package tomo

import "image"

// ParentHooks is a struct that contains callbacks that let child elements send
// information to their parent element without the child element knowing
// anything about the parent element or containing any reference to it. When a
// parent element adopts a child element, it must set these callbacks. They are
// allowed to be nil.
type ParentHooks struct {
	// Draw is called when a part of the child element's surface is updated.
	// The updated region will be passed to the callback as a sub-image.
	Draw func (region Canvas)

	// MinimumSizeChange is called when the child element's minimum width
	// and/or height changes. When this function is called, the element will
	// have already been resized and there is no need to send it a resize
	// event.
	MinimumSizeChange func (width, height int)

	// FlexibleHeightChange is called when the parameters affecting the
	// element's felxible height have changed.
	FlexibleHeightChange func ()

	// ContentBoundsChange is called by scrollable elements when the
	// element's content bounds have changed. When this function is called,
	// the element will have already done any drawing necessary. This is
	// only intended for updating things like scrollbar positions.
	ContentBoundsChange func ()
	
	// SelectionRequest is called when the child element element wants
	// itself to be selected. If the parent element chooses to grant the
	// request, it must send the child element a selection event and return
	// true.
	SelectionRequest func () (granted bool)

	// SelectionMotionRequest is called when the child element wants the
	// parent element to select the previous/next element in relation to it.
	SelectionMotionRequest func (direction SelectionDirection) (granted bool)
}

// RunDraw runs the Draw hook if it is not nil. If it is nil, it does nothing.
func (hooks ParentHooks) RunDraw (region Canvas) {
	if hooks.Draw != nil {
		hooks.Draw(region)
	}
}

// RunMinimumSizeChange runs the MinimumSizeChange hook if it is not nil. If it
// is nil, it does nothing.
func (hooks ParentHooks) RunMinimumSizeChange (width, height int) {
	if hooks.MinimumSizeChange != nil {
		hooks.MinimumSizeChange(width, height)
	}
}

// RunFlexibleHeightChange runs the ExpandingHeightChange hook if it is not
// nil. If it is nil, it does nothing.
func (hooks ParentHooks) RunFlexibleHeightChange () {
	if hooks.FlexibleHeightChange != nil {
		hooks.FlexibleHeightChange()
	}
}

// RunSelectionRequest runs the SelectionRequest hook if it is not nil. If it is
// nil, it does nothing.
func (hooks ParentHooks) RunSelectionRequest () (granted bool) {
	if hooks.SelectionRequest != nil {
		granted = hooks.SelectionRequest()
	}
	return
}

// RunSelectionMotionRequest runs the SelectionMotionRequest hook if it is not
// nil. If it is nil, it does nothing.
func (hooks ParentHooks) RunSelectionMotionRequest (
	direction SelectionDirection,
) (
	granted bool,
) {
	if hooks.SelectionMotionRequest != nil {
		granted = hooks.SelectionMotionRequest(direction)
	}
	return
}

// Element represents a basic on-screen object.
type Element interface {
	// Element must implement the Canvas interface. Elements should start
	// out with a completely blank buffer, and only allocate memory and draw
	// on it for the first time when sent an EventResize event.
	Canvas

	// MinimumSize specifies the minimum amount of pixels this element's
	// width and height may be set to. If the element is given a resize
	// event with dimensions smaller than this, it will use its minimum
	// instead of the offending dimension(s).
	MinimumSize () (width, height int)

	// Resize resizes the element. This should only be called by the
	// element's parent.
	Resize (width, height int)

	// SetParentHooks gives the element callbacks that let it send
	// information to its parent element without it knowing anything about
	// the parent element or containing any reference to it. When a parent
	// element adopts a child element, it must set these callbacks.
	SetParentHooks (callbacks ParentHooks)
}

