Monitoring HTTP Traffic with Instruments
Xcode comes bundled with Instruments, a tool that helps measure app metrics like memory usage, network activity and time profile. Using Instruments to profile your app provides valuable insights into app behavior and performance. Instruments works with all Apple platforms and allows live profiling and inspection while testing the app on a device!
In this tutorial, you’ll learn:
- How to use the HTTP Traffic Instrument in Xcode.
- About tasks and transactions in the instrument, and profiling networking activities.
- How to archive and share HTTP traffic traces with your fellow web developers.
Use the Download Materials button at the top or bottom of this tutorial to download the starter project.
The starter project is a tab-based app called Shuffle built using Swift UI. Shuffle has three tabs:
- Today: Shows the Quote of the Day along with a Shuffle button.
- Explore: A list of quotes the user can select, view and bookmark.
- Profile: Lets the user login and displays user activity on login.
Open the starter project. Then build and run it on your device. Navigate around the app to familiarize yourself with the UI.
You’ll use the APIs provided by https://favqs.com/ and https://picsum.photos/ for this tutorial. You can use these test credentials to login to the app or signup for an account at https://favqs.com/login.
Explore the project. Pay particular attention to DataProvider.swift. This file is the networking helper class that manages all the API requests and responses in the app. Check out the data models, views and view models to understand the implementation.
Try using the app, and you’ll notice a few issues that need addressing. Instruments can help to identify the underlying issues. The instrument you’ll use here is the HTTP Traffic Instrument.
What is the HTTP Traffic Instrument?
In its Network profiling template, Instruments 13 introduces a new HTTP Traffic Instrument which lets you track and inspect all the HTTP traffic flowing in the app.
HTTP Traffic not only helps you see what’s happening with the request/response you receive while using a web service API, it also helps you see what’s happening with the connection. In this way, it’s unlike other proxy web debugging tools available, such as Charles Proxy or Proxyman.
HTTP Traffic also enables network debugging without needing to install any certificates on the device. It even works when SSL pinning is enforced or when using a VPN, all while showing the entire requests and responses in a pretty format for HTTP/s connections.
While it’s a great tool, it has limitations. Currently, the HTTP Traffic Instrument doesn’t support simulator, mocking requests or responses, injecting or modifying payloads.
Now that you have a better understanding of the tool, next you’ll use it to inspect the app.
Inspecting the App
Before testing the app on a device, change the bundle identifier if necessary and set the provisioning profile. Follow the steps below to profile using Instruments:
- Connect your iOS device to your Mac.
- Open the starter project and set the run target as your device.
- Choose Product ▸ Profile from the menu or press Command-I.
- Choose Network when the profiling template selection dialog appears.
- To begin recording, press Record in the toolbar or press Command-R.
- While the recording is in progress, use the app and visit different screens.
- To stop the recording, press Record again.
You can remove the Network Connections Instrument as it’s unnecessary for this tutorial.
Now you can inspect and examine the data collected while you use the app. The data includes details related to each of the networking tasks and transactions that occurred during the session.
A typical inspection session looks like this:
HTTP Traffic Instrument shows all the HTTP activity in a timeline of Tracks. Each track is specific to a URL Session.
A. Selected Instrument: HTTP Traffic
B. Process: Application
C. Session: URLSession — Shared
D. Domain: URLSession Task — HTTP Request domain
Now, observe the filters:
- Filter 1: Lets you choose whether to display tasks or transactions in the track. You’ll learn about these later in this tutorial.
- Filter 2: Has options to choose a list of URL Session Tasks or HTTP Transactions or a summary of transaction durations.
Next, you’ll learn how HTTP Traffic Instrument actually works.
How the HTTP Traffic Instrument Works
For communicating with web services, you use higher-level networking APIs such as URLSession. But, Instruments relies on the lower-level networking stack of Apple’s core frameworks. Thus, it works on all Apple devices and all HTTP traffic passing through the app when using any networking API.
Task and Transaction
A Task is akin to a URLSessionDataTask or URLSessionDownloadTask. It begins when you call
resume and calls the respective completion closures before it ends.
A Transaction refers to a single instance of communication with a web service. It includes:
- Establishing the connection.
- Performing any cache lookup.
- Spinning up a thread.
- Sending an HTTP request.
- Blocking the thread while waiting.
- And receiving the incoming response.
In other words, and in its simplest terms, a transaction starts when you call
resume() on a task, and it ends when the associated completion block is called.
However, in some instances, a task may have more than one transaction, for example when there are redirects or URL forwarding.
Now that you know about all the information the tool provides, it’s time to profile the app and see how it fares!
Profiling and Inspecting the App
First, profile the starter app as a new session and start recording. In the Today tab, tap Shuffle and wait for a moment. Then stop recording the session.
Now that you’ve gathered the data, observe the timeline and tasks. The Instrument timeline will look something like this:
As seen in the image above, there are two tasks in the Today tab.
Check the task to get the quote of the day. It calls the favqs.com web service API.
The detail area and tooltip show all the important information related to this task:
- Endpoint and Query, if any: api/qotd
- HTTP Version: 1
- HTTP Method: GET
- Time taken/Duration/em>: example (100 ms)
- Response — Status code: 200
- Response mime type: application/json
As you can see, this task was successful and marked in green. Perfect, isn’t it? :]
Now, under Shuffle, choose the Track Display as HTTP Transactions. Then, choose the filter option in the detail area and select List: HTTP Transactions to view the transaction’s details.
You’ll see the request/response details in the inspection area for the selected transaction.
There’s one problem, though: The call to picsum.photos shows as canceled and is marked in grey. Not so good, is it? :[
To find out what went wrong, choose List: URLSession Task as the filter in the detail area. Then scroll right to see more information:
- Error domain
- Error code
- Localized error description
Select the particular task in the detail list to inspect it. The inspection detail to the right shows the backtrace.
getRandomPicture(completion:) in DataProvider.swift. Now you’ll see where the task starts, which helps you quickly check for possible problems.
Fortunately, in this case, you’ll see a
.cancel sent to the task dataTaskFetchImage in
getQOTD(completion:). It must be a typo that the developer inadvertently added that canceled this task when trying to get the quote of the day!
Well, how interesting is a quote without a nice picture in the background?! :] You’ll fix the missing picture in the next section.