Android显示系统——基本概念梳理
DisplayState
struct DisplayState {
// 这里定义了Display变更类型，说明Display可能发生的变化类型
enum {
eSurfaceChanged = 0x01,
eLayerStackChanged = 0x02,
eDisplayProjectionChanged = 0x04,
eDisplaySizeChanged = 0x08,
eFlagsChanged = 0x10
};
DisplayState();
void merge(const DisplayState& other);
void sanitize(int32_t permissions); // 清除无效（无权限）的变换标记，即若无权限进行某些变更，则拦截对应的变更操作
uint32_t what = 0; // 变化标记，表示发生了什么变化
uint32_t flags = 0;
sp<IBinder> token;
sp<IGraphicBufferProducer> surface;
ui::LayerStack layerStack = ui::DEFAULT_LAYER_STACK; // 属于此display的layer栈，即需要显示在此display上的所有图层
ui::Rotation orientation = ui::ROTATION_0; // display的显示方向
Rect layerStackSpaceRect = Rect::EMPTY_RECT; // 表示原始layers区域大小，即图层的原始大小和位置
Rect orientedDisplaySpaceRect = Rect::EMPTY_RECT; // 表示进行一系列变换后，图层应该显示在的区域(大小、位置)，即图层应该显示在屏幕的哪个位置及大小
uint32_t width = 0; // display宽度
uint32_t height = 0; // display高度
status_t write(Parcel& output) const;
status_t read(const Parcel& input);
};
- orientation表示屏幕显示的方向，一般为0/90/180/270。
- layerStackSpaceRect定义的矩形区域用于表示原始layers区域大小，即图层的原始大小和位置。
- orientedDisplaySpaceRect定义的矩形区域用于表示进行一系列变换后，图层应该显示在的区域(大小、位置)，即图层应该显示在屏幕的哪个位置及大小
DisplayState指定了如何将图层显示到物理屏幕上，首先要先将从layerslayerStackSpaceRect平移(translate)和缩放到orientedDisplaySpaceRect区域，然后再根据orientation/width/height进行旋转，将图层显示到物理屏上指定的位置。
例如，假设定义了一个Rect(0, 0, 200, 100)的layerStackSpaceRect区域(大小为200x100)，而orientedDisplaySpaceRect区域为Rect(20, 10, 420, 210)，其大小为400x200，物理屏display的尺寸为WxH。当orientation为0时，首先需要将layers进行放大2倍以匹配目标显示区域的大小，然后将其从(0,0)点移动到(20，10)点。当orientation为1时，在进行完缩放和平衡动作后，还需要顺时钟旋转90度，再平衡(W, 0)后，就变换到了指定的物理屏上的显示位置。
ComposerState
ComposeState的定义如下，其中最关键的是layer_state_t结构体：
struct ComposerState {
layer_state_t state;
status_t write(Parcel& output) const;
status_t read(const Parcel& input);
};
layer_state_t结构体内容较多，拆开来看
首先，定义了三个权限，含义比较简单
enum Permission {
ACCESS_SURFACE_FLINGER = 0x1, // 访问权限
ROTATE_SURFACE_FLINGER = 0x2, // 旋转权限
INTERNAL_SYSTEM_WINDOW = 0x4, // 系统窗口权限
};
接口定义一组layer相关类型，一般用于创建图层时指定layer的类型
enum {
eLayerHidden = 0x01, // 隐藏此图层
eLayerOpaque = 0x02, // Layer不透明
eLayerSkipScreenshot = 0x40, // 截图(屏)时跳过此layer，即截屏截不到此图层
eLayerSecure = 0x80, // 是否为安全图层，安全图层投屏时不会被投到外接屏
// Queue up BufferStateLayer buffers instead of dropping the oldest buffer when this flag is
// set. This blocks the client until all the buffers have been presented. If the buffers
// have presentation timestamps, then we may drop buffers.
eEnableBackpressure = 0x100, // ENABLE_BACKPRESSURE
eLayerIsDisplayDecoration = 0x200, // 装饰图层，如屏幕顶部和底部的圆角图层，隐私指示器图层等
// Ignore any destination frame set on the layer. This is used when the buffer scaling mode
// is freeze and the destination frame is applied asynchronously with the buffer submission.
// This is needed to maintain compatibility for SurfaceView scaling behavior.
// See SurfaceView scaling behavior for more details.
