App的启动可以分为2种

App启动时间优化,主要针对冷启动进行优化

1.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

第1次运行Xcode打印时间

// 总共需要时间
Total pre-main time: 760.53 milliseconds (100.0%)
// 动态库加载时间
dylib loading time: 100.44 milliseconds (13.2%)
rebase/binding time: 496.51 milliseconds (65.2%)
// ObjC结构体准备
ObjC setup time: 119.91 milliseconds (15.7%)
// 初始化
initializer time:  43.54 milliseconds (5.7%)
// 比较慢的加载(比较耗时的)
slowest intializers :
// 动态库
libSystem.B.dylib :   8.72 milliseconds (1.1%)
libMainThreadChecker.dylib :  23.59 milliseconds (3.1%)

启动App时,dyld所做的事情有:

启动App时,runtime所做的事情有:

到此为止,可执行文件和动态库中所有的符号(Class,Protocol,Selector, IMP,…)都已经按格式成功加载到内存中,被runtime所管理

总结一下: 1.App的启动有dyld主导,将可执行文件加载到内存,加载所有依赖的动态库 2.由runtime负责加载成Objective-C定义的结构

所有初始化工作结束后,dyld就会调用main函数

接下来就是UIApplicationMain函数,AppDegate的application: didFinishLaunchingWithOptions:方法

第2次运行Xcode打印时间

dylib loading time:  72.89 milliseconds (37.5%)
rebase/binding time:  34.46 milliseconds (17.7%)
ObjC setup time:  55.50 milliseconds (28.5%)
initializer time:  31.31 milliseconds (16.1%)
slowest intializers :
libSystem.B.dylib :   4.24 milliseconds (2.1%)
libMainThreadChecker.dylib :  18.00 milliseconds (9.2%)

第3次运行Xcode打印时间

Total pre-main time: 304.79 milliseconds (100.0%)
dylib loading time: 182.09 milliseconds (59.7%)
rebase/binding time:  40.21 milliseconds (13.1%)
ObjC setup time:  52.77 milliseconds (17.3%)
initializer time:  29.52 milliseconds (9.6%)
slowest intializers :
libSystem.B.dylib :   2.71 milliseconds (0.8%)
libMainThreadChecker.dylib :  17.79 milliseconds (5.8%)

可以看到每次运行,这个时间并不准确,只能说给我们参考

需要更详细的信息,DYLD_PRINT_STATISTICS_DETAILS 设置为1

total time: 635.54 milliseconds (100.0%)
// 镜像加载
total images loaded:  257 (0 from dyld shared cache)
// 模块映射
total segments mapped: 764, into 103180 pages with 7208 pages pre-fetched
total images loading time: 453.85 milliseconds (71.4%)
total load time in ObjC:  57.09 milliseconds (8.9%)
total debugger pause time: 359.81 milliseconds (56.6%)
total dtrace DOF registration time:   0.17 milliseconds (0.0%)
total rebase fixups:  2,513,282
total rebase fixups time:  63.67 milliseconds (10.0%)
total binding fixups: 282,663
total binding fixups time:  31.07 milliseconds (4.8%)
total weak binding fixups time:   0.48 milliseconds (0.0%)
total redo shared cached bindings time:  37.20 milliseconds (5.8%)
total bindings lazily fixed up: 0 of 0
total time in initializers and ObjC +load:  29.17 milliseconds (4.5%)
libSystem.B.dylib :   2.60 milliseconds (0.4%)
libBacktraceRecording.dylib :   2.69 milliseconds (0.4%)
CoreFoundation :   1.64 milliseconds (0.2%)
Foundation :   2.04 milliseconds (0.3%)
libMainThreadChecker.dylib :  18.14 milliseconds (2.8%)
libLLVMContainer.dylib :   0.74 milliseconds (0.1%)
total symbol trie searches:    132646
total symbol table binary searches:    0
total images defining weak symbols:  20
total images using weak symbols:  61

App的冷启动可以概括为3大阶段

归根到底可以总结为main之前,和main之后

App的冷启动优化

按照不同的阶段

dyld

runtime

main

在不影响用户体验的前提下,尽可能将一些操作延迟,不要全部都放在finishLaunching方法中,按需加载

当我们点击了 build 之后，做了什么事情呢？