Cocos2d-x屏幕适配

资源分辨率，设计分辨率，屏幕分辨率

• Resources width 以下简写为RW，Resources height 以下简写为RH
• Design width 以下简写为DW，Design height 以下简写为DH
• Screen width 以下简写为SW，Screen height 以下简写为SH

接口setContentScaleFactor()和setSearchPaths()控制着第一个转换过程。
而setDesignResolutionSize()控制第二个过程。两个过程结合在一起，影响最终的显示效果。

从资源分辨率到设计分辨率

setSearchPaths()需要根据当前屏幕分辨率做恰当的设置。
setContentScaleFactor()决定了图片显示到屏幕的缩放因子，但是这个接口的参数不是通过资源图片的宽、高比屏幕宽、高得来。Cocos2d-x引擎设计试图屏蔽游戏开发者直接去关注屏幕，所以这个因子是资源宽、高比设计分辨率宽、高。
setContentScaleFactor()通常有两个方式来设置参数。 RH/DH或RW/DW，不同的因子选择有不同的缩放负作用。 先看一张图:

用高度比作为内容缩放因子，保证了背景资源的垂直方向在设计分辨率范围内的全部显示。
用宽度比作为内容缩放因子，保证了背景资源的水平方向在设计分辨率范围内的全部显示。

从设计分辨率到屏幕分辨率

首先，我们通过一张图了解下Cocos2d-X的几种屏幕适配规则：

1. EXACT_FIT将设计尺寸映射到屏幕尺寸，长宽按各自比例拉伸，可能造成图像拉伸变形。
2. SHOW_ALL和NO_BORDER都是长宽等比例拉伸，图像不会被拉伸变形；但可能造成黑边和资源显示不全的问题。
3. FIXED_HEIGHT和FIXED_WIDTH和之前的三种策略不同，EXACT_FIT、SHOW_ALL和NO_BORDER都不会修正设计尺寸，而FIXED_HEIGHT和FIXED_WIDTH在设计阶段就修正设计尺寸，让可视区域就是设计尺寸区域。
4. NO_BORDER是唯一可视区域小于最终设计区域（FIXED_HEIGHT和FIXED_WIDTH会修正传入的设计尺寸）的一种策略。其他几种策略的设计区域等于设计区域。
5. NO_BORDER、FIXED_HEIGHT和FIXED_WIDTH三种策略根据设计或可视区域的改变需要做相应的坐标适配。

FIXED_HEIGHT:
1. 如果屏幕尺寸长高比>资源尺寸长高比，进行坐标偏差纠正，让资源居中显示，左右两边留黑边。
2. 如果屏幕尺寸长高比<资源尺寸长高比，进行坐标偏差纠正，让资源居中显示，左右两边被裁剪无法显示。

所以，屏幕尺寸长高比略小于资源尺寸长高比，显示效果最佳。

源码面前无秘密

class CC_DLL GLView : public Ref
{
 /** * Get the frame size of EGL view. * In general,it returns the screen size since the EGL view is a fullscreen view. * * @return The frame size of EGL view. */
    const Size& GLView::getFrameSize() const
    {
        return _screenSize;
    }

    /** * Set the frame size of EGL view. * * @param width The width of the fram size. * @param height The height of the fram size. */
    void GLView::setFrameSize(float width,float height)
    {
        _screenSize = Size(width,height);
    }
      /** * Get the visible area size of opengl viewport. * * @return The visible area size of opengl viewport. */
    Size GLView::getVisibleSize() const
    {
        if (_resolutionPolicy == ResolutionPolicy::NO_BORDER)
        {
            return Size(_screenSize.width/_scaleX,_screenSize.height/_scaleY);
        }
        else 
        {
            return _designResolutionSize;
        }
    }

    /** * Get the visible origin point of opengl viewport. * * @return The visible origin point of opengl viewport. */
    Vec2 GLView::getVisibleOrigin() const
    {
        if (_resolutionPolicy == ResolutionPolicy::NO_BORDER)
        {
            return Vec2((_designResolutionSize.width - _screenSize.width/_scaleX)/2,(_designResolutionSize.height - _screenSize.height/_scaleY)/2);
        }
        else 
        {
            return Vec2::ZERO;
        }
    }
    /** * Set the design resolution size. * @param width Design resolution width. * @param height Design resolution height. * @param resolutionPolicy The resolution policy desired,you may choose: * [1] EXACT_FIT Fill screen by stretch-to-fit: if the design resolution ratio of width to height is different from the screen resolution ratio,your game view will be stretched. * [2] NO_BORDER Full screen without black border: if the design resolution ratio of width to height is different from the screen resolution ratio,two areas of your game view will be cut. * [3] SHOW_ALL Full screen with black border: if the design resolution ratio of width to height is different from the screen resolution ratio,two black borders will be shown. */
    void GLView::setDesignResolutionSize(float width,float height,ResolutionPolicy resolutionPolicy)
    {
        CCASSERT(resolutionPolicy != ResolutionPolicy::UNKNOWN,"should set resolutionPolicy");

