嵌入式linux之go语言开发（十三）LittlevGL，漂亮的嵌入式GUI的go语言绑定

LittleVgl，一款开源嵌入式图形用户界面库。https://littlevgl.cn/

使用简单小巧，界面也漂亮。很适合用在嵌入式上。在Stm32单片机上都能跑，只要实现了基础的函数很好移植。

看下stm32单片机下的效果：

业余时间移植到了嵌入式linux上，使用帧缓冲Frambuffer,显示效果还不错。

计划选用LittleVgl作为嵌入式的漂亮的UI。

其他GUI如QT，miniGUI,周立功的AWTK也都有了解过。界面UI是强大，但是就是有点儿庞大，在简单的嵌入式界面上不想花太多精力和时间。毕竟不是手机，不需要太多人机交互，要的是简单小巧，够用即可。

之前本想在Ubuntu14上玩儿一下AWTK,但是编译依赖一大堆，最后放弃了。查看官网才知道需要使用Ubuntu16以上系统且安装好几个不知道什么用途的依赖，后续有机会再尝试吧。他们能不能出个纯c代码的精简版？可能太强大和支持N多平台的缘故吧，代码依赖很多三方库，这点儿有点儿不让人喜欢，不够小巧。

如图：

但是，还是不够好用。

计划使用go加上面向对象的思想，对其进行进一步的封装，让其更简单好用一点儿。

如果使用上类似于python上的绑定那样简单的话，该是多么美好的事情，如：

$ python3
Python 3.6.5 (default, May  9 2018, 10:02:20) 
[GCC 4.2.1 Compatible Apple LLVM 9.1.0 (clang-902.0.39.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 
>>> from pylvgl import Demo, lvgl
>>> d = Demo()
>>>
>>> b1 = lvgl.Btn(lvgl.scr_act())        
>>> b1.set_size(200,50)
>>> b1.align(b1.get_parent(), lvgl.ALIGN_IN_LEFT_MID, 0, 0)
>>>
>>> l1 = lvgl.Label(b1)
>>> l1.set_text('LittlevGL')
>>>
>>> lm = lvgl.Lmeter(lvgl.scr_act())
>>> lm.align(cal.get_parent(), lvgl.ALIGN_IN_LEFT_MID, 300, 0)
>>> lm.set_range(0,100)
>>> lm.set_value(10)
>>>
>>> style_lm1.line_width = 4
>>> style_lm1.body_main_color = 0x2200
>>> style_lm1.body_grad_color = 0x0022
>>> lm.set_style(style_lm1)
>>>
>>> lb2 = lvgl.Label(lm)
>>> lb2.set_text('10%')
>>> lb2.align(lm, lvgl.ALIGN_CENTER, 0, 0)

已经初步验证了，go的绑定可行，使用也变得简单啦

如：

package main

import (
	"golvgl/lvgl"
	"log"
	"time"
)

func main() {
	log.Println("Hello Go,LittlevGL")
	lb := lvgl.Label(lvgl.ScrAct(), nil)
	lb.SetText("hello world,go test 222")
	lb.Align(nil, lvgl.LV_ALIGN_CENTER, 0, 0)
	log.Println("Hello Go Over 222")
	for true {
		lvgl.TickInc(5)
		lvgl.TaskHandler()
		time.Sleep(5000)
	}
}

随后代码会放在 github上，希望感兴趣的有志之士能一块儿参与完善下封装。

package lvgl

/*

#cgo CFLAGS: -Iinclude/lvgl
#include "lvgl.h"
#cgo LDFLAGS: -Llibs -llvgl
#include <stdlib.h>

extern void disp_init(void);
extern void fbdev_init(void);
extern void fbdev_flush(void);

void disp_init(){
	lv_disp_drv_t disp_drv;
    lv_disp_drv_init(&disp_drv);
    disp_drv.disp_flush = fbdev_flush;
    lv_disp_drv_register(&disp_drv);
}
*/
import "C"
import "unsafe"

// Lvobj ...
type Lvobj C.struct__lv_obj_t

const (
	LV_ALIGN_CENTER C.uchar = C.LV_ALIGN_CENTER
)

func init() {
	C.lv_init()
	C.fbdev_init()
	C.disp_init()
}

// ScrAct ...
func ScrAct() *Lvobj {
	return (*Lvobj)(unsafe.Pointer(C.lv_scr_act()))
}

