74 lines
2.7 KiB
Go
74 lines
2.7 KiB
Go
package pixelgl
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// Doer is an interface for manipulating OpenGL state.
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//
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// OpenGL is a state machine. Every object can 'enter' it's state and 'leave' it's state. For example,
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// you can bind a buffer and unbind a buffer, bind a texture and unbind it, use shader and unuse it, and so on.
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//
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// This interface provides a clever and flexible way to do it. A typical workflow of an OpenGL object is that
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// you enter (load, bind) that object's state, then do something with it, and then leave the state. That 'something'
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// in between, let's call it sub (as in subroutine).
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//
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// The recommended way to implement a Doer is to wrap another Doer (vertex array wraps texture and so on), let's call
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// it parent. Then the Do method will look like this:
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//
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// func (o *MyObject) Do(sub func(Context)) {
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// o.parent.Do(func(ctx Context) {
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// // enter the object's state
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// sub(ctx)
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// // leave the object's state
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// })
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// }
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//
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// It might seem difficult to grasp this kind of recursion at first, but it's really simple. What it's basically saying
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// is: "Hey parent, enter your state, then let me enter mine, then I'll do whatever I'm supposed to do in the middle.
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// After that I'll leave my state and please leave your state too parent."
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//
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// Also notice, that the functions are passing a Context around. This context contains the most important state variables.
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// Usually, you just pass it as you received it. If you want to pass a changed context to your child (e.g. your a shader),
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// use ctx.With* methods.
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//
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// If possible and makes sense, Do method should be reentrant.
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type Doer interface {
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Do(sub func(Context))
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}
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// Context takes state from one object to another. OpenGL is a state machine, so we have to approach it like that.
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// However, global variables are evil, so we have Context, that transfers important OpenGL state from one object to another.
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//
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// This type does *not* represent an OpenGL context in the OpenGL terminology.
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type Context struct {
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shader *Shader
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}
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// Shader returns the current shader.
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func (c Context) Shader() *Shader {
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return c.shader
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}
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// WithShader returns a copy of this context with the specified shader.
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func (c Context) WithShader(s *Shader) Context {
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return Context{
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shader: s,
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}
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}
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// ContextHolder is a root Doer with no parent. It simply forwards a context to a child.
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type ContextHolder struct {
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Context Context
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}
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// Do calls sub and passes it the held context.
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func (ch *ContextHolder) Do(sub func(ctx Context)) {
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sub(ch.Context)
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}
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type noOpDoer struct{}
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func (noOpDoer) Do(sub func(ctx Context)) {
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sub(Context{})
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}
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// NoOpDoer is a Doer that just passes an empty context to the caller of Do.
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var NoOpDoer Doer = noOpDoer{}
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