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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
+
+