Tutorial 3a - multi-layer keyboard
==================================
When you finish this tutorial you will be able to be able to modify a multi-layer keybrd sketch to write your very own multi-layer keyboard firmware.

Multi-layer nomenclature
------------------------
**[layers](http://deskthority.net/wiki/Layer)** - are key bindings provided by the keyboard firmware.  For example,
* The classic [IBM PC keyboard](http://en.wikipedia.org/wiki/IBM_PC_keyboard) has one layer.
* Many compact keyboards have an additional [Fn layer](http://en.wikipedia.org/wiki/Fn_key).
* The [Neo layout](http://neo-layout.org/index_en.html) has 6 layers.

**layer id** - is an integer used to identify a layer.

**active layer** - is the layer currently used by the keyboard.

**layer scheme** - is a system for changing the active layer while typing (a single-layer scheme does not change layers).

A simple multi-layer keybrd sketch
----------------------------------
The [keybrd_3a_multi-layer.ino](keybrd_3a_multi-layer/keybrd_3a_multi-layer.ino) sketch is for a simple two-layer keyboard.
It will run on the basic breadboard keyboard described in [tutorial_1_breadboard_keyboard.md](tutorial_1_breadboard_keyboard.md).

![basic breadboard keyboard](keybrd_1_breadboard/breadboard_keyboard_2x2.JPG "basic breadboard keyboard")

The sketch annotations explain how multi-layer keyboards work.
The sketch uses three layer-scheme classes:
* LayerState
* Code_LayerHold
* Key_LayeredKeys

The internal workings of these three classes are revealed in the next section.

Pseudo code for simple layer scheme
-----------------------------------
The following pseudo code is of three keybrd library classes.
It has just enough detail to show the internal workings of layer schemes.

**Code_Layer** objects change the active layer.
When a Code_Layer object is pressed, it tells LayerState to update the active layer.
```
class Code_Layer
{
    int layerId;
    LayerState& refLayerState;
    press() { refLayerState.setActiveLayer(layerId); }
};
```

**LayerState** objects keep track of the activeLayer.
A LayerState's activeLayer is always up to date.
```
class LayerState
{
    int activeLayer;
    setActiveLayer(int layerId) { activeLayer = layerId; }
    getActiveLayer() { return activeLayer; }
};
```

**Key_LayeredKeys** objects contain an array of keys, one key for each layer.
Key_LayeredKeys objects use layerIds as Key_LayeredKeys indexes.
When a Key_LayeredKeys object is pressed, it gets the active layerId from LayerState, and sends the corresponding key.
```
class Key_LayeredKeys
{
    Key** ptrsKeys;         //array of Key pointers, one Key pointer per layer
    LayerState& refLayerState;
    press() { layerId = refLayerState.getActiveLayer();
              ptrsKeys[layerId]->press(); }
};
```

Dependency diagram
```
       +------------+
       | Code_Layer |
       +------------+
             |
             |setActiveLayer()
             |
             v
       +------------+
       | LayerState |
       +------------+
             ^
             |
             |getActiveLayer()
             |
     +-----------------+
     | Key_LayeredKeys |
     +-----------------+
```
Layer-scheme classes
--------------------
There are several layer scheme-classes to choose from.
You can view all the class definitions in the [keybrd library](../src/).

Code_Layer classes include:
* Code_LayerHold
* Code_LayerLock

A basic LayerState class is:
* LayerState

Key_Layered classes include:
* Key_LayeredKeys
* Key_LayeredScSc
* Key_LayeredCodeSc

The basic LayerState provided by the keybrd library is sufficient for implementing ordinary layer schemes.
For experimental layer schemes, you would need to create a custom LayerState class, and possibly custom Code_Layer and Key_Layered classes as well.

Single-layer Codes
------------------
Most Code objects only have one scancode or code.
Example single-layer Code classes include:
* Code_Sc
* Code_ScS
* Code_ScNS
* Code_Shift
* Code_LayerHold
* Code_LayerLock

Exercises
---------
1) Modify the keybrd_3a_multi-layer.ino sketch to use two Code_LayerLock objects.

| Layout | **0**  | **1**  |
|:------:|:------:|:------:|
|  **0** | a   1  | b   2  |
|  **1** | layer0 | layer1 |

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