TL;DR Using functional programming techniques, we can easily build fancy terminals with dynamic components in C++. is a C++ library that provides some of the necessary building blocks for implementing this concept. RxTerm We applied the same ideas in . This is the result: Buckaroo Motivation Imagine that we want to build a text-based application that updates the console as its state changes. A great example of this is Curl, which gives a live progress bar for downloads. Try this in your teminal: Such interfaces are implemented using . These are invisible characters that have side-effects, such as erasing characters and moving the cursor. ANSI escape codes We can use these escape codes to create live text interfaces. A simple implementation might update a progress bar like this: So far so good, but what if we have more complicated case? Ideally, we would leverage a library that figures these things out for us. We should only have to specify what the terminal output should look like, and the library functions would figure out the characters to print to get us there. So, at a high-level, we need three things: 1. A variable to track the previous state of the console 2. A function for rendering the current application state 3. A function for transforming the console from the previous state to the next state developers will be familiar with this pattern. It’s a clever strategy that can actually be applied to any I/O device! React Additionally, we want to be able leverage reusable UI components for things like progress-bars, lists and so on. Something like HTML, but simpler, would be ideal. A component would be any object than can be rendered to console output. Simple API Example To see where we are going, let’s take a look at an example in . With RxTerm, we can turn basic terminal components into more complex ones using composition. If the app-state changes, then we compute a new view for the terminal and replace the currently visible output. RxTerm This example shows how we can design a new component called , which prints frames like this: fancyCounter Here’s the code: As you can see, the interface is very high-level. ANSI Escape Codes Let’s start with the basics. How can we use ANSI escape codes to change the color and delete a line? This function prints in red type followed by in the default color. The magic escape sequences ( and ) modify how the terminal displays characters. Hello World \e[31m \e[0m An escape sequence is introduced with followed by a semicolon-separated-list of modifiers, terminated with . For instance, would print in red italics on a blue background. \e[ m \e[3;31;42mTEXT TEXT We can also print to reset the terminal back to its default state. \e[0m You can find a whole list on . bash-hackers.org Abstracting the Terminal State To be able to compose components, we need a high-level representation of the state of the console. Since we want to support colors, an intuitive representation of this is a map from coordinates to pixels: Once we have the basic abstraction for components,we can take advantage of type-erasure to maintain value-semantics and make inheritance an implementation detail: Now, we can build higher order components like a object: Text You can find the actual implementation . here Performing State Transitions Imagine we want to transition from this: … to this: The simplest way would be to delete the latest 3 lines and print the new lines. We can delete the latest line by printing ( ; ; ). \e[2K\r\e[1A delete line move cursor to the start of the line move cursor up one line A more sophisticated approach would be to compute the difference and move to the target position and edit only the terminal pixels that changed. For simplicity, we will stick to the first approach and combine the moving parts into one class: The . actual implementation is hosted on GitHub Verdict We saw how easy it is to write a reactive terminal framework and manage the state transitions without too many headaches! You can find the implemention of some basic components in our . We would love to see more people doing beautiful terminal interfaces! repo on GitHub Teaser: FRP Style Components With RxCpp Our next article will be about how we can leverage to write highly concurrent applications with complex state management. RxCpp More About Buckaroo We created to make C++ code-reuse easier. Read more about it : Buckaroo on Medium _Buck is a cross-platform, cross-language build system made for large scale compilation at Facebook. All Buckaroo…_hackernoon.com 7 Reasons to Use Buck Build _C++ is an unusual language in that it does not yet have a dominant package manager (we’re working on it!). As a result…_hackernoon.com Approaches to C++ Dependency Management, or Why We Built Buckaroo _When writing C++ applications, you will inevitably make use of external libraries. This is a good thing! Code re-use…_hackernoon.com 6 Reasons Why We Distribute C++ Libraries as Source-Code
