Android core analysis (28) -- Android GDI surface & canvas

Surface & canvas

Canvas indicates the meaning of the canvas. The upper-layer plotting of Android is almost done through the canvas instance. In fact, canvas is more of an interface packaging. Drawpaints, drawpoints, drawrect, drawbitmap...

1 Essential Relationship Between canvas and Surface

For this section, we will not study the skia graphics engine itself. What we need to know is how the image we make is placed in that place and how the canvas is connected to the surface.

Canvas (Java) has a native canvas C ++ object in the C ++ native layer.

Lockcanvas () @ Java

Surface_lockCanvas@android_view_Surface.cpp

Surfacecontrol-> New Surface (Control) @ surface. cpp

Surface: Lock operation:

Graphicbuffer: Lock

Getbuffermapper (). Lock graphicbuffermapper: Lock

Mallocmod-> lockgralloc_module_t: Lock

Surfacelock can be used to obtain the graphic buffer address corresponding to the surface (mlockedbuffe.

(1) create a bitmap device connected to skcanvas. The bitmap uses the image buffer address obtained above as its own bitmap memory.

(2) set the drawing target device of skcanvas to this bitmap.

Through this process, the connection between surfacecontrol and canvas is established.

2 view: ondraw Source

When OpenGL is not used for rendering, the view attribute of Android changes. When a new view or Z-order changes, you need to re-draw the view on the System screen, at this time, our view will execute ondraw (canvas). Where is the root cause?

Viewroot. Java

Extends mtraversals (..)

...

Draw ()

Canvas = surface. lockcanvas (dirty );

...

Mview. Draw (canvas );

Draw (CAVAS) @ view. Java

Background. Draw (canvas );

Ondraw (CAVAS)

Dispatchdraw (CAVAS)

Ondrawscrolbars (CAVAS)

Surface. unlockcanvasandpost (canvas );