Shift

There are three types of Shifts: After, Before, and By.

After

Mixin injections happen the line before a method is called. So, what do we do if we want to inject after a method call?

Say we had the following method:

public void foo() {
    doSomething1();
    doSomething2();
    doSomething4();
}

Let's say we wanted to call doSomething3() after doSomething2. This could be done using a Shift.AFTER as such:

@Inject(method = "foo", at = @At(value = "INVOKE", target = "doSomething2()V", at = At.Shift.AFTER))
private void doSomethingAfter() {
    doSomething3();
}

This gives us the following code:

public void foo() {
    doSomething1();
    doSomething2();
    doSomething3();
    doSomething4();
}

Before

Danger

Shifting before is not recommended.

You can shift backwards by an instruction by using shift = At.Shift.BEFORE. However, since mixin injections already inject before the method call, shifting like this is unecessary and britle in most cases.

By

Danger

Shifting by is not recommended.

You can specify a certain number of instructions to shift by by using shift = At.Shift.BY = i, where i is a number that you choose to shift by. This method of shifting is brittle and should be avoided.

Practical Example

Shifts have many practical use cases. Say we had the following code:

public void render(ResourceLocation guiTexture, ResourceLocation otherTexture, int x, int y) {
    int a = x / 2;
    int b = y / 2;
    drawGuiTexture(guiTexture, a, b);
    int c = a + 10;
    int d = b + 10;
    drawOtherTexture(otherTexture, c, d);
}

And we want to surround only the drawGuiTexture call in calls to RenderSystem.setShaderColor to overlay a color or apply opacity. However, we want to reset the shader color before rendering anything else. To do this, we can surround the drawGuiTexture call with our first regular Inject, and a second shifted Inject as so:

@Inject(method = "render", at = @At(value = "INVOKE", target = "drawTexture(Lnet/minecraft/resources/ResourceLocation)V"))
private void drawGuiTexturePre() {
    RenderSystem.setShaderColor(1f, 1f, 1f, 0.5f); // set an alpha of 0.5
}

@Inject(method = "render", at = @At(value = "INVOKE", target = "drawTexture(Lnet/minecraft/resources/ResourceLocation)V", at = At.Shift.AFTER))
private void drawGuiTexturePost() {
        RenderSystem.setShaderColor(1f, 1f, 1f, 1f); // reset alpha after drawing the texture
}

This would result in the following code:

public void render(ResourceLocation guiTexture, ResourceLocation otherTexture, int x, int y) {
    int a = x / 2;
    int b = y / 2;
    RenderSystem.setShaderColor(1f, 1f, 1f, 0.5f);
    drawGuiTexture(guiTexture);
    RenderSystem.setShaderColor(1f, 1f, 1f, 1f);
    int c = a + 10;
    int d = b + 10;
    drawOtherTexture(otherTexture);
}