Thinking about how you will observe whether things are working correctly or not ahead of time can also have a big impact on the quality of the code you write.

The practice of test-driven development is where you actually write the tests first, then write the code to pass more and more tests until all of them pass.

It’s still a good idea to keep your code in three different buckets, and keep these buckets isolated from each other:Display/UI ComponentsProgram logic/business rules — the stuff that deals with the problem you’re solving for the user.Side effects (I/O, network, disk, etc.)

Test first development, also known as Test Driven Development (TDD) is a development style in which you write the unit tests before you write the code to test.

We shouldn’t actually care what the target/action values on the bar button item are. We should only care about what happens when it is pressed. Everything else is an implementation detail

Isolate the dependencies Write the test first Draw the boundary Implement the logic