【Android显示系统初探】surface初相识

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本文从实践应用角度窥探Android显示系统。通过C++程序获取surface与SurfaceFlinger交互，在应用中操作surface获取内存地址并写入color值实现显示。还使用dumpsys查看SurfaceFlinger信息，解释了状态栏覆盖绘制图层的原因是Z - order值不同。

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Android显示系统非常复杂，从早期版本演化至今有很大的变化和改进，所以从当前的版本直接去查看会很困难。

初学者看到繁多的概念和类会感到无从下手。

这里我们将从实践应用的方式来展开这一系列，试图对Android的显示系统进行窥探。

Android应用通过surface绘图和SurfaceFlinger进行图像数据交互，Java层的surface绘图流程比较繁琐，

这里我们直接通过一个C++程序来得到一个surface来和SurfaceFlinger进行交互，来达到一个直观的效果。


#define LOG_TAG "bindertest"

#include <stdio.h>
#include <binder/IInterface.h>
#include <binder/Parcel.h>
#include <binder/IBinder.h>
#include <binder/Binder.h>
#include <binder/ProcessState.h>
#include <binder/IPCThreadState.h>
#include <binder/IServiceManager.h>

#include <iostream>
#include <iomanip>

#include <unistd.h>

//for ALOGD
#include <log/log.h>

#include <cutils/memory.h>

//test socketpair
#include <sys/types.h>
#include <error.h>
#include <errno.h>

#include <gui/Surface.h>
#include <gui/SurfaceComposerClient.h>
#include <android/native_window.h>


using namespace android;
using namespace std;
//

int main(int argc, char** argv)
{
	//the color is ABGR
	int color = 0xFF0000FF;
//这个用来确定当前屏幕的ARGB显示顺序
	if (argc == 2 && argv[1][0] == '1') color = 0xFFFF0000;
	if (argc == 2 && argv[1][0] == '2') color = 0xFF000000;
	if (argc == 2 && argv[1][0] == '3') color = 0xFF00FF00;
	if (argc == 2 && argv[1][0] == '4') color = 0x00FF00FF;
	if (argc == 2 && argv[1][0] == 'b') color = 0x00000000;
	if (argc == 2 && argv[1][0] == 'a') color = 0x00FF00FF;
		
    // set up the thread-pool
    sp<ProcessState> proc(ProcessState::self());
    ProcessState::self()->startThreadPool();

    // create a client to surfaceflinger
    sp<SurfaceComposerClient> client = new SurfaceComposerClient();
    
    sp<SurfaceControl> surfaceControl = client->createSurface(String8("testDraw"),
            160, 240, PIXEL_FORMAT_RGBA_8888, 0);

    sp<Surface> surface = surfaceControl->getSurface();

    SurfaceComposerClient::openGlobalTransaction();
    surfaceControl->setLayer(100000);
    SurfaceComposerClient::closeGlobalTransaction();

    ANativeWindow_Buffer outBuffer;
    surface->lock(&outBuffer, NULL);
    ssize_t bpr = outBuffer.stride * bytesPerPixel(outBuffer.format);
    android_memset32((uint32_t*)outBuffer.bits, color, bpr*outBuffer.height);
    surface->unlockAndPost();

		cout << "outBuffer.stride = " << outBuffer.stride << ", bytesPerPixel(outBuffer.format) = "
		 << bytesPerPixel(outBuffer.format) << ", outBuffer.height = " << outBuffer.height << endl;

 
    IPCThreadState::self()->joinThreadPool();
    
    return 0;
}

Android.mk

LOCAL_PATH := $(call my-dir)

#for service

include $(CLEAR_VARS)

LOCAL_SRC_FILES:= \

service.cpp

LOCAL_SHARED_LIBRARIES := \

libbase \

libutils \

liblog \

libbinder \

libcutils libskia libui libgui

LOCAL_CFLAGS += -Wall -Werror -Wunused -Wunreachable-code

LOCAL_MODULE:= testDraw

include $(BUILD_EXECUTABLE)

这里通过对surface的操作，在应用中获取到了在SurfaceFlinger中分配的内存地址，

写入color值，在界面上进行了显示

ANativeWindow_Buffer outBuffer;

surface->lock(&outBuffer, NULL);

ssize_t bpr = outBuffer.stride * bytesPerPixel(outBuffer.format);

android_memset32((uint32_t*)outBuffer.bits, color, bpr*outBuffer.height);

surface->lock(&outBuffer, NULL);这句话是冰山一角，里面有一系列操作，

这篇的目的是对surface有个初步的概念，混个脸熟，所以在这里并不去深入的分析各处理流程，

这个程序的执行结果

在手机上显示160*240的surface

使用我们的老朋友dumpsys来和SurfaceFlinger来个心灵对话，可以看到在SurfaceFlinger中生成了相应的Layer

dumpsys SurfaceFlinger

+ Layer 0x76b8edd000 (testDraw#0)

Region transparentRegion (this=0x76b8edd380, count=1)

[ 0, 0, 0, 0]

Region visibleRegion (this=0x76b8edd010, count=1)

[ 0, 0, 160, 240]

Region surfaceDamageRegion (this=0x76b8edd088, count=1)

[ 0, 0, 0, 0]

layerStack= 0, z= 100000, pos=(0,0), size=( 160, 240), crop=( 0, 0, -1, -1), finalCrop=( 0, 0, -1, -1), isOpaque=0, invalidate=0, dataspace=Default (0), pixelformat=RGBA_8888 alpha=1.000, flags=0x00000000, tr=[1.00, 0.00][0.00, 1.00]

client=0x76ba1aa2c0

format= 1, activeBuffer=[ 160x 240: 160, 1], queued-frames=0, mRefreshPending=0

mSecure=0, mProtectedByApp=0, mFiltering=0, mNeedsFiltering=0 mDestroyCalled=0

mTexName=8 mCurrentTexture=0

mCurrentCrop=[0,0,0,0] mCurrentTransform=0

mAbandoned=0

- BufferQueue mMaxAcquiredBufferCount=1 mMaxDequeuedBufferCount=2

mDequeueBufferCannotBlock=0 mAsyncMode=0

default-size=[160x240] default-format=1 transform-hint=00 frame-counter=1

FIFO(0):

this=0x76ba1c9000 (mConsumerName=testDraw#0, mConnectedApi=2, mConsumerUsageBits=2304, mId=19, mPid=8411, producer=[12222:./testDraw], consumer=[8411:/system/bin/surfaceflinger])

Slots:

>[00:0x76bd8407a0] state=ACQUIRED 0x76bd840f80 frame=1 [ 160x 240: 160, 1]

在上面的图片中，你会发现，状态栏（statusbar）覆盖在我们所绘制的图层上面，为什么呢，

查看下dumpsys中打印的信息就清楚了，

Display 0 HWC layers:

-------------------------------------------------------------------------------

Layer name

Z | Comp Type | Disp Frame (LTRB) | Source Crop (LTRB)

-------------------------------------------------------------------------------

com.google.android.googlequicksearch[...]ps.gsa.searchnow.SearchNowActivity#1

21005 | Device | 0 0 720 1280 | 0.0 0.0 720.0 1280.0

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -