Add documentation on how to add barcode support (#198)

src/BinaryKits.Zpl.Viewer/README.md

@@ -21,4 +21,85 @@ foreach (var labelInfo in analyzeInfo.LabelInfos)
 	var imageData = drawer.Draw(labelInfo.ZplElements);
 	//imageData is bytes of png image
 }
-```
+```
+
+# Adding Barcode support
+Also applies to other "drawables", like shapes and graphics.
+
+## Common barcodes still missing support
+### 1D
+- [ ] Code 93: Supported by Xzing.net
+- [ ] MSI: Supported by Xzing.net
+- [ ] Codabar/NW7: Supported by Xzing.net
+- [ ] Databar/RSS-14: Not supported by Xzing.net
+- [ ] [Code 11](https://web.archive.org/web/20070202060711/http://www.barcodeisland.com/code11.phtml): Not supported by Xzing.net, simple to implement
+
+### 2D
+- [ ] Maxicode: Not supported by Xzing.net, required for UPS labels
+- [ ] Aztec: Supported by Xzing.net
+- [ ] Micro pdf417: Not supported by Xzing.net
+	- ascii data only
+	- appears to be a subset of pdf417
+- [ ] Micro QrCode: Not supported by Xzing.net
+	- appears to be a subset of QrCode
+- [ ] Composite: Not supported by Xzing.net, simple to implement
+
+## Examples
+- [EAN Barcode](https://github.com/BinaryKits/BinaryKits.Zpl/commit/3fac409732e19be9e047ee71f942ba1f68c6fa5c)
+- [Datamatrix Barcode](https://github.com/BinaryKits/BinaryKits.Zpl/commit/f79d01512eee7e3d16246e932877ca6d4aa4e306)
+- [PDF417 Barcode](https://github.com/BinaryKits/BinaryKits.Zpl/pull/190/files)
+
+## Steps
+Generally, looking at files in the directory for each step should give you a clue on how to implement a file.
+
+### 1: Create the abstract element
+This is a class used to represent the barcode with all it's settings, mostly to be used for zpl generation.
+We are currently in the middle of a (stalled) refactor which leads to some amount of code duplication.
+A class should be created in these `<project>/<directory>`:
+- `BinaryKits.Zpl.Label/Elements/`
+- `BinaryKits.Zpl.Protocol/Commands/`
+
+For 2d barcodes, implement `ZplPositionedElementBase` and `IFormatElement`, for 1d barcodes, subclass `ZplBarcode`
+
+### 2: Create a model in `BinaryKits.Zpl.Viewer/Models/`
+This one is only used for the viewer project and should contain mostly the same properties as the class created in the previous step.
+
+### 3: Create the command analyzer in `BinaryKits.Zpl.Viewer/CommandAnalyzers/`
+This is where you write the code for parsing zpl to the Model/Element.
+
+- Subclass from `ZplCommandAnalyzerBase`
+- Use `this.SplitCommand` get all the zpl arguments
+- Remember to check that the length of args array is long enough before trying to parse at an index
+- Remember to set the defaults each parameter before parsing
+	- There are parameters stored in the virtual printer such as `^BY`
+	- Otherwise use the defaults as described in the zpl programming manual
+- Because there is always data to encode, set the next field data encountered by the virtual printer to return the content to the model
+
+### 4: Add an entry to `BinaryKits.Zpl.Viewer/CommandAnalyzers/FieldDataZplCommandAnalyzer.cs`
+This is to make sure that we grab the `text`, the model from the previous step it into a new instance of the abstract element.
+
+Find the massive `if` chain following `if (this.VirtualPrinter.NextElementFieldData != null)` and add an entry for your mode.
+
+Architecturally, this type of hard binding is probably not good. We should probably fix this.
+
+### 5: Add an entry to `BinaryKits.Zpl.Viewer/ZPLAnalyzer.cs`
+Find the List `elementAnalyzers` and instantiate your analyzer there.
+
+### 6: Create a drawer in `BinaryKits.Zpl.Viewer/ElementDrawers`
+- Subclass `BarcodeDrawerBase`
+- Implement `CanDraw` by checking against the abstract element in step 1.
+- Implement `Draw`. Working from the end:
+	- The end goal is to write some pixels (a skia bitmap) to the base canvas with `this.DrawBarcode`
+	- Those pixels will be obtained by converting an abstract barcode drawing to a png `SKBitmap`
+	- The abstract drawing will be generated by calling an external library with the correct parameters obtained from the abstract element
+		- Zxing.net uses hints to pass in extra parameters
+	- Some adaptative helper functions may needed to convert between the library types and skia bitmaps or binarykit.zpl models
+	- `BarcodeDrawerBase` may have some helper functions needed
+
+### 7: Add an entry to `BinaryKits.Zpl.Viewer/ZPLElementDrawer.cs`
+Find the array `this._elementDrawers` and instantiate your drawer there.
+
+### 8: Create a test in to `BinaryKits.Zpl.Viewer.UnitTest/DrawerTest.cs`
+- todo: load data from BinaryKits.Zpl.Viewer.UnitTest/Data/zpl like in the webapi project
+- Use the `DefaultPrint` function when possible, you can pass dimensions, density and drawer options
+- todo: an empty custom file `BinaryKits.Zpl.Viewer.UnitTest/Data/zpl/custom.zpl2` is not under vcs to enable quick testing