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 viewportfloat 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 rectauto 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