How can I turn on parenthesis matching in Xcode?

on Xcode Version 6.1 (6A1052d), you just need to double click on one of the parenthesis and the entire code that the parenthesis is scoping out is highlighted.

for example, if you have a code like the one below and you double click '{' or '}' bracket the entire code in their scope is highlighted :

if(condition == true) { //do something }

Edit

In XCode 9 the only solution is to put mouse over parentheses while pressing command button. Only this works.

Previous

To extend the other answers it is possible to roll in/out code inside brackets.

Choose Editor > Structure > Balance Delimiter

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Nevertheless, as already written earlier, the major feature is

Editor > Code Folding > Focus Follows Selection

Choose editor -> code folding -> focus