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 Model M 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.

**default layer** - is the active layer when the keyboard starts up.

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

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

Single-layer keys contain one Code object.
Multi-layer keys contain multiple Code objects, one code for each layer.

A simple multi-layer keybrd sketch
----------------------------------
The [keybrd_3a_multi-layerHold.ino](keybrd_3a_multi-layerHold/keybrd_3a_multi-layerHold.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/basic_breadboard_keyboard_front.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
{
    const 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 arrays of keys, one key for each layer.
Key_LayeredKeys objects use layerIds as array 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   (covered in next tutorial)
* Key_LayeredCodeSc

The association between Codes, Keys, and Rows
---------------------------------------------
Codes, Keys, and Rows are associated by class compositions:

```
Each Code object contains one scancode or one layercode.

Each Key contains either
* one Code object (single-layer)
* multiple Code objects (multi-layer)
* Key object (key nested in key)

Each Row contains Key objects.
```

You may have been wondering why Code pointers are in Key pointers arrays.
You don't need to know the reasons to write a sketch.
For the curious, two reasons are explained below.

1) Single-layer keys is the first reason you see Code pointers in a Key pointers array.
Rows contain keys.  The keys can be Single-layer or Multi-layer.
Wrapping a code in a single-layer key before placing it a row is tedious.
It is more convenient to place a code directly in a row.
Codes are a kind of Key (polymorphism), so that rows can contain codes and keys.

From keybrd_3a_multi-layerHold.ino:

```
Key* const ptrsKeys_0[] = { &k_00, &k_01 };
Row row_0(scanner, 0, ptrsKeys_0, keyCount_0);

Key* const ptrsKeys_1[] = { &l_fn, &s_shift };
Row row_1(scanner, 1, ptrsKeys_1, keyCount_1);
```
row0's ptrsKeys_0[] array contains pointers to Keys.
row1's ptrsKeys_1[] array contains pointers to Codes.

2) Sublayers (nested keys) is the second reason you see Code pointers in a Key pointers array.
Layered keys usually contain just codes.  When nesting keys, layered keys contain keys.
Codes are a kind of Key (polymorphism), so that layered keys can contain both codes and keys.

From keybrd_3d_sublayerNested.ino:

```
Key* const ptrsKeys_sub00[] = { &s_minus, &s_1 };
Key_LayeredKeys1 k_sub00(ptrsKeys_sub00);

Key* const ptrsKeys_00[] = { &s_a, &k_sub00 };
Key_LayeredKeys k_00(ptrsKeys_00);
```
k_00's ptrsKeys_00[] array contains pointers to code s_a and key k_sub00.

Exercises
---------
1) Compile and run [keybrd_3a_multi-layerHold.ino](keybrd_3a_multi-layerHold/keybrd_3a_multi-layerHold.ino)
and [keybrd_3b_multi-layerLock.ino](keybrd_3b_multi-layerLock/keybrd_3b_multi-layerLock.ino).
Notice how Code_LayerHold and Code_LayerLock objects behave.

2) Modify the keybrd_3a_multi-layerHold.ino sketch to make a 3-layer keyboard with two Code_LayerHold keys.

| Layout | **0** | **1** |
|:------:|:-----:|:-----:|
|  **0** | a - 1 | b = 2 |
|  **1** |  sym  |  num  |

<br>
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