在iOS上调试时，如果我暂停执行，我经常会看到对“__workq_kernreturn”的多个引用。如果我的应用碰巧崩溃了，我经常会看到如下所示的多个线程:Thread19:0libsystem_kernel.dylib0x332d0cd4__workq_kernreturn+8Thread20:0libsystem_kernel.dylib0x332d0cd4__workq_kernreturn+8Thread21:0libsystem_kernel.dylib0x332d0cd4__workq_kernreturn+8“__workq_kernreturn”是表示等待退出的线程，还
