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# Layout Specifications
## overview
This document is the specification of layout system.
The layout system imitates WPF and Android layout system.
## rules
### about `width` and `height` in `LayoutParams`
There is three mode in measure: `Content`, `Exactly`, `Stretch`.
- `Exactly` means the control should be of an exact size.
- `Content` means the control has the size that contains the children.
- `Stretch` means the control stretch to the max size that it can fill. If parent is `Content`, it occupies all available room parent provides, which means its parent will stretch as well.
### about `max_size`, `min_size`
`max_size` specifies the max size and `min_size` specifies the min size. They are of higher priority of `width` and `height`. Calculated size should be adjusted according to the four properties.
## structure
### enum `MeasureMode`
``` c++
enum class MeasureMode
{
Content,
Stretch,
Exactly
};
```
### struct `MeasureLength`
``` c++
struct MeaureLength
{
float length;
MeasureMode mode;
};
```
### struct `MeaureSize`
``` c++
struct MeasureSize
{
MeasureLength width;
MeasureLength height;
};
```
### struct `OptionalSize`
``` c++
struct OptionalSize
{
optional<float> width;
optional<float> height;
}
```
### struct `BasicLayoutParams`
``` c++
struct BasicLayoutParams
{
MeasureSize size;
OptionalSize max_size;
OptionalSize min_size;
}
```
### interface `ILayoutable`
``` c++
struct ILayoutable : virtual Interface
{
virtual void Measure(const Size&) = 0;
virtual void Layout(const Rect&) = 0;
virtual BasicLayoutParams* GetLayoutParams() = 0;
virtual void SetLayoutParams(BasicLayoutParams* params) = 0;
virtual Size GetDesiredSize() = 0;
virtual void SetDesiredSize(const Size& size) = 0;
/*
protected:
virtual Size OnMeasure(const Size& size);
virtual void OnLayout(const Rect& rect);
*/
};
```
## process
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