How do I post code to be run on the Android main thread from a separate thread in C++?

Question

I have a separate thread running in C++ in the background and I want it to be able to post code to be run on another thread that's already running an android.os.Looper (e.g. the main thread). By 'post', I mean something akin to View#post where a Runnable is enqueued to be run on the event loop. The code that would be executed is also written in C++.

I found the ALooper API (http://developer.android.com/ndk/reference/group___looper.html), but the docs aren't great and it's unclear to me whether getting the ALooper associated with the destination thread, adding another FD, and signaling it will make my code maintain correct ordering in the event queue with respect to the other enqueued Runnables.

I'd prefer not to have to go through Java and get a Handler, etc. -- it just seems unnecessary since both the code I'm trying to run and the code that's posting it are in c++.

Answer 1

A thread can only have one Looper associated with it, a Looper has only one message queue, so mixing Java and native callbacks will maintain ordering.

With that, I don't think there is any contractual obligation in Android today that post() is guaranteed to execute in particular order, i.e.

getHandler().post(new Runnable() {     @Override     public void run() {         mTextView.setText("first");     } }); getHandler().post(new Runnable() {     @Override     public void run() {         mTextView.setText("second");     } }); 

is not formally guaranteed to leave mTextView displaying second. Definitely nothing is set in stone when two posts are issued from different threads, or delayed.

You can find an Android messaging and concurrency framework for native code development desctibed in a great blog post.

Update

Here is the required proof. The stacktrace below was received while working on an unrelated problem:

A/art: art/runtime/check_jni.cc:65]   native: #00 pc 0000484c  /system/lib/libbacktrace_libc++.so (UnwindCurrent::Unwind(unsigned int, ucontext*)+23) A/art: art/runtime/check_jni.cc:65]   native: #01 pc 00003031  /system/lib/libbacktrace_libc++.so (Backtrace::Unwind(unsigned int, ucontext*)+8) A/art: art/runtime/check_jni.cc:65]   native: #02 pc 002441f9  /system/lib/libart.so (art::DumpNativeStack(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, int, char const*, art::mirror::ArtMethod*)+68) A/art: art/runtime/check_jni.cc:65]   native: #03 pc 002285a1  /system/lib/libart.so (art::Thread::Dump(std::__1::basic_ostream<char, std::__1::char_traits<char> >&) const+144) A/art: art/runtime/check_jni.cc:65]   native: #04 pc 000afe9b  /system/lib/libart.so (art::JniAbort(char const*, char const*)+582) A/art: art/runtime/check_jni.cc:65]   native: #05 pc 000b05d1  /system/lib/libart.so (art::JniAbortF(char const*, char const*, ...)+60) A/art: art/runtime/check_jni.cc:65]   native: #06 pc 000b299d  /system/lib/libart.so (art::ScopedCheck::Check(bool, char const*, ...) (.constprop.129)+672) A/art: art/runtime/check_jni.cc:65]   native: #07 pc 000bab87  /system/lib/libart.so (art::CheckJNI::CallVoidMethodV(_JNIEnv*, _jobject*, _jmethodID*, std::__va_list)+50) A/art: art/runtime/check_jni.cc:65]   native: #08 pc 00060817  /system/lib/libandroid_runtime.so (???) A/art: art/runtime/check_jni.cc:65]   native: #09 pc 000a5b29  /system/lib/libandroid_runtime.so (???) A/art: art/runtime/check_jni.cc:65]   native: #10 pc 00010fd7  /system/lib/libutils.so (android::Looper::pollInner(int)+482) A/art: art/runtime/check_jni.cc:65]   native: #11 pc 00011081  /system/lib/libutils.so (android::Looper::pollOnce(int, int*, int*, void**)+92) A/art: art/runtime/check_jni.cc:65]   native: #12 pc 0007fbe5  /system/lib/libandroid_runtime.so (android::NativeMessageQueue::pollOnce(_JNIEnv*, int)+22) A/art: art/runtime/check_jni.cc:65]   native: #13 pc 00051b8b  /system/framework/arm/boot.oat (Java_android_os_MessageQueue_nativePollOnce__JI+102) A/art: art/runtime/check_jni.cc:65]   at android.os.MessageQueue.nativePollOnce(Native method) A/art: art/runtime/check_jni.cc:65]   at android.os.MessageQueue.next(MessageQueue.java:143) A/art: art/runtime/check_jni.cc:65]   at android.os.Looper.loop(Looper.java:122) A/art: art/runtime/check_jni.cc:65]   at android.app.ActivityThread.main(ActivityThread.java:5411) A/art: art/runtime/check_jni.cc:65]   at java.lang.reflect.Method.invoke!(Native method) A/art: art/runtime/check_jni.cc:65]   at java.lang.reflect.Method.invoke(Method.java:372) A/art: art/runtime/check_jni.cc:65]   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:916) A/art: art/runtime/check_jni.cc:65]   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:709)