小编Dif*_*hen的帖子

我尝试pod install在我的 Xcode 项目中运行，但出现以下错误

    /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/dependency.rb:311:in `to_specs': Could not find 'ffi' (>= 1.3.0) among 84 total gem(s) (Gem::MissingSpecError)
Checked in 'GEM_PATH=/Users/dylan/.gem/ruby/2.6.0:/Library/Ruby/Gems/2.6.0:/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/gems/2.6.0', execute `gem env` for more information

当我跑步时gem env，我得到了

    RubyGems Environment:
  - RUBYGEMS VERSION: 3.0.3
  - RUBY VERSION: 2.6.3 (2019-04-16 patchlevel 62) [universal.x86_64-darwin20]
  - INSTALLATION DIRECTORY: /Library/Ruby/Gems/2.6.0
  - USER INSTALLATION DIRECTORY: /Users/dylan/.gem/ruby/2.6.0
  - RUBY EXECUTABLE: /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby
  - GIT EXECUTABLE: /usr/bin/git
  - EXECUTABLE DIRECTORY: /usr/local/bin
  - SPEC CACHE DIRECTORY: /Users/dylan/.gem/specs
  - SYSTEM CONFIGURATION DIRECTORY: /Library/Ruby/Site
  - RUBYGEMS PLATFORMS: …

在 Swift 5 中从 Firebase 获取所有添加的子项后，如何执行一些代码？

我尝试过使用 DispatchGroup 并观察 .value，但它们都没有有效地工作。

let dispatchGroup = DispachGroup()

ref.child("path").observe(.childAdded, with: { (snapshot) in
     self.dispatchGroup.enter()
     //store snapshot data into an object
     self.dispatchGroup.leave()
})

dispatchGroup.notify(queue: .main) {
    //code to execute after all children are fetched
}

在这种情况下，代码将在获取数据之前执行。

当只有回调块到达最后一个子节点时，如何执行代码？