我們知道RunLoop在不斷的切換Mode處理其中的Source0，Source1，Observers，Timers，如果沒有相關(guān)的觸發(fā)就會(huì)休眠，進(jìn)行線程阻塞，等到有相關(guān)的觸發(fā)繼續(xù)干活。

那是通過

while(1){};

來實(shí)現(xiàn)的嗎？

我們來看下相關(guān)的源碼

 __CFRunLoopServiceMachPort(waitSet, &msg, sizeof(msg_buffer), &livePort, poll ? 0 : TIMEOUT_INFINITY, &voucherState, &voucherCopy);

static Boolean __CFRunLoopServiceMachPort(mach_port_name_t port, mach_msg_header_t **buffer, size_t buffer_size, mach_port_t *livePort, mach_msg_timeout_t timeout, voucher_mach_msg_state_t *voucherState, voucher_t *voucherCopy) {
    Boolean originalBuffer = true;
    kern_return_t ret = KERN_SUCCESS;
    for (;;) {      /* In that sleep of death what nightmares may come ... */
        mach_msg_header_t *msg = (mach_msg_header_t *)*buffer;
        msg->msgh_bits = 0;
        msg->msgh_local_port = port;
        msg->msgh_remote_port = MACH_PORT_NULL;
        msg->msgh_size = buffer_size;
        msg->msgh_id = 0;
        if (TIMEOUT_INFINITY == timeout) { CFRUNLOOP_SLEEP(); } else { CFRUNLOOP_POLL(); }
        ret = mach_msg(msg, MACH_RCV_MSG|(voucherState ? MACH_RCV_VOUCHER : 0)|MACH_RCV_LARGE|((TIMEOUT_INFINITY != timeout) ? MACH_RCV_TIMEOUT : 0)|MACH_RCV_TRAILER_TYPE(MACH_MSG_TRAILER_FORMAT_0)|MACH_RCV_TRAILER_ELEMENTS(MACH_RCV_TRAILER_AV), 0, msg->msgh_size, port, timeout, MACH_PORT_NULL);

        // Take care of all voucher-related work right after mach_msg.
        // If we don't release the previous voucher we're going to leak it.
        voucher_mach_msg_revert(*voucherState);
        
        // Someone will be responsible for calling voucher_mach_msg_revert. This call makes the received voucher the current one.
        *voucherState = voucher_mach_msg_adopt(msg);
        
        if (voucherCopy) {
            if (*voucherState != VOUCHER_MACH_MSG_STATE_UNCHANGED) {
                // Caller requested a copy of the voucher at this point. By doing this right next to mach_msg we make sure that no voucher has been set in between the return of mach_msg and the use of the voucher copy.
                // CFMachPortBoost uses the voucher to drop importance explicitly. However, we want to make sure we only drop importance for a new voucher (not unchanged), so we only set the TSD when the voucher is not state_unchanged.
                *voucherCopy = voucher_copy();
            } else {
                *voucherCopy = NULL;
            }
        }

        CFRUNLOOP_WAKEUP(ret);
        if (MACH_MSG_SUCCESS == ret) {
            *livePort = msg ? msg->msgh_local_port : MACH_PORT_NULL;
            return true;
        }
        if (MACH_RCV_TIMED_OUT == ret) {
            if (!originalBuffer) free(msg);
            *buffer = NULL;
            *livePort = MACH_PORT_NULL;
            return false;
        }
        if (MACH_RCV_TOO_LARGE != ret) break;
        buffer_size = round_msg(msg->msgh_size + MAX_TRAILER_SIZE);
        if (originalBuffer) *buffer = NULL;
        originalBuffer = false;
        *buffer = realloc(*buffer, buffer_size);
    }
    HALT;
    return false;
}

mach_msg 是比較底層的函數(shù)，讓線程直接阻塞休眠，實(shí)現(xiàn)真正的休眠。等到有相關(guān)的消息觸發(fā)，又會(huì)蘇醒干活。
以while(1){};這種形式是可以阻塞線程，但是CPU還是在不斷的工作，不過是死循環(huán)而已。

區(qū)別：
當(dāng)用mach_msg的時(shí)候，會(huì)從用戶態(tài)轉(zhuǎn)化到內(nèi)核態(tài)去讓線程阻塞，休眠
當(dāng)用while(1){};的時(shí)候，還是在用戶態(tài)，只不過是代碼邏輯阻塞，線程沒有真正的休眠

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