Your Swift Playgrounds app won't run or the preview is stuck? Here's how to diagnose and fix the most common build and runtime errors, from typos to state crashes.
When an app won't run, the first move is always the same: read the exact error Swift Playgrounds shows.
The app surfaces errors inline, often pointing at the specific line. That line, or the one just above it, is usually where the problem lives.
Resist the urge to randomly change code. Errors are information, and Swift's messages are more helpful than beginners expect.
Most failures fall into a handful of categories, and once you recognize the category, the fix is quick. The rest of this guide walks those categories.
The most common cause of a non-running app is a simple syntax mistake.
Look for unbalanced brackets and parentheses. A missing closing brace is the classic culprit, and it often makes the error appear on a line far from the real problem.
Check for misspelled view or type names. SwiftUI is case-sensitive, so text is not the same as Text.
Confirm every string has matching quotes and every block opens and closes cleanly. Swift Playgrounds' code completion helps here: accept its suggestions instead of typing names by hand.
Swift is strongly typed, so passing the wrong type triggers a build error.
A frequent example is handing a number where a String is expected, or vice versa. The error will mention the expected type and the one you provided.
Convert explicitly when needed. To show a number in a Text view, interpolate it into a string rather than passing the raw number.
Read the type names in the error carefully. They tell you exactly what Swift wanted and what it got, which usually points straight at the fix.
A view's body must return a view. Some errors come from a body that accidentally tries to return something else or nothing.
A common trap is writing regular statements, like a loop or a print, directly in the body where SwiftUI expects view-building code.
If you need logic, compute values before the view content or use SwiftUI's view-building constructs like ForEach instead of a plain for loop.
When the error mentions the body or a closure not returning a view, this is usually the cause. Keep the body focused on describing UI.
Sometimes the code is fine but the live preview seems frozen or stale.
First, stop and re-run the app. A fresh run clears many transient glitches.
If it stays stuck, check for an infinite loop or extremely heavy work running on launch. Code that never finishes will hang the preview.
Also confirm your iPad or Mac is not low on resources. Closing other heavy apps and restarting Swift Playgrounds resolves a surprising number of stuck-preview cases.
An app that builds but crashes when running points to a runtime error rather than a syntax one.
A classic cause is force-unwrapping an optional that is nil. If you used an exclamation mark to unwrap a value that turned out to be missing, the app crashes.
Replace force-unwraps with safe handling: use optional binding with if let or guard, or provide a default with the nil-coalescing operator.
Array index out of range is another common runtime crash. Confirm any index you use is actually within the array's bounds before accessing it.
When you cannot spot the cause, shrink the problem.
Comment out recent additions and run. If the app works again, you have narrowed the issue to the code you removed.
Add code back in small pieces, running after each, until the error returns. The last addition is your culprit.
This binary-search approach feels slow but is faster than staring. It is also exactly how professionals debug, and it works in Swift Playgrounds just as well as in Xcode.
Occasionally the error you see does not match the code in front of you, which usually means something stale is hanging around.
Start by stopping the running app fully and running again from scratch. A clean run clears a lot of transient confusion.
If that does not help, quit Swift Playgrounds entirely and reopen it. A fresh launch resets the editor and the live preview state.
When an error stubbornly persists across edits that should have fixed it, suspect a stale state rather than your latest code. A clean restart is a quick, low-cost thing to try before digging deeper, and it resolves a surprising share of phantom errors.
A subtle class of bugs is not a crash at all: the app runs, but the screen does not update when you expect it to.
The usual cause is changing a value that is not stored in state. If a plain property changes but the view does not refresh, it likely needs the @State attribute so SwiftUI watches it.
The opposite mistake is editing a copy instead of the source of truth. If a child view changes a value but the parent never sees it, you probably need a @Binding so both sides share one value.
When the UI feels frozen even though no error appears, look first at how your data is declared. Nine times out of ten the fix is moving a value into @State or wiring up a binding correctly.
When you cannot tell what your code is actually doing, make it tell you.
Add print statements at key points to log values and confirm whether a branch of code even runs. Seeing the output is often faster than guessing.
For SwiftUI specifically, you can temporarily show a value on screen in a Text view to watch how it changes as you interact with the app.
This kind of lightweight inspection turns a mysterious bug into an observable one. Once you can see the values flowing through your code, the cause usually becomes obvious.
Most run and build problems are solvable inside Swift Playgrounds, but some are not.
If you need deep debugging, breakpoints with rich inspection, or detailed performance analysis, Xcode on a Mac offers far more tooling.
Because your app project is Xcode-compatible, you can open it on a Mac to chase a stubborn bug, then continue wherever you like.
Knowing that escape hatch exists takes the pressure off. Solve what you can in Swift Playgrounds, and reach for Xcode only when the problem genuinely demands it.
Unbalanced brackets often make Swift report the error on a later line. Check for a missing closing brace above where the error appears.
Usually putting plain statements like loops or prints directly in the body, where SwiftUI expects view-building code. Use ForEach and compute values outside the view content.
Common runtime crashes come from force-unwrapping a nil optional or accessing an array index out of range. Use safe unwrapping and bounds checks.
Stop and re-run first. If it stays stuck, check for an infinite loop or heavy launch work, and restart Swift Playgrounds if resources are low.
When you need breakpoints, deep inspection, or performance profiling. Your project is Xcode-compatible, so you can open it on a Mac anytime.