eIgnoreDestinationFrame = 0x400,
};
这一组定义含义比较明确，用于表示layer发生了什么变化，一般配合transaction的各种setXXX方法进行使用，当transaction提交到SurfaceFlinger时，SurfaceFlinger于依据不同类型的变化进行相应的处理。
enum {
ePositionChanged = 0x00000001,
eLayerChanged = 0x00000002,
eSizeChanged = 0x00000004,
eAlphaChanged = 0x00000008,
eMatrixChanged = 0x00000010,
eTransparentRegionChanged = 0x00000020,
eFlagsChanged = 0x00000040,
eLayerStackChanged = 0x00000080,
eDimmingEnabledChanged = 0x00000400,
eShadowRadiusChanged = 0x00000800,
/* unused 0x00001000, */
eBufferCropChanged = 0x00002000,
eRelativeLayerChanged = 0x00004000,
eReparent = 0x00008000,
eColorChanged = 0x00010000,
eDestroySurface = 0x00020000,
eTransformChanged = 0x00040000,
eTransformToDisplayInverseChanged = 0x00080000,
eCropChanged = 0x00100000,
eBufferChanged = 0x00200000,
/* unused 0x00400000, */
eDataspaceChanged = 0x00800000,
eHdrMetadataChanged = 0x01000000,
eSurfaceDamageRegionChanged = 0x02000000,
eApiChanged = 0x04000000,
eSidebandStreamChanged = 0x08000000,
eColorTransformChanged = 0x10000000,
eHasListenerCallbacksChanged = 0x20000000,
eInputInfoChanged = 0x40000000,
eCornerRadiusChanged = 0x80000000,
eDestinationFrameChanged = 0x1'00000000,
/* unused = 0x2'00000000, */
eBackgroundColorChanged = 0x4'00000000,
eMetadataChanged = 0x8'00000000,
eColorSpaceAgnosticChanged = 0x10'00000000,
eFrameRateSelectionPriority = 0x20'00000000,
eFrameRateChanged = 0x40'00000000,
eBackgroundBlurRadiusChanged = 0x80'00000000,
eProducerDisconnect = 0x100'00000000,
eFixedTransformHintChanged = 0x200'00000000,
/* unused 0x400'00000000, */
eBlurRegionsChanged = 0x800'00000000,
eAutoRefreshChanged = 0x1000'00000000,
eStretchChanged = 0x2000'00000000,
eTrustedOverlayChanged = 0x4000'00000000,
eDropInputModeChanged = 0x8000'00000000
};
struct layer_state_t {
// ......
layer_state_t();
void merge(const layer_state_t& other);
status_t write(Parcel& output) const;
status_t read(const Parcel& input);
bool hasBufferChanges() const;
bool hasValidBuffer() const;
void sanitize(int32_t permissions);
struct matrix22_t {
float dsdx{0};
float dtdx{0};
float dtdy{0};
float dsdy{0};
status_t write(Parcel& output) const;
status_t read(const Parcel& input);
};
sp<IBinder> surface;
int32_t layerId;
uint64_t what;
float x; // layer位置的x坐标
float y; // layer位置的y坐标
int32_t z; // layer的z轴顺序
uint32_t w; // layer的宽度w
uint32_t h; // layer的高度h
ui::LayerStack layerStack = ui::DEFAULT_LAYER_STACK;
float alpha; // layer的透明度
uint32_t flags; // layer发生变化的flags标记，参考前面layer发生变化的enum定义
uint32_t mask;
uint8_t reserved;
matrix22_t matrix;
float cornerRadius; // layer的圆角半径
uint32_t backgroundBlurRadius;
sp<SurfaceControl> reparentSurfaceControl;
sp<SurfaceControl> relativeLayerSurfaceControl;
sp<SurfaceControl> parentSurfaceControlForChild;
half3 color;
Region transparentRegion; // non POD must be last. see write/read
uint32_t transform;
bool transformToDisplayInverse;
Rect crop;
std::shared_ptr<BufferData> bufferData = nullptr;
ui::Dataspace dataspace;
HdrMetadata hdrMetadata;
Region surfaceDamageRegion;
int32_t api;
sp<NativeHandle> sidebandStream;
mat4 colorTransform;
std::vector<BlurRegion> blurRegions;
sp<gui::WindowInfoHandle> windowInfoHandle = new gui::WindowInfoHandle();
LayerMetadata metadata;
float bgColorAlpha; // The following refer to the alpha, and dataspace, respectively of the background color layer
ui::Dataspace bgColorDataspace;
bool colorSpaceAgnostic; // A color space agnostic layer means the color of this layer can be interpreted in any color space.
std::vector<ListenerCallbacks> listeners;
float shadowRadius; // 此layer的阴影半径
int32_t frameRateSelectionPriority;
float frameRate; // 此layer设定的预期刷新率
int8_t frameRateCompatibility;
int8_t changeFrameRateStrategy;
// Set by window manager indicating the layer and all its children are
// in a different orientation than the display. The hint suggests that
// the graphic producers should receive a transform hint as if the
// display was in this orientation. When the display changes to match
// the layer orientation, the graphic producer may not need to allocate
// a buffer of a different size. -1 means the transform hint is not set,
// otherwise the value will be a valid ui::Rotation.
ui::Transform::RotationFlags fixedTransformHint;
// Indicates that the consumer should acquire the next frame as soon as it
// can and not wait for a frame to become available. This is only relevant
// in shared buffer mode.
bool autoRefresh;
// An inherited state that indicates that this surface control and its children
// should be trusted for input occlusion detection purposes
bool isTrustedOverlay;
StretchEffect stretchEffect; // 此layer的拉伸效果
Rect bufferCrop;
Rect destinationFrame;
gui::DropInputMode dropInputMode; // 强制inputflinger忽略所有此图层及其子图层接收到的input事件，即此图层不处理input事件。
bool dimmingEnabled;
};
