext context = {0};
    context.perform = DoNothingRunLoopCallback;

    CFRunLoopSourceRef source = CFRunLoopSourceCreate(NULL, 0, &context);
    CFRunLoopAddSource(CFRunLoopGetCurrent(), source, kCFRunLoopCommonModes);

    do {
      [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode
                               beforeDate:[NSDate distantFuture]];
    } while (TRUE);

    CFRunLoopRemoveSource(CFRunLoopGetCurrent(), source, kCFRunLoopCommonModes);
    CFRelease(source);
  }
}

The exit condition

Now that the thread sleeps forever, how can we ensure a clean shutdown of the thread? It's entirely possible to use

+[NSThread
 exit]

to kill the currently running thread. But, this does not clean up stack references to heap objects (like the runloop source that is created), drain the top level autorelease pool, nor allows the thread to flush any remaining work. Instead, we
need to make the runloop stop sleeping in order to exit

runMode:beforeDate:

,
and we need a condition the thread can check in order to know it's time to shut down.

NSRunLoop

's conditions for exiting

runMode:beforeDate:

are
somewhat limited. Its return values being

YES

(when the runloop processed
an input source or if the timeout was reached) or

NO

(when the runloop
could not be started) aren't great. Given that the documentation states that

NO

is
returned only when there are no sources or timers, our code will never see

NO

returned
since we added an input source to keep the runloop from spinning!

Luckily,

NSRunLoop

controls the same object that

CFRunLoop

APIs
do. The CoreFoundation alternative is to use

CFRunLoopRunInMode

, which
provides a more specific reason for the runloop's exit. Specifically,

kCFRunLoopRunStopped

indicates
the runloop was stopped by

CFRunLoopStop

. This is also the reason that

CFRunLoopRun

exits
(other than having no sources or times, but that will not occur due to our fake source), so we don't need to bother with

CFRunLoopRunInMode

,
nor check a condition.

It is best to execute

CFRunLoopStop

on the target thread itself. We can
do this by using

performSelector:onThread:withObject:waitUntilDone:

.

The new

threadProc:

looks like this:

- (void)threadProc:(id)ignored
{
  @autoreleasepool {
    CFRunLoopSourceContext context = {0};
    context.perform = DoNothingRunLoopCallback;

    CFRunLoopSourceRef source = CFRunLoopSourceCreate(NULL, 0, &context);
    CFRunLoopAddSource(CFRunLoopGetCurrent(), source, kCFRunLoopCommonModes);

    // Keep processing events until the runloop is stopped.
    CFRunLoopRun();

    CFRunLoopRemoveSource(CFRunLoopGetCurrent(), source, kCFRunLoopCommonModes);
    CFRelease(source);
  }
}

We can use code like the following to exit the thread quickly from any thread, including the
target thread itself:

- (void)stop
{
  [self performSelector:@selector(_stop) onThread:_thread withObject:nil waitUntilDone:NO];
  _thread = nil;
}

- (void)_stop
{
  CFRunLoopStop(CFRunLoopGetCurrent());
}

Synchronizing thread startup and exit

Two more problems to solve: when starting the thread, can we ensure that it is ready? When shutting down a thread, can we ensure it is gone?

I would note that there are better threading patterns than attempting to ensure the state of the target thread. For example, a thread should be able to accept work, but simply not process it until it is ready. Resources outside of the thread's runloop should
be minimal such that ensuring the thread is no longer executing should be above and beyond the desired knowledge of the thread's state.

It is tempting to simply add

waitUntilDone:YES

to the

performSelector

statement
in order to wait until the target thread has exited, but that would only wait until the

_stop

selector
was invoked, and not wait for the thread's cleanup code to be run. In order to do that, we need to make a new assumption: the target thread is being shut down from another thread. It would be impossible for a thread to wait for itself to shut down.

In order for the target thread to signal to the control thread that it is done, a condition must be shared between them.

NSCondition

provides
convenient semantics for our purposes.

