APP的启动可以分为2种：

冷启动（Cold Launch）：从零开始启动APP
热启动（Warm Launch）：APP已经在内存中，在后台存活着，再次点击图标启动APP

APP启动时间的优化

那我们通常所说的启动时间优化都是再说的冷启动的时间优化，其实Xcode是提供给我们一种分析启动时间的方式，我们接下来试一试

通过添加环境变量可以打印出APP的启动时间分析（Edit Scheme -> Run -> Arguments)

DYLD_PRINT_STATISTICS设置为1

我们可以看到控制台打印出了

Code

Total pre-main time: 238.11 milliseconds (100.0%)
       dylib loading time: 173.78 milliseconds (72.9%)
      rebase/binding time: 126687488.9 seconds (15889931.7%)
          ObjC setup time:  15.16 milliseconds (6.3%)
         initializer time:  62.59 milliseconds (26.2%)
         slowest intializers :
           libSystem.B.dylib :   6.66 milliseconds (2.7%)
 libBacktraceRecording.dylib :   7.30 milliseconds (3.0%)
  libMainThreadChecker.dylib :  40.12 milliseconds (16.8%)

如果需要更详细的信息，那就将DYLD_PRINT_STATISTICS_DETAILS设置为1

我们看到这次输出的信息比上次要详细很多

Code

  total time: 981.15 milliseconds (100.0%)
  total images loaded:  334 (327 from dyld shared cache)
  total segments mapped: 21, into 370 pages
  total images loading time: 664.10 milliseconds (67.6%)
  total load time in ObjC:  20.03 milliseconds (2.0%)
  total debugger pause time: 402.89 milliseconds (41.0%)
  total dtrace DOF registration time:   0.22 milliseconds (0.0%)
  total rebase fixups:  17,951
  total rebase fixups time:   2.27 milliseconds (0.2%)
  total binding fixups: 460,826
  total binding fixups time: 229.13 milliseconds (23.3%)
  total weak binding fixups time:   0.03 milliseconds (0.0%)
  total redo shared cached bindings time: 254.48 milliseconds (25.9%)
  total bindings lazily fixed up: 0 of 0
  total time in initializers and ObjC +load:  65.34 milliseconds (6.6%)
                         libSystem.B.dylib :   9.14 milliseconds (0.9%)
               libBacktraceRecording.dylib :   7.73 milliseconds (0.7%)
                           libobjc.A.dylib :   1.55 milliseconds (0.1%)
                            CoreFoundation :   2.24 milliseconds (0.2%)
                libMainThreadChecker.dylib :  38.86 milliseconds (3.9%)
                    libLLVMContainer.dylib :   2.13 milliseconds (0.2%)
total symbol trie searches:    1116978
total symbol table binary searches:    0
total images defining weak symbols:  37
total images using weak symbols:  92