// SelectionDirection represents a keyboard navigation direction.
type SelectionDirection int

const (
	SelectionDirectionNeutral  SelectionDirection =  0
	SelectionDirectionBackward SelectionDirection = -1
	SelectionDirectionForward  SelectionDirection =  1
)

// Canon returns a well-formed direction.
func (direction SelectionDirection) Canon () (canon SelectionDirection) {
	if direction > 0 {
		return SelectionDirectionForward
	} else if direction == 0 {
		return SelectionDirectionNeutral
	} else {
		return SelectionDirectionBackward
	}
}

// Selectable represents an element that has keyboard navigation support. This
// includes inputs, buttons, sliders, etc. as well as any elements that have
// children (so keyboard navigation events can be propagated downward).
type Selectable interface {
	Element

	// Selected returns whether or not this element is currently selected.
	Selected () (selected bool)

	// Select selects this element, if its parent element grants the
	// request.
	Select ()

	// HandleSelection causes this element to mark itself as selected, if it
	// can currently be. Otherwise, it will return false and do nothing.
	HandleSelection (direction SelectionDirection) (accepted bool)

	// HandleDeselection causes this element to mark itself and all of its
	// children as deselected.
	HandleDeselection ()
}

// KeyboardTarget represents an element that can receive keyboard input.
type KeyboardTarget interface {
	Element

	// HandleKeyDown is called when a key is pressed down while this element
	// has keyboard focus. It is important to note that not every key down
	// event is guaranteed to be paired with exactly one key up event. This
	// is the reason a list of modifier keys held down at the time of the
	// key press is given.
	HandleKeyDown (key Key, modifiers Modifiers, repeated bool)

	// HandleKeyUp is called when a key is released while this element has
	// keyboard focus.
	HandleKeyUp (key Key, modifiers Modifiers)
}

// MouseTarget represents an element that can receive mouse events.
type MouseTarget interface {
	Element

	// Each of these handler methods is passed the position of the mouse
	// cursor at the time of the event as x, y.

	// HandleMouseDown is called when a mouse button is pressed down on this
	// element.
	HandleMouseDown (x, y int, button Button)

	// HandleMouseUp is called when a mouse button is released that was
	// originally pressed down on this element.
	HandleMouseUp (x, y int, button Button)

	// HandleMouseMove is called when the mouse is moved over this element,
	// or the mouse is moving while being held down and originally pressed
	// down on this element.
	HandleMouseMove (x, y int)

	// HandleScroll is called when the mouse is scrolled. The X and Y
	// direction of the scroll event are passed as deltaX and deltaY.
	HandleScroll (x, y int, deltaX, deltaY float64)
}

// Flexible represents an element who's preferred minimum height can change in
// response to its width.
type Flexible interface {
	Element

	// HeightForWidth returns what the element's minimum height would be if
	// resized to a specified width. This does not actually alter the state
	// of the element in any way, but it may perform significant work, so it
	// should be called sparingly.
	//
	// It is reccomended that parent containers check for this interface and
	// take this method's value into account in order to support things like
	// flow layouts and text wrapping, but it is not absolutely necessary.
	// The element's MinimumSize method will still return the absolute
	// minimum size that the element may be resized to.
	//
	// It is important to note that if a parent container checks for
	// flexible chilren, it itself will likely need to be flexible.
	MinimumHeightFor (width int) (height int)
}

// Scrollable represents an element that can be scrolled. It acts as a viewport
// through which its contents can be observed.
type Scrollable interface {
	Element

	// ScrollContentBounds returns the full content size of the element.
	ScrollContentBounds () (bounds image.Rectangle)

	// ScrollViewportBounds returns the size and position of the element's
	// viewport relative to ScrollBounds.
	ScrollViewportBounds () (bounds image.Rectangle)

	// ScrollTo scrolls the viewport to the specified point relative to
	// ScrollBounds.
	ScrollTo (position image.Point)

	// ScrollAxes returns the supported axes for scrolling.
	ScrollAxes () (horizontal, vertical bool)
}