        if (width == 0.0f || height == 0.0f)
        {
            return;
        }

        _designResolutionSize.setSize(width,height);
        _resolutionPolicy = resolutionPolicy;

        updateDesignResolutionSize();
     }

     void GLView::updateDesignResolutionSize()
     {
        if (_screenSize.width > 0 && _screenSize.height > 0
            && _designResolutionSize.width > 0 && _designResolutionSize.height > 0)
        {
            _scaleX = (float)_screenSize.width / _designResolutionSize.width;
            _scaleY = (float)_screenSize.height / _designResolutionSize.height;

            if (_resolutionPolicy == ResolutionPolicy::NO_BORDER)
            {
                _scaleX = _scaleY = MAX(_scaleX,_scaleY);
            }

            else if (_resolutionPolicy == ResolutionPolicy::SHOW_ALL)
            {
                _scaleX = _scaleY = MIN(_scaleX,_scaleY);
            }

            else if ( _resolutionPolicy == ResolutionPolicy::FIXED_HEIGHT) {
                _scaleX = _scaleY;
                _designResolutionSize.width = ceilf(_screenSize.width/_scaleX);
            }

            else if ( _resolutionPolicy == ResolutionPolicy::FIXED_WIDTH) {
                _scaleY = _scaleX;
                _designResolutionSize.height = ceilf(_screenSize.height/_scaleY);
            }

            // calculate the rect of viewport
            float viewPortW = _designResolutionSize.width * _scaleX;
            float viewPortH = _designResolutionSize.height * _scaleY;

            _viewPortRect.setRect((_screenSize.width - viewPortW) / 2,(_screenSize.height - viewPortH) / 2,viewPortW,viewPortH);


            // reset director's member variables to fit visible rect
            auto director = Director::getInstance();
            director->_winSizeInPoints = getDesignResolutionSize();
            director->_isStatusLabelUpdated = true;
            director->setProjection(director->getProjection());

            // Github issue #16139
            // A default viewport is needed in order to display the FPS,
            // since the FPS are rendered in the Director,and there is no viewport there.
            // Everything,including the FPS should renderer in the Scene.
            glViewport(0,0,_screenSize.width,_screenSize.height);
        }
    }

}；

设置/获取屏幕的大小（主要是desktop系统上设置，也用于调试各屏幕适配的情况是否符合我们的预期等）：
void GLView:: setFrameSize(float width,float height)
const Size& GLView::getFrameSize() const

设置/获取设计尺寸大小（相当于我们设定了我们的创作区域大小、最终如何显示到屏幕上，通过ResolutionPolicy指定。
注意：SHOW_ALL、EXACT_FIT、NO_BORDER三种策略的设计分辨率都是传入值，内部不做修正。FIXED_HEIGHT和FIXED_WIDTH都是会在内部修正传入设计分辨率，以保证屏幕分辨率到设计分辨率无拉伸铺满屏幕，所以通过getDesignResolutionSize()获取的值可能与传入的值不一致。
void GLView:: setDesignResolutionSize(float width,float height,ResolutionPolicy resolutionPolicy)
const Size& GLView::getDesignResolutionSize() const

获取可视区域的大小和原点,即我们设定的setDesignResolutionSize大小有多少是可以被显示的。由于适配策略NO_BORDER，有可能设计区域有部分最终无法在屏幕上显示，所以这里需要计算，可视原点也可能不是(0，0)。还有由于我们通过setDesignResolutionSize设置的设计尺寸大小，FIXED_HEIGHT、FIXED_WIDTH因为需要裁剪，所以这里的可视区域大小也是经过裁剪后的大小。
Size GLView::getVisibleSize() const
Vec2 Director::getVisibleOrigin() const

这里获取的WinSize，即setDesignResolutionSize设置后经过计算裁剪（可能）的_designResolutionSize。 const Size& Director::getWinSize(void) const