Tweaks is an easy way to fine-tune an iOS app.

The best way to improve an app is to use it every day. Even when ideas can be tested out in advance — for example, with Origami — it can still take some time with the app to see how it works in practice.

Occasionally, it's perfect the first try. Sometimes, the idea doesn't work at all. But often, it just needs a few minor adjustments. That last case is where Tweaks fits in. Tweaks makes those small adjustments easy: with no code changes and no computer, you can try out different options and decide which works best.

Some of the most useful parameters to adjust are animation timings, velocity thresholds, colors, and physics constants. At Facebook, we also use tweaks to temporarily disable new features during development. That way, the designers and engineers involved can enable it on just their devices, without getting in the way of others testing the app.

Tweaks was invaluable for building Paper. We hope it can be useful for your app too.

Each configurable value is called a tweak. There's a few ways to set them up, found in FBTweakInline.h.

The simplest way to create a tweak is to replace a constant with FBTweakValue:

CGFloat animationDuration = FBTweakValue(@"Category", @"Group", @"Duration", 0.5);

The first three parameters are where the tweak is listed and what it's called, and the last one is the default value. You can pass in many types of constants for the default:

if (FBTweakValue(@"Category", @"Feature", @"Enabled", YES)) {
  label.text = FBTweakValue(@"Category", @"Group", @"Text", @"Tweaks example.");
}

In release builds, the FBTweakValue macro expands to just the default value, so there's no performance impact. In debug builds, though, it fetches the latest value of the tweak.

For numeric tweaks (NSInteger, CGFloat, and others), you can pass an extra two parameters which are used as the minimum and maximum value for the tweak:

self.red = FBTweakValue(@"Header", @"Colors", @"Red", 0.5, 0.0, 1.0);

To make tweaks update live, you can use FBTweakBind:

FBTweakBind(self.headerView, alpha, @"Main Screen", @"Header", @"Alpha", 0.85);

The first parameter is the object to bind to, and the second is the property. Whenever the tweak is changed, self.headerView's alpha property is updated to match. A few more examples:

FBTweakBind(audioPlayer, volume, @"Player", @"Audio", @"Volume", 0.9);
FBTweakBind(webView.scrollView, scrollEnabled, @"Browser", @"Scrolling", @"Enabled", YES);

As with FBTweakValue, in release builds FBTweakBind expands to just setting the property to the default value.

To configure your tweaks, you need a way to show the configuration UI. There's two options for that:

• Traditionally, tweaks is activated by shaking your phone. To use that, just replace your root UIWindow with a FBTweakShakeWindow.
• You can present a FBTweakViewController from anywhere in your app. Be sure to restrict the activation UI to debug builds!

You can also access the objects that make up the macros mentioned above. That can be useful for more complex scenarios, like adjusting members of a C structure.

For example, to manually create a tweak:

FBTweak *tweak = [[FBTweak alloc] initWithIdentifier:@"com.tweaks.example.advanced"];
tweak.name = @"Advanced Settings";
tweak.defaultValue = @NO;

FBTweakStore *store = [FBTweakStore sharedInstance];
FBTweakCategory *category = [store tweakCategoryWithName:@"Settings"];
FBTweakCollection *collection = [category tweakCollectionWithName:@"Enable"];
[collection addTweak:tweak];

[tweak addObserver:self];

Then, you can watch for when the tweak changes:

- (void)tweakDidChange:(FBTweak *)tweak
{
  self.advancedSettingsEnabled = ![tweak.currentValue boolValue];
}

To override when tweaks are enabled, you can define the FB_TWEAK_ENABLED macro. It's suggested to avoid including them when submitting to the App Store.

In debug builds, the tweak macros use __attribute__((section)) to statically store data about each tweak in the __FBTweak section of the mach-o. Tweaks loads that data at startup and loads the latest values from NSUserDefaults.

In release builds, the macros just expand to the default value. Nothing extra is included in the binary.

There are two options:

1. Tweaks is available as Tweaks in Cocoapods.
2. Manually add the files from FBTweak/ into your Xcode project. Slightly simpler, but updates are also manual.

Tweaks requires iOS 6 or later.

There's also a demo project available. To use it, make sure to open FBTweakExample.xcworkspace (rather than the .xcodeproj) so the dependencies build correctly.

See the CONTRIBUTING file for how to help out.

Tweaks is BSD-licensed. We also provide an additional patent grant.