Aspects v1.4.2

Delightful, simple library for aspect oriented programming in Objective-C and Swift.
Under MIT License
By steipete

swift hooks objective-c aspects objectivec

Aspects v1.4.2

A delightful, simple library for aspect oriented programming by @steipete.

Think of Aspects as method swizzling on steroids. It allows you to add code to existing methods per class or per instance, whilst thinking of the insertion point e.g. before/instead/after. Aspects automatically deals with calling super and is easier to use than regular method swizzling.

Aspects hooks deep into the class hierarchy and creates dynamic subclasses, much like KVO. There's known issues with this approach, and to this date (February 2019) I STRICTLY DO NOT RECOMMEND TO USE Aspects IN PRODUCTION CODE. We use it for partial test mocks in, PSPDFKit, an iOS PDF framework that ships with apps like Dropbox or Evernote, it's also very useful for quickly hacking something up.

Aspects uses _objc_msgForward which causes issues with other code that uses message forwarding.

Aspects extends NSObject with the following methods:

`` objc
/// Adds a block of code before/instead/after the current
selectorfor a specific class.
/// @param block Aspects replicates the type signature of the method being hooked.
/// The first parameter will be
id, followed by all parameters of the method.
/// These parameters are optional and will be filled to match the block signature.
/// You can even use an empty block, or one that simple gets
/// @note Hooking static methods is not supported.
/// @return A token which allows to later deregister the aspect.
+ (id)aspect_hookSelector:(SEL)selector
error:(NSError **)error;

/// Adds a block of code before/instead/after the current selector for a specific instance.
- (id)aspect_hookSelector:(SEL)selector
error:(NSError **)error;

/// Deregister an aspect.
/// @return YES if deregistration is successful, otherwise NO.
id aspect = ...;
[aspect remove];

Adding aspects returns an opaque token of type AspectToken which can be used to deregister again. All calls are thread-safe.

Aspects uses Objective-C message forwarding to hook into messages. This will create some overhead. Don't add aspects to methods that are called a lot. Aspects is meant for view/controller code that is not called 1000 times per second.

Aspects calls and matches block arguments. Blocks without arguments are supported as well. The first block argument will be of type id<AspectInfo>.

When to use Aspects

Aspect-oriented programming (AOP) is used to encapsulate "cross-cutting" concerns. These are the kind of requirements that cut-across many modules in your system, and so cannot be encapsulated using normal object oriented programming. Some examples of these kinds of requirements:

If we implemented the above requirements using regular OOP there'd be some drawbacks:

Good OOP says a class should have a single responsibility, however adding on extra cross-cutting requirements means a class that is taking on other responsibilites. For example you might have a StoreClient that is supposed to be all about making purchases from an online store. Add in some cross-cutting requirements and it might also have to take on the roles of logging, security and recommendations. This is not great because:

AOP lets us modularize these cross-cutting requirements, and then cleanly identify all of the places they should be applied. As shown in the examples above cross-cutting requirements can be either technical or business focused in nature.

Here are some concrete examples:

Aspects can be used to dynamically add logging for debug builds only:

[UIViewController aspect_hookSelector:@selector(viewWillAppear:) withOptions:AspectPositionAfter usingBlock:^(id<AspectInfo> aspectInfo, BOOL animated) {
NSLog(@"View Controller %@ will appear animated: %tu", aspectInfo.instance, animated);
} error:NULL];

It can be used to greatly simplify your analytics setup:

You can check if methods are really being called in your test cases:
``` objc
- (void)testExample {
TestClass testClass = [TestClass new];
testClass2 = [TestClass new];

__block BOOL testCallCalled = NO;
[testClass aspect_hookSelector:@selector(testCall) withOptions:AspectPositionAfter usingBlock:^{
testCallCalled = YES;
} error:NULL];

[testClass2 testCallAndExecuteBlock:^{
[testClass testCall];
} error:NULL];
XCTAssertTrue(testCallCalled, @"Calling testCallAndExecuteBlock must call testCall");


It can be really useful for debugging. Here I was curious when exactly the tap gesture changed state:

[_singleTapGesture aspect_hookSelector:@selector(setState:) withOptions:AspectPositionAfter usingBlock:^(id<AspectInfo> aspectInfo) {
NSLog(@"%@: %@", aspectInfo.instance, aspectInfo.arguments);
} error:NULL];

Another convenient use case is adding handlers for classes that you don't own. I've written it for use in PSPDFKit, where we require notifications when a view controller is being dismissed modally. This includes UIKit view controllers like MFMailComposeViewController and UIImagePickerController. We could have created subclasses for each of these controllers, but this would be quite a lot of unnecessary code. Aspects gives you a simpler solution for this problem:

``` objc
@implementation UIViewController (DismissActionHook)

// Will add a dismiss action once the controller gets dismissed.
- (void)pspdf_addWillDismissAction:(void (^)(void))action {
PSPDFAssert(action != NULL);

[self aspect_hookSelector:@selector(viewWillDisappear:) withOptions:AspectPositionAfter usingBlock:^(id<AspectInfo> aspectInfo) {
if ([aspectInfo.instance isBeingDismissed]) {
} error:NULL];




Aspects identifies itself nicely in the stack trace, so it's easy to see if a method has been hooked:

Using Aspects with non-void return types

You can use the invocation object to customize the return value:

``` objc
[PSPDFDrawView aspect_hookSelector:@selector(shouldProcessTouches:withEvent:) withOptions:AspectPositionInstead usingBlock:^(id info, NSSet touches, UIEvent event) {
// Call original implementation.
BOOL processTouches;
NSInvocation *invocation = info.originalInvocation;
[invocation invoke];
[invocation getReturnValue:&processTouches];

    if (processTouches) {
processTouches = pspdf_stylusShouldProcessTouches(touches, event);
[invocation setReturnValue:&processTouches];
} error:NULL];



The simplest option is to use pod "Aspects".

You can also add the two files Aspects.h/m to your project. There are no further requirements.

Compatibility and Limitations

Aspects uses quite some runtime trickery to achieve what it does. You can mostly mix this with regular method swizzling.

An important limitation is that for class-based hooking, a method can only be hooked once within the subclass hierarchy. See #2
This does not apply for objects that are hooked. Aspects creates a dynamic subclass here and has full control.

KVO works if observers are created after your calls aspect_hookSelector: It most likely will crash the other way around. Still looking for workarounds here - any help appreciated.

Because of ugly implementation details on the ObjC runtime, methods that return unions that also contain structs might not work correctly unless this code runs on the arm64 runtime.


The idea to use _objc_msgForward and parts of the NSInvocation argument selection is from the excellent ReactiveCocoa from the GitHub guys. This article explains how it works under the hood.

Supported iOS & SDK Versions


MIT licensed, Copyright (c) 2014 Peter Steinberger, [email protected], @steipete

Release Notes

Version 1.4.2

Version 1.4.1

Version 1.4.0

Version 1.3.1

Version 1.3.0

Version 1.2.0

Version 1.1.0

Version 1.0.1

Version 1.0.0