The thread management code is below. This pattern keeps a thread with no work asleep for long periods of time, while allowing for a fast and clean exit. It also allows for startup and shutdown to be synchronized.

- (void)start
{
  if (_thread) {
    return;
  }

  _thread = [[NSThread alloc] initWithTarget:self selector:@selector(threadProc:) object:nil];

  // _condition was created in -init
  [_condition lock];
  [_thread start];
  [_condition wait];
  [_condition unlock];
}

- (void)stop
{
  if (!_thread) {
    return;
  }

  [_condition lock];
  [self performSelector:@selector(_stop) onThread:_thread withObject:nil waitUntilDone:NO];
  [_condition wait];
  [_condition unlock];
  _thread = nil;
}

- (void)threadProc:(id)object
{
  @autoreleasepool {
    CFRunLoopSourceContext context = {0};
    context.perform = DoNothingRunLoopCallback;

    CFRunLoopSourceRef source = CFRunLoopSourceCreate(NULL, 0, &context);
    CFRunLoopAddSource(CFRunLoopGetCurrent(), source, kCFRunLoopCommonModes);

    [_condition lock];
    [_condition signal];
    [_condition unlock];

    // Keep processing events until the runloop is stopped.
    CFRunLoopRun();

    CFRunLoopRemoveSource(CFRunLoopGetCurrent(), source, kCFRunLoopCommonModes);
    CFRelease(source);

    [_condition lock];
    [_condition signal];
    [_condition unlock];
  }
}

Ensuring Destruction of Thread Resources

There is still another issue with this code: when the thread has signaled its exit, the autorelease pool has not yet been drained. Without ensuring that the thread's memory resources have been released, the purpose of synchronizing the thread's exit becomes
much less appealing.

There's a bit of a catch-22.

NSCondition

makes no promise that it is free
from using

-autorelease

in its implementations of

-lock

,

-signal

,
and

-unlock

. That means there should be a valid

NSAutoreleasePool

when
using these APIs. We have two solutions available to us. We can either manually drain the autorelease pool, or use a different way to synchronize the thread's exit that waits until

threadProc:

has
exited. The first is somewhat messy. The second has two variants of its own.

Manual Autorelease

In order to use

NSAutoreleasePool

directly,
you must disable ARC.

Remember that

-[NSAutoreleasePool drain]

is effectively the same as

-[NSAutoreleasePool
 release]

and that the pool is no longer valid after draining it. So, manually draining an autorelease pool means creating another one to ensure the

NSCondition

APIs
have the right environment.

- (void)threadProc:(id)object
{
  NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

  {
    CFRunLoopSourceContext context = {0};
    context.perform = DoNothingRunLoopCallback;

    CFRunLoopSourceRef source = CFRunLoopSourceCreate(NULL, 0, &context);
    CFRunLoopAddSource(CFRunLoopGetCurrent(), source, kCFRunLoopCommonModes);

    [_condition lock];
    [_condition signal];
    [_condition unlock];

    // Keep processing events until the runloop is stopped.
    CFRunLoopRun();

    CFRunLoopRemoveSource(CFRunLoopGetCurrent(), source, kCFRunLoopCommonModes);
    CFRelease(source);

    // Release all accumulated resources, but make sure NSCondition has the
    // right environment.
    [pool drain];
    pool = [[NSAutoreleasePool alloc] init];

    [_condition lock];
    [_condition signal];
    [_condition unlock];
  }
  [pool drain];
}

Using NSThreadWillExitNotification

NSThreadWillExitNotification

is a notification sent by

NSThread

when
the thread's main function has finished and the thread is about to finish execution. This must happen after

threadProc:

,
so this ensures the thread's top level autorelease pool has been drained. Since the notification fires on the exiting thread,

NSCondition

is
still used to synchronize the state of the thread.