// Label ...
func Label(par, copy *Lvobj) *Lvobj {
	var p1 *C.struct__lv_obj_t
	var p2 *C.struct__lv_obj_t
	p1 = (*C.struct__lv_obj_t)(unsafe.Pointer(par))
	p2 = (*C.struct__lv_obj_t)(unsafe.Pointer(copy))
	return (*Lvobj)(unsafe.Pointer(C.lv_label_create(p1, p2)))
}

// SetText ...
func (obj *Lvobj) SetText(str string) {
	C.lv_label_set_text((*C.struct__lv_obj_t)(unsafe.Pointer(obj)), C.CString(str))
}

// Align ...
func (obj *Lvobj) Align(base *Lvobj, align C.uchar, x int, y int) {
	var ba *C.struct__lv_obj_t
	ba = (*C.struct__lv_obj_t)(unsafe.Pointer(base))
	C.lv_obj_align((*C.struct__lv_obj_t)(unsafe.Pointer(obj)), ba, align, C.short(x), C.short(y))
}

// TickInc ...
func TickInc(tick int) {
	C.lv_tick_inc(C.uint(tick))
}

// TaskHandler ...
func TaskHandler() {
	C.lv_task_handler()
}

其中遇到的问题总结：

void * 对应go中的unsafe.Pointer

c结构体和 go之间的转换为：

var obj *Lvobj

var p *C.struct__lv_obj_t

p = (*C.struct__lv_obj_t)(unsafe.Pointer(obj))

byte[]和 c的数组之间转换为：

var img_bubble_pattern_map = []byte{0,1,2}

(*C.uchar)(unsafe.Pointer(&img_bubble_pattern_map[0])

如果是struct, union和enum的话，需要加上如下前缀，struct_、union_和enum_，比如 C.struct_MSG。

可以用注释符//和/**/包围C代码
import “C” 和包含C代码之间是没有空行的
动态库的导入和编译选项通过LDFLAGS、CFLAGS/CXXFLAGS来设置
还可以用pkg-config #cgo pkg-config : xxxxname
编译宏定义指定#cgo CFLAGS: -DNDEBUG -DXXXX=2

const (

   LV_ALIGN_CENTER C.uchar = C.LV_ALIGN_CENTER

   //  error:LV_PROTECT_POS C.enum_ = C.enum_LV_PROTECT_POS

)

对于c中的enum，没有enum{} name,名字的，它的类型是什么呢？经过摸索发现，其实就是个int或byte

ok: LV_PROTECT_POS byte = C.LV_PROTECT_POS

package main
 
// #include <stdio.h>
// #include <stdlib.h>
/*
void print(char *str) {
    printf("%s\n", str);
}
*/
import "C"
 
import "unsafe"
 
func main() {
    s := "Hello Cgo"
    cs := C.CString(s)
    C.print(cs)
    C.free(unsafe.Pointer(cs))
}

c结构体的位域和go之间转换为：

var img_bubble_pattern C.lv_img_dsc_t

var img_bubble_pattern_map = []byte{
	0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53,
	0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53,
	0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53}

const (
	LV_ALIGN_CENTER C.uchar = C.LV_ALIGN_CENTER
)

func init() {
	fmt.Println(img_bubble_pattern)
	fmt.Println(reflect.TypeOf(img_bubble_pattern))
	//fmt.Println(reflect.TypeOf(img_bubble_pattern.header.always_zero))
	bt := [4]byte{0, 1, 2, 3}
	img_bubble_pattern = C.lv_img_dsc_t{
		header:    C.lv_img_header_t{bt},
		data_size: 112320 * C.LV_COLOR_SIZE / 8,
		data:      (*C.uchar)(unsafe.Pointer(&img_bubble_pattern_map[0])),
	}
}

未完待续，敬请期待....

