iOS app查看启动时间

以main()函数作为分界点，main()函数之前部分叫做pre-main。

pre-main

我们可以在 Xcode 中配置环境变量 DYLD_PRINT_STATISTICS 为 1（Edit Scheme → Run → Arguments → Environment Variables → +）。

这时在 iOS 10 以上系统中运行一个 TestDemo，main()函数之前的启动时间会在控制台中打印出来。

Total pre-main time: 354.21 milliseconds (100.0%)
         dylib loading time:  25.52 milliseconds (7.2%)
        rebase/binding time:  12.70 milliseconds (3.5%)
            ObjC setup time: 152.74 milliseconds (43.1%)
           initializer time: 163.24 milliseconds (46.0%)
           slowest intializers :
             libSystem.B.dylib :   7.98 milliseconds (2.2%)
   libBacktraceRecording.dylib :  13.53 milliseconds (3.8%)
    libMainThreadChecker.dylib :  41.11 milliseconds (11.6%)
                      TestDemo :  88.76 milliseconds (25.0%)

如果要更详细的信息，就设置 DYLD_PRINT_STATISTICS_DETAILS 为 1。

  total time: 1.6 seconds (100.0%)
  total images loaded:  388 (381 from dyld shared cache)
  total segments mapped: 23, into 413 pages
  total images loading time: 805.78 milliseconds (48.6%)
  total load time in ObjC: 152.74 milliseconds (9.2%)
  total debugger pause time: 780.26 milliseconds (47.1%)
  total dtrace DOF registration time:   0.00 milliseconds (0.0%)
  total rebase fixups:  54,265
  total rebase fixups time:  20.77 milliseconds (1.2%)
  total binding fixups: 527,211
  total binding fixups time: 513.54 milliseconds (31.0%)
  total weak binding fixups time:   0.31 milliseconds (0.0%)
  total redo shared cached bindings time: 521.93 milliseconds (31.5%)
  total bindings lazily fixed up: 0 of 0
  total time in initializers and ObjC +load: 163.24 milliseconds (9.8%)
                         libSystem.B.dylib :   7.98 milliseconds (0.4%)
               libBacktraceRecording.dylib :  13.53 milliseconds (0.8%)
                libMainThreadChecker.dylib :  41.11 milliseconds (2.4%)
              libViewDebuggerSupport.dylib :   6.68 milliseconds (0.4%)
                                  TestDemo :  88.76 milliseconds (5.3%)
total symbol trie searches:    1306942
total symbol table binary searches:    0
total images defining weak symbols:  41
total images using weak symbols:  105

main() 函数之后

手动插入代码，进行耗时计算。

// 第一步：在 main() 函数里用变量 MainStartTime 记录当前时间
CFAbsoluteTime MainStartTime;
int main(int argc, char * argv[]) {
    MainStartTime = CFAbsoluteTimeGetCurrent();
    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

// 第二步：在 AppDelegate.m 文件中用 extern 声明全局变量 MainStartTime
extern CFAbsoluteTime MainStartTime;

// 第三步：在 didFinishLaunchingWithOptions 方法结束前，再获取一下当前时间，与 MainStartTime 的差值就是 main() 函数阶段的耗时
double mainLaunchTime = (CFAbsoluteTimeGetCurrent() - MainStartTime);
NSLog(@"main() 阶段耗时：%.2fms", mainLaunchTime * 1000);