Code Coverage is a tool in Xcode that is used to gain insights into how much of your code you are testing with your test suite. It tells you exactly which parts of your code were executed during a test and which parts of your code were not. This is extremely useful because you can take focused action based on the information provided by Code Coverage.
To enable Code Coverage, open the scheme editor through the (Product | Scheme) menu:
Select the Test action and make sure the Gather coverage checkbox on the Options tab is checked:
After doing this, close the scheme editor and run your tests. This time, Xcode will monitor which parts of your code were executed during this test, and which parts weren't. This information can give you some good insights about which parts of your code could use some more testing. To see the coverage data, open the Report navigator in the left sidebar in Xcode...