想完善其他的封装太难了，几度想放弃。上面的c结构体的位域和go之间转换问题解决了，又遇到其他问题，如：

panic: runtime error: cgo argument has Go pointer to Go pointer
大意是 Go调用C Code时，Go传递给C Code的Go指针所指的Go Memory中不能包含任何指向Go Memory的Pointer。

比如以下样例代码举例：我本意想在go层读取图片二进制内容，传给给c层，调用c的显示图片的接口。

package main

/*
#include <stdio.h>

//模拟图片信息的定义
typedef struct {
int a;
int b;
unsigned char* data;
}st;

//模拟 c层的获取图片内容并显示图片的接口
void testc(void *p){
   char * p1;
   p1 = p;
   printf("a is:%d\n",p1[0]);
}
*/
import "C"
import (
	"fmt"
	"unsafe"
)

var img_bubble_pattern C.st

//模拟go层二进制图片数据
var dat = []byte{
	1, 2, 3,
}

//模拟封装的调用c层的图片显示接口
func test(p *C.st) {
	//fmt.Printf("%#v\n", p)
	C.testc(unsafe.Pointer(p))
}
func main() {
	fmt.Println("Hello, World!")
	fmt.Printf("%#v\n", img_bubble_pattern)
	//对结构体赋值
	img_bubble_pattern = C.st{
		a: 1, b: 2, data: (*C.uchar)(unsafe.Pointer(&dat[0])),
	}
	//调用
	test(&img_bubble_pattern)
}

var dat是从go应用层读取到的图片的二进制数据，调c的接口完成图片显示。c的显示图片的接口就是一个结构体指针。

但是如果调试这段代码就发现，报panic: runtime error: cgo argument has Go pointer to Go pointer，

这段代码有问题吗？肯定有问题的，但是实际的用途上没错，就想这么干，让cgo达到我这调用的目的，这问题该怎么破？

果然，还是有高人的，这个问题问到了go语言交流群里的群主苏州-狄，昵称曦晨。在这里表示感谢！

他的解决方案如下，把这个指针转换的问题换了一种方式，绕过去了，但也达到了目的。

package main

/*
#include <stdio.h>

//模拟图片信息的定义
typedef struct {
int a;
int b;
unsigned char* data;
}st;

//模拟 c层的获取图片内容并显示图片的接口
void testc(void *p){
	st* p1 = p;
   printf("a is:%d\n",p1->data[0]);
}
*/
import "C"
import (
	"fmt"
	"unsafe"
)

var img_bubble_pattern C.st

//模拟go层二进制图片数据
var dat = []byte{
	1, 2, 3,
}

//模拟封装的调用c层的图片显示接口
func test(p C.st) {
	//fmt.Printf("%#v\n", p)
	C.testc(unsafe.Pointer(&p))
}

func main() {
	fmt.Println("Hello, World!")
	fmt.Printf("%#v\n", img_bubble_pattern)
	//对结构体赋值
	img_bubble_pattern = C.st{
		a: 1, b: 2, data: (*C.uchar)(unsafe.Pointer(&dat[0])),
	}
	//调用
	// C.testc(unsafe.Pointer(&img_bubble_pattern))
	test(img_bubble_pattern)
}

对比两份代码发现，在go层不传指针了，改为结构体的值传递，解决了这个问题。

可以实现CGO和C之间的指针传递，且不再报错 cgo argument has Go pointer to Go pointer

如下：

C.testc(pointer.Save(p))

package main

/*
#include <stdio.h>
#include <stdlib.h>
//模拟图片信息的定义
typedef struct {
int a;
int b;
unsigned char* data;
}st;

//模拟 c层的获取图片内容并显示图片的接口
void testc(void *p){
	st* p1 = p;
   printf("a is:%d\n",p1->data[0]);
}
*/
import "C"
import (
	"fmt"
	"unsafe"

	"github.com/mattn/go-pointer"
)

var img_bubble_pattern C.st

//模拟go层二进制图片数据
var dat = []byte{
	1, 2, 3,
}

//模拟封装的调用c层的图片显示接口
// func test(p C.st) {
// 	//fmt.Printf("%#v\n", p)
// 	C.testc(unsafe.Pointer(&p))
// }

//模拟封装的调用c层的图片显示接口
func test(p *C.st) {
	//fmt.Printf("%#v\n", p)
	//C.pass_pointer(pointer.Save(p))
	//v := *(pointer.Restore(C.get_from_pointer()).(*C.st))
	C.testc(pointer.Save(p))
}

func main() {
	fmt.Println("Hello, World!")
	fmt.Printf("%#v\n", img_bubble_pattern)
	//对结构体赋值
	img_bubble_pattern = C.st{
		a: 1, b: 2, data: (*C.uchar)(unsafe.Pointer(&dat[0])),
	}
	//调用
	// C.testc(unsafe.Pointer(&img_bubble_pattern))
	test(&img_bubble_pattern)
}