The Podfile is a specification that describes the dependencies of the targets of one or more Xcode projects.A Podfile can be very simple:

target'MyApp'

pod 'AFNetworking', '~> 1.0'

An example of a more complex Podfile can be:

platform :ios, '9.0'
inhibit_all_warnings!

target 'MyApp' do
  pod 'ObjectiveSugar', '~> 0.5'

  target "MyAppTests" do
    inherit! :search_paths
    pod 'OCMock', '~> 2.0.1'
  end
end

post_install do |installer|
  installer.pods_project.pod_targets.each do |target|
    puts "#{target.name}"
  end
end

常见写入Podfile的写法：

xcodeproj 'CacheDemo.xcodeproj'

platform:ios,'7.0'

pod 'FMDB'

pod 'AFNetworking','~>2.6'

pod install vs. pod update

Introduction

Many people starting with CocoaPods seem to think pod install is only used the first time you setup a project using CocoaPods and pod update is used afterwards. But that's not the case at all.

The aim of this guide is to explain when you should use pod install and when you should use pod update.

• Use pod install to install new pods in your project. Even if you already have a Podfileand ran pod install before; so even if you are just adding/removing pods to a project already using CocoaPods.
• Use pod update [PODNAME] only when you want to update pods to a newer version.

<Detailed presentation of the commands

Note: the vocabulary of install vs. update is not actually specific to CocoaPods. It is inspired by a lot of other dependency managers like bundler, RubyGems or composer, which have similar commands, with the exact same behavior and intents as the one described in this document.

<

pod install

This is to be used the first time you want to retrieve the pods for the project, but also every time you edit your Podfile to add, update or remove a pod.

• Every time the pod install command is run — and downloads and install new pods — it writes the version it has installed, for each pods, in the Podfile.lock file. This file keeps track of the installed version of each pod and locks those versions.
• When you run pod install, it only resolve dependencies for pods that are not already listed in the Podfile.lock.
  • For pods listed in the Podfile.lock, it downloads the explicit version listed in the Podfile.lock without trying to check if a newer version is available
  • For pods not listed in the Podfile.lock yet, it searches for the version that matches what is described in the Podfile (like in pod 'MyPod', '~>1.2')

<

pod outdated

When you run pod outdated, CocoaPods will list all pods which have newer versions than the ones listed in the Podfile.lock (the versions currently installed for each pod). This means that if you run pod update PODNAME on those pods, they will be updated — as long as the new version still matches the restrictions like pod 'MyPod', '~>x.y' set in your Podfile.

<pod update

When you run pod update PODNAME, CocoaPods will try to find an updated version of the pod PODNAME, without taking into account the version listed in Podfile.lock. It will update the pod to the latest version possible (as long as it matches the version restrictions in your Podfile).

If you run pod update with no pod name, CocoaPods will update every pod listed in your Podfile to the latest version possible.

<Intended usage

With pod update PODNAME, you will be able to only update a specific pod (check if a new version exists and update the pod accordingly). As opposed to pod install which won't try to update versions of pods already installed.

When you add a pod to your Podfile, you should run pod install, not pod update — to install this new pod without risking to update existing pod in the same process.

You will only use pod update when you want to update the version of a specific pod (or all the pods).

<Commit your Podfile.lock

As a reminder, even if your policy is not to commit the Pods folder into your shared repository, you should always commit & push your Podfile.lock file.

Otherwise, it would break the whole logic explained above about pod install being able to lock the installed versions of your pods.

<Scenario Example

Here is a scenario example to illustrate the various use cases one might encounter during the life of a project.

<Stage 1: User1 creates the project

user1 creates a project and wants to use pods A,B,C. They create a Podfile with those pods, and run pod install.

This will install pods A,B,C, which we'll say are all in version 1.0.0.

The Podfile.lock will keep track of that and note that A,B and C are each installed as version 1.0.0.

Incidentally, because that's the first time they run pod install and the Pods.xcodeproj project doesn't exist yet, the command will also create the Pods.xcodeproj and the .xcworkspace, but that's a side effect of the command, not its primary role.

<Stage 2: User1 adds a new pod

Later, user1 wants to add a pod D into their Podfile.

They should thus run pod install afterwards, so that even if the maintener of pod B released a version 1.1.0 of their pod since the first execution of pod install, the project will keep using version 1.0.0 — because user1 only wants to add pod D, without risking an unexpected update to pod B.

That's where some people get it wrong, because they use pod update here — probably thinking this as "I want to update my project with new pods"? — instead of using pod install — to install new pods in the project.

<Stage 3: User2 joins the project

Then user2, who never worked on the project before, joins the team. They clone the repository then use pod install.

The contents of Podfile.lock (which should be committed onto the git repo) will guarantee they will get the exact same pods, with the exact same versions that user1 was using.

Even if a version 1.2.0 of pod C is now available, user2 will get the pod C in version 1.0.0. Because that's what is registered in Podfile.lock. pod C is locked to version 1.0.0 by the Podfile.lock (hence the name of this file).

<Stage 4: Checking for new versions of a pod

Later, user1 wants to check if any updates are available for the pods. They run pod outdatedwhich will tell them that pod B have a new 1.1.0 version, and pod C have a new 1.2.0 version released.

user1 decides they want to update pod B, but not pod C; so they will run pod update B which will update B from version 1.0.0 to version 1.1.0 (and update the Podfile.lock accordingly) but will keep pod C in version 1.0.0 (and won't update it to 1.2.0).

<Using exact versions in the Podfile is not enough

Some might think that by specifying exact versions of their pods in their Podfile, like pod 'A', '1.0.0', is enough to guarantee that every user will have the same version as other people on the team.

Then they might even use pod update, even when just adding a new pod, thinking it would never risk to update other pods because they are fixed to a specific version in the Podfile.

But in fact, that is not enough to guarantee that user1 and user2 in our above scenario will always get the exact same version of all their pods.

One typical example is if the pod A has a dependency on pod A2 — declared in A.podspec as dependency 'A2', '~> 3.0'. In such case, using pod 'A', '1.0.0' in your Podfile will indeed force user1 and user2 to both always use version 1.0.0 of the pod A, but:

• user1 might end up with pod A2 in version 3.4 (because that was A2's latest version at that time)
• while when user2 runs pod install when joining the project later, they might get pod A2 in version 3.5 (because the maintainer of A2 might have released a new version in the meantime).

That's why the only way to ensure every team member work with the same versions of all the pod on each's computer is to use the Podfile.lock and properly use pod install vs. pod update.

区别在这里：pod install会按照 pod 文件指定的版本安装，而pod update会安装最新版本。如果不是特别需要指定版本的话，一般情况下用pod update就可以啦。

https://guides.cocoapods.org/using/pod-install-vs-update.html