- (void)stop
{
  if (!_thread) {
    return;
  }

  NSNotificationCenter *nc = [NSNotificationCenter defaultCenter];
  [nc addObserver:self selector:@(_signal) name:NSThreadWillExitNotification object:_thread];

  [_condition lock];
  [self performSelector:@selector(_stop) onThread:_thread withObject:nil waitUntilDone:NO];
  [_condition wait];
  [_condition unlock];

  [nc removeObserver:self name:NSThreadWillExitNotification object:_thread];
  _thread = nil;
}

- (void)threadProc:(id)object
{
  @autoreleasepool {
    CFRunLoopSourceContext context = {0};
    context.perform = DoNothingRunLoopCallback;

    CFRunLoopSourceRef source = CFRunLoopSourceCreate(NULL, 0, &context);
    CFRunLoopAddSource(CFRunLoopGetCurrent(), source, kCFRunLoopCommonModes);

    [_condition lock];
    [_condition signal];
    [_condition unlock];

    // Keep processing events until the runloop is stopped.
    CFRunLoopRun();

    CFRunLoopRemoveSource(CFRunLoopGetCurrent(), source, kCFRunLoopCommonModes);
    CFRelease(source);
  }
}

- (void)_signal
{
  [_condition lock];
  [_condition signal];
  [_condition unlock];
}

Using pthreads

All of the above solutions have one slight problem: the thread hasn't quite exited when the control thread believes it has. The target thread is very close to being done, but that is not the same as done.

To ensure that, we must go outside the realm of

NSThread

and use pthreads
instead.

pthread_join

guarantees the target thread has terminated. Using
pthreads makes the code a bit more verbose, and some of the memory management must be done carefully. In particular,

self

is
retained when specifying it as an argument of

NSThread

's initializer, but
will not be retained when using it as an argument to

pthread_create

. Note
that we still need an

NSThread

reference to use

performSelector:onThread:withObject:waitUntilDone:

,
but there is no way to convert a

pthread_t

to an

NSThread

.
Luckily,

+[NSThread currentThread]

can obtain the correct object reference.

NSCondition

is still used to synchronize thread startup. Because it is not
used for any other purpose, it is not necessary to lock the condition before starting the thread. However, to be consistent with previous code, we will follow the previous pattern of creating the thread in a suspended state and resuming it with the condition's
lock held.

static void *ThreadProc(void *arg)
{
  ThreadedComponent *component = (__bridge_transfer ThreadedComponent *)arg;
  [component threadProc:nil];
  return 0;
}

- (void)start
{
  if (_thread) {
    return;
  }

  if (pthread_create_suspended_np(&_pthread, NULL, &ThreadProc, (__bridge_retained void *)self) != 0) {
    return;
  }

  // _condition was created in -init
  [_condition lock];
  mach_port_t mach_thread = pthread_mach_thread_np(_pthread);
  thread_resume(mach_thread);
  [_condition wait];
  [_condition unlock];
}

- (void)stop
{
  if (!_thread) {
    return;
  }

  [self performSelector:@selector(_stop) onThread:_thread withObject:nil waitUntilDone:NO];
  pthread_join(_pthread, NULL);
  _thread = nil;
}

- (void)threadProc:(id)object
{
  @autoreleasepool {
    CFRunLoopSourceContext context = {0};
    context.perform = DoNothingRunLoopCallback;

    CFRunLoopSourceRef source = CFRunLoopSourceCreate(NULL, 0, &context);
    CFRunLoopAddSource(CFRunLoopGetCurrent(), source, kCFRunLoopCommonModes);

    // Obtain the current NSThread before signaling startup is complete.
    _thread = [NSThread currentThread];

    [_condition lock];
    [_condition signal];
    [_condition unlock];

    // Keep processing events until the runloop is stopped.
    CFRunLoopRun();

    CFRunLoopRemoveSource(CFRunLoopGetCurrent(), source, kCFRunLoopCommonModes);
    CFRelease(source);
  }
}

Code Samples

Head on over to github to download code samples for all four working versions of the code
above.