带着问题认识Flutter SDK系列2——命令从哪开始运行

注：本文基于Flutter1.12.13+Mac版本。
    问题一，当在命令终端输入flutter run或flutter doctor诸如此类的命令或从vscode之类的IDE运行的时候SDK发生了什么？
    当我们安装Flutter SDK时主要是在系统环境变量里配置了Flutter的bin路径下的一个可执行文件，在Mac上叫Unix可执行文件，这个文件名叫flutter的文件就是命令flutter对应的入口文件。
    用vscode（其它文本编辑器也可以）打开bin/flutter文件让我们来看看bin下面的flutter文件里都做了什么事情：

文件的开头 #!/usr/bin/env bash 说明这是一个Shell脚本文件；以下代码补充了我们更详细的中文注解，为更好地查看代码，建议你把以下代码复制到你的IDE如vscode里查看：

#!/usr/bin/env bash
#
#【0】确保所有运行错误都是fatal error随时退出，直接跳到【1】处查看
#
set -e
#
#删除环境变量CDPATH
#
unset CDPATH

function follow_links() {
  cd -P "${1%/*}"
  local file="$PWD/${1##*/}"
  while [[ -h "$file" ]]; do
    # On Mac OS, readlink -f doesn't work.
    cd -P "${file%/*}"
    file="$(readlink "$file")"
    cd -P "${file%/*}"
    file="$PWD/${file##*/}"
  done
  echo "$PWD/${file##*/}"
}

# Convert a filesystem path to a format usable by Dart's URI parser.
function path_uri() {
  # Reduce multiple leading slashes to a single slash.
  echo "$1" | sed -E -e "s,^/+,/,"
}

function _rmlock () {
  [ -n "$FLUTTER_UPGRADE_LOCK" ] && rm -f "$FLUTTER_UPGRADE_LOCK"
}

function retry_upgrade {
  local total_tries="10"
  local remaining_tries=$((total_tries - 1))
  while [[ "$remaining_tries" -gt 0 ]]; do
    (cd "$FLUTTER_TOOLS_DIR" && "$PUB" upgrade "$VERBOSITY" --no-precompile) && break
    echo "Error: Unable to 'pub upgrade' flutter tool. Retrying in five seconds... ($remaining_tries tries left)"
    remaining_tries=$((remaining_tries - 1))
    sleep 5
  done

  if [[ "$remaining_tries" == 0 ]]; then
    echo "Command 'pub upgrade' still failed after $total_tries tries, giving up."
    return 1
  fi
  return 0
}

function upgrade_flutter () {
#
#【10】全路径创建cache文件夹，用来放置生成的snapshot等，提高效率
#
  mkdir -p "$FLUTTER_ROOT/bin/cache"

  # This function is executed with a redirect that pipes the source of
  # this script into file descriptor 3.
  #
  # To ensure that we don't simultaneously update Dart in multiple
  # parallel instances, we try to obtain an exclusive lock on this
  # file descriptor (and thus this script's source file) while we are
  # updating Dart and compiling the script. To do this, we try to use
  # the command line program "flock", which is available on many
  # Unix-like platforms, in particular on most Linux distributions.
  # You give it a file descriptor, and it locks the corresponding
  # file, having inherited the file descriptor from the shell.
  #
  # Complicating matters, there are two major scenarios where this
  # will not work.
  #
  # The first is if the platform doesn't have "flock", for example on Mac.
  # There is not a direct equivalent, so on platforms that don't have flock,
  # we fall back to using a lockfile and spinlock with "shlock".  This
  # doesn't work as well over NFS as it relies on PIDs. Any platform
  # without either of these tools has no locking at all. To determine if we
  # have "flock" or "shlock" available, we abuse the "hash" shell built-in.
  #
  # The second complication is NFS. On NFS, to obtain an exclusive
  # lock you need a file descriptor that is open for writing, because
  # NFS implements exclusive locks by writing, or some such. Thus, we
  # ignore errors from flock. We do so by using the '|| true' trick,
  # since we are running in a 'set -e' environment wherein all errors
  # are fatal, and by redirecting all output to /dev/null, since
  # users will typically not care about errors from flock and are
  # more likely to be confused by them than helped.
  #
  # For "flock", the lock is released when the file descriptor goes out of
  # scope,  i.e. when this function returns.  The lock is released via
  # a trap when using "shlock".
  if hash flock 2>/dev/null; then
    flock 3 2>/dev/null || true
  elif hash shlock 2>/dev/null; then
    FLUTTER_UPGRADE_LOCK="$FLUTTER_ROOT/bin/cache/.upgrade_lock"
    while ! shlock -f "$FLUTTER_UPGRADE_LOCK" -p $$ ; do sleep .1 ; done
    trap _rmlock EXIT
  fi

  local revision="$(cd "$FLUTTER_ROOT"; git rev-parse HEAD)"

