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1. Thread synchronization is multiple threads accessing the same resource at the same time, waiting for resource access to end, wasting time, inefficient, serial execution of tasksThread Async: Access resources while idle wait while accessing other resources, implement multithreading mechanism, execute task 1.2.3 mode in parallelUse GCD's primary queue. Use Nsoperationqueue to set the maximum number of threads (concurrency) to 1 1.1.1How

First, create the PCH file. Click Command+n. such as OperationNaming conventions are generally: project name-prefixStep TwoOK, you have finished making the PCH file here. Then you can use it.PCH files generally write data that can be used globally.Like what:The width and color of the screen (no punctuation behind the macro)#define RGB (r,g,b,a) [Uicolor colorwithred: (r/255.0) Green: (g/255.0) Blue: (b/255.0) Alpha: (a)] #define SCREEN_W [ UIScreen mainscreen].bounds.size.width#define Screen_h