ModifyArg

ModifyArg provides a simple way to change a single argument of a method call.

Say we had the following code:

public int foo(float f) {
    String a = f.toString();
    int b = f / 4;
    doSomething(a, b, f);
    return b + 5;
}

Let's say we wanted to change b to be b / 2 instead only in the doSomething call. We can ModifyArg the call using the correct index:

@ModifyArg(method = "foo", at = @At(value = "INVOKE", target = "doSomething(Ljava/lang/String;IF)V"), index = 1) // gets the 1st (0-indexed) variable
private int changeMethodParam(int b) {
    return b / 2;
}

This results in:

public void foo(float f) {
    String a = f.toString();
    int b = f / 4;
    doSomething(a, b / 2, f);
    return b + 5
}

However, in this case, specifying an index is actually not needed. Mixin is able to tell that there is only one int argument in the method call, so we can (and should) omit the index here! Only use an index if there is more than one of the type.