  # Invalidate cache if:
  #  * SNAPSHOT_PATH is not a file, or
  #  * STAMP_PATH is not a file with nonzero size, or
  #  * Contents of STAMP_PATH is not our local git HEAD revision, or
  #  * pubspec.yaml last modified after pubspec.lock
#
#【11】判断是否需要更新cache，如果不需要则执行【9】
#
  if [[ ! -f "$SNAPSHOT_PATH" || ! -s "$STAMP_PATH" || "$(cat "$STAMP_PATH")" != "$revision" || "$FLUTTER_TOOLS_DIR/pubspec.yaml" -nt "$FLUTTER_TOOLS_DIR/pubspec.lock" ]]; then
    rm -f "$FLUTTER_ROOT/version"
    touch "$FLUTTER_ROOT/bin/cache/.dartignore"
    "$FLUTTER_ROOT/bin/internal/update_dart_sdk.sh"
    VERBOSITY="--verbosity=error"

    echo Building flutter tool...
    if [[ "$CI" == "true" || "$BOT" == "true" || "$CONTINUOUS_INTEGRATION" == "true" || "$CHROME_HEADLESS" == "1" ]]; then
      PUB_ENVIRONMENT="$PUB_ENVIRONMENT:flutter_bot"
      VERBOSITY="--verbosity=normal"
    fi
    export PUB_ENVIRONMENT="$PUB_ENVIRONMENT:flutter_install"

    if [[ -d "$FLUTTER_ROOT/.pub-cache" ]]; then
      export PUB_CACHE="${PUB_CACHE:-"$FLUTTER_ROOT/.pub-cache"}"
    fi
#
#【12】下载相关依赖重试次数，有时候外网不给力的时候会从这里报错
#
    retry_upgrade
#
#【13】重新打包snapshot并执行
#
    "$DART" $FLUTTER_TOOL_ARGS --snapshot="$SNAPSHOT_PATH" --packages="$FLUTTER_TOOLS_DIR/.packages" "$SCRIPT_PATH"
    echo "$revision" > "$STAMP_PATH"
  fi
  # The exit here is duplicitous since the function is run in a subshell,
  # but this serves as documentation that running the function in a
  # subshell is required to make sure any lockfile created by shlock
  # is cleaned up.
#
#【14】退出
#
  exit $?
}
#
#【1】脚本开始执行的地方
#
PROG_NAME="$(path_uri "$(follow_links "$BASH_SOURCE")")"
BIN_DIR="$(cd "${PROG_NAME%/*}" ; pwd -P)"
export FLUTTER_ROOT="$(cd "${BIN_DIR}/.." ; pwd -P)"
#
#【2】SDK源代码存放的地方
#
FLUTTER_TOOLS_DIR="$FLUTTER_ROOT/packages/flutter_tools"
#
#【3】SDK快照存放的地方，即源代码编译成二进制后的代码
#
SNAPSHOT_PATH="$FLUTTER_ROOT/bin/cache/flutter_tools.snapshot"
#
#【4】SDK的时间戳，用来和git版本比较，如果不相同则重新生成snapshot
#
STAMP_PATH="$FLUTTER_ROOT/bin/cache/flutter_tools.stamp"
#
#【5】SDK执行的入口文件
#
SCRIPT_PATH="$FLUTTER_TOOLS_DIR/bin/flutter_tools.dart"
#
#【6】SDK自带的Dart，运行snapshot，下载第三方插件及环境相关就在这里
#
DART_SDK_PATH="$FLUTTER_ROOT/bin/cache/dart-sdk"

DART="$DART_SDK_PATH/bin/dart"
#
#【7】下载pub上的第三方插件就在这里
#
PUB="$DART_SDK_PATH/bin/pub"

# If running over git-bash, overrides the default UNIX
# executables with win32 executables
case "$(uname -s)" in
  MINGW32*)
    DART="$DART.exe"
    PUB="$PUB.bat"
    ;;
esac

# Test if running as superuser – but don't warn if running within Docker
if [[ "$EUID" == "0" && ! -f /.dockerenv ]]; then
  echo "   Woah! You appear to be trying to run flutter as root."
  echo "   We strongly recommend running the flutter tool without superuser privileges."
  echo "  /"
  echo "📎"
fi

# Test if Git is available on the Host
if ! hash git 2>/dev/null; then
  echo "Error: Unable to find git in your PATH."
  exit 1
fi
# Test if the flutter directory is a git clone (otherwise git rev-parse HEAD would fail)
if [[ ! -e "$FLUTTER_ROOT/.git" ]]; then
  echo "Error: The Flutter directory is not a clone of the GitHub project."
  echo "       The flutter tool requires Git in order to operate properly;"
  echo "       to set up Flutter, run the following command:"
  echo "       git clone -b stable https://github.com/flutter/flutter.git"
  exit 1
fi

# To debug the tool, you can uncomment the following lines to enable checked mode and set an observatory port:
# FLUTTER_TOOL_ARGS="--enable-asserts $FLUTTER_TOOL_ARGS"
# FLUTTER_TOOL_ARGS="$FLUTTER_TOOL_ARGS --observe=65432"
#
#【8】生成cache里的文件如snapshot、stamp等并执行，如果cache不需要生成则执行【9】的命令，否则不执行
#
(upgrade_flutter) 3< "$PROG_NAME"

# FLUTTER_TOOL_ARGS isn't quoted below, because it is meant to be considered as
# separate space-separated args.
#
#【9】如果【8】不执行则从$SCRIPT_PATH开始执行命令
#
"$DART" --packages="$FLUTTER_TOOLS_DIR/.packages" $FLUTTER_TOOL_ARGS "$SNAPSHOT_PATH" "$@"

了解了SDK的入口文件，接下来将带你进入具体的代码看看flutter是怎么一步步打包的。从flutter run开