ios - 当数据为 NULL 时，访问链表的数据元素时应用程序崩溃

Question

我正在使用以下代码遍历 objective-c 中的链表

   const MSList *calls = linphone_core_get_calls(LC);
            if (calls == NULL)
            {
                                [self dismissCtrl];
                //how to check current is which screen is on
                //                while ((currentView == CallView.compositeViewDescription) ||
                //                       (currentView == CallIncomingView.compositeViewDescription) ||
                //                       (currentView == CallOutgoingView.compositeViewDescription)) {
                //                    [self popCurrentView];
                //                }

            } else {
                linphone_call_resume((LinphoneCall *)calls->data);
                while (calls)
                {

                    if(calls->data != NULL && calls->data != nil && calls->data != (__bridge void *)((id)[NSNull null]))
                    {
                        //crash
                    if (linphone_call_get_state((LinphoneCall *)calls->data) == LinphoneCallIncomingReceived ||
                        linphone_call_get_state((LinphoneCall *)calls->data) == LinphoneCallIncomingEarlyMedia) {
                        [self displayIncomingCall:(LinphoneCall *)calls->data];
                        break;
                    }
                   }
                    calls = calls->next;



                }

当整个列表不为空但其数据、前一个或下一个值为 NULL 时，应用程序崩溃。我添加了代码来检查数据是否为 ​​NULL，但如果它(数据)为 NULL，那么我将无法访问它，并且在这种情况下应用程序本身就会崩溃。如何防止这种情况发生？我已附上应用程序崩溃位置以及列表当时包含的值的屏幕截图。

Answer 1

if (calls == NULL) {
                [self dismissCtrl];
                //how to check current is which screen is on
                //                while ((currentView == CallView.compositeViewDescription) ||
                //                       (currentView == CallIncomingView.compositeViewDescription) ||
                //                       (currentView == CallOutgoingView.compositeViewDescription)) {
                //                    [self popCurrentView];
                //                }

            }
            else
            {

                size_t count = bctbx_list_size(calls);

                linphone_call_resume((LinphoneCall *)calls->data);


                int i = 0;

                while (calls )
                {


                    if(i < count)
{



                    if ( calls->data == (__bridge void *)((id)[NSNull null]) || calls->data == NULL || calls->data == nil )
                    {

                        return;
                    }


                    LinphoneCall *objCall =  (LinphoneCall *)calls->data;



                    LinphoneCallState state = (objCall != NULL) ? linphone_call_get_state(call) : 0;



                    if (state == LinphoneCallIncomingReceived ||
                        state == LinphoneCallIncomingEarlyMedia)
                    {
                        [self displayIncomingCall:(LinphoneCall *)calls->data];
                        break;
                    }

                    //calls ? calls->data : NULL
                   // calls =  calls ? calls->next : NULL;
                    calls = calls->next;
                        i = i+1;
                    }
                    else
                    {
                        break;
                    }
                }