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====== Mixin Injects ======

===== Introduction =====
Injects allows you to place custom code at a specified position inside an existing method. For a working example, view the [[tutorial:mixin_injects#practical_example|Practical Example]] category at the bottom of this page. The standard form of an inject is as shown:

<code java>
@Inject(method = "METHOD NAME OR SIGNATURE", at = @At("INJECTION POINT REFERENCE"))
private void injectMethod(METHOD ARGS, CallbackInfo info) {

}
</code>

The [[https://github.com/SpongePowered/Mixin/wiki/Injection-Point-Reference|Injection Point Reference]] defines where the code inside the method body is injected inside the target method. The following table describes a few of the options:

^ Name ^ Description ^
| HEAD | Top of the method |
| RETURN | Before every return statement |
| INVOKE | At a method call |
| TAIL | Before the final return statement |

In the case of injection points that reference statements or members, the target value can be set inside //@At//. Target value is specified using JVM bytecode descriptors.

Oracle defines the following [[https://docs.oracle.com/javase/specs/jvms/se14/html/jvms-4.html#jvms-4.3.2|field descriptors]]:

^ Descriptor ^ Primitive ^ Description ^
| B | byte | signed byte |
| C | char | Unicode character code point in the Basic Multilingual Plane, encoded with UTF-16 |
| D | double | double-precision floating-point value |
| F | float | single-precision floating-point value |
| I | int | signed integer |
| J | long | signed long integer |
| L//ClassName//; | reference | an instance of //ClassName// |
| S | short | signed short |
| Z | boolean | ''true'' or ''false'' |
| [ | reference | one array dimension |

A method descriptor is comprised of the method name, followed by a set of parentheses containing the parameter types, followed by the return type. A method defined in Java as ''Object m(int i, double[] d, Thread t)'' would have the method descriptor ''m(I[DLjava/lang/Thread;)Ljava/lang/Object;''.

In the case that the return type is void, you need to use V (Void Descriptor Type) as the type (for example, ''void foo(String bar)'' would become ''foo(Ljava/lang/String;)V'').

Generics' types are left out, as generics don't exist on runtime. So ''Pair<Integer, ? extends Task<? super VillagerEntity>‍>'' would become ''Lcom/mojang/datafixers/util/Pair''.

''@Inject'' methods always have a void return type. The method name does not matter and neither does the access modifier; using something that describes what the inject does is best. The target method's arguments are placed first in the method's header, followed by a ''CallbackInfo'' object. If the target method has a return type (T), ''CallbackInfoReturnable<T>'' is used instead of ''CallbackInfo''.

=== Returning & Cancelling from Inject ===
To cancel or return early inside a method, use ''CallbackInfo#cancel'' or ''CallbackInfoReturnable<T>#setReturnValue(T)''. Note that ''cancel'' does not have to be called after ''setReturnValue''. In both instances, ''cancellable'' will have to be set to true in the inject annotation:
<code java>
@Inject(method = "...", at = @At("..."), cancellable = true)
</code>


=== Injecting into Constructors ===
To inject into a constructor, use ''<init>()V'' as the method target, with ''()'' containing the constructor argument descriptors. When injecting into constructors, ''@At'' must be set to either ''TAIL'' or ''RETURN''. No other forms of injection are officially supported. Note that some classes have methods named ''init'' which are different from ''<init>''. Don't get confused!

To inject into a static constructor, use ''<clinit>'' as the method name.

===== Practical Example =====
The following example injects a print statement at the top of ''TitleScreen#init'' (note: the method ''init'' is a normal method and not a constructor).

<code java [enable_line_numbers="false"]>
@Mixin(TitleScreen.class)
public class ExampleMixin {
	@Inject(at = @At("HEAD"), method = "init()V")
	private void init(CallbackInfo info) {
		System.out.println("This line is printed by an example mod mixin!");
	}
}
</code>

For more information on this particular example, view its usage in the [[https://github.com/FabricMC/fabric-example-mod/blob/master/src/main/java/net/fabricmc/example/mixin/ExampleMixin.java|Fabric Example Mod repo]].