skia2/include/core/SkDrawLooper.h

/*
 * Copyright 2011 The Android Open Source Project
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */


#ifndef SkDrawLooper_DEFINED
#define SkDrawLooper_DEFINED

#include "SkBlurTypes.h"
#include "SkFlattenable.h"
#include "SkPoint.h"
#include "SkColor.h"

class SkCanvas;
class SkPaint;
struct SkRect;
class SkString;

/** \class SkDrawLooper
    Subclasses of SkDrawLooper can be attached to a SkPaint. Where they are,
    and something is drawn to a canvas with that paint, the looper subclass will
    be called, allowing it to modify the canvas and/or paint for that draw call.
    More than that, via the next() method, the looper can modify the draw to be
    invoked multiple times (hence the name loop-er), allow it to perform effects
    like shadows or frame/fills, that require more than one pass.
*/
class SK_API SkDrawLooper : public SkFlattenable {
public:
    /**
     *  Holds state during a draw. Users call next() until it returns false.
     *
     *  Subclasses of SkDrawLooper should create a subclass of this object to
     *  hold state specific to their subclass.
     */
    class SK_API Context : ::SkNoncopyable {
    public:
        Context() {}
        virtual ~Context() {}

        /**
         *  Called in a loop on objects returned by SkDrawLooper::createContext().
         *  Each time true is returned, the object is drawn (possibly with a modified
         *  canvas and/or paint). When false is finally returned, drawing for the object
         *  stops.
         *
         *  On each call, the paint will be in its original state, but the
         *  canvas will be as it was following the previous call to next() or
         *  createContext().
         *
         *  The implementation must ensure that, when next() finally returns
         *  false, the canvas has been restored to the state it was
         *  initially, before createContext() was first called.
         */
        virtual bool next(SkCanvas* canvas, SkPaint* paint) = 0;
    };

    /**
     *  Called right before something is being drawn. Returns a Context
     *  whose next() method should be called until it returns false.
     *  The caller has to ensure that the storage pointer provides enough
     *  memory for the Context. The required size can be queried by calling
     *  contextSize(). It is also the caller's responsibility to destroy the
     *  object after use.
     */
    virtual Context* createContext(SkCanvas*, void* storage) const = 0;

    /**
      *  Returns the number of bytes needed to store subclasses of Context (belonging to the
      *  corresponding SkDrawLooper subclass).
      */
    virtual size_t contextSize() const = 0;


    /**
     * The fast bounds functions are used to enable the paint to be culled early
     * in the drawing pipeline. If a subclass can support this feature it must
     * return true for the canComputeFastBounds() function.  If that function
     * returns false then computeFastBounds behavior is undefined otherwise it
     * is expected to have the following behavior. Given the parent paint and
     * the parent's bounding rect the subclass must fill in and return the
     * storage rect, where the storage rect is with the union of the src rect
     * and the looper's bounding rect.
     */
    bool canComputeFastBounds(const SkPaint& paint) const;
    void computeFastBounds(const SkPaint& paint, const SkRect& src, SkRect* dst) const;

    struct BlurShadowRec {
        SkScalar        fSigma;
        SkVector        fOffset;
        SkColor         fColor;
        SkBlurStyle     fStyle;
        SkBlurQuality   fQuality;
    };
    /**
     *  If this looper can be interpreted as having two layers, such that
     *      1. The first layer (bottom most) just has a blur and translate
     *      2. The second layer has no modifications to either paint or canvas
     *      3. No other layers.
     *  then return true, and if not null, fill out the BlurShadowRec).
     *
     *  If any of the above are not met, return false and ignore the BlurShadowRec parameter.
     */
    virtual bool asABlurShadow(BlurShadowRec*) const;

    SK_TO_STRING_PUREVIRT()
    SK_DEFINE_FLATTENABLE_TYPE(SkDrawLooper)

protected:
    SkDrawLooper() {}

private:
    typedef SkFlattenable INHERITED;
};

#endif