一、Surface是什么
Handle onto a raw buffer that is being managed by the screen compositor.
大概意思是处理由屏幕合成器管理的原理缓存区。
二、Surface实现原理
在Surface类里有一个Canvas对象，在Canvas里有一个Bitmap，Bitmap是真正的画布。
Bitmap是什么
Bitmap缩写是BMP，是一种存储像素的数据结构。
三、Surface跨进程间传递
源码：frameworks/base/core/java/android/view/Surface.java
/**
* Handle onto a raw buffer that is being managed by the screen compositor.
*
* <p>A Surface is generally created by or from a consumer of image buffers (such as a
* {@link android.graphics.SurfaceTexture}, {@link android.media.MediaRecorder}, or
* {@link android.renderscript.Allocation}), and is handed to some kind of producer (such as
* {@link android.opengl.EGL14#eglCreateWindowSurface(android.opengl.EGLDisplay,android.opengl.EGLConfig,java.lang.Object,int[],int) OpenGL},
* {@link android.media.MediaPlayer#setSurface MediaPlayer}, or
* {@link android.hardware.camera2.CameraDevice#createCaptureSession CameraDevice}) to draw
* into.</p>
*
* <p><strong>Note:</strong> A Surface acts like a
* {@link java.lang.ref.WeakReference weak reference} to the consumer it is associated with. By
* itself it will not keep its parent consumer from being reclaimed.</p>
*/
public class Surface implements Parcelable {
……
}
Surface继承自Parcelable，Parcelable是一个序列化接口，将对象转化为二进制流（二进制序列）的过程。二进制流可以通过Socket传输或者保存到本地。
在Android中Surface是怎么在进程间传递的，应用如何将Surface传递给SurfaceFlinger进程绘制的。
Java层实现：
在Parcelable中有两个关键函数：
/**
* 将数据写入Parcel对象
*/
public void writeToParcel(Parcel dest, int flags) {}
/**
* 从Parcel对象中读取数据
*/
public void readFromParcel(Parcel source) {}
先看Parcelable.writeToParcel()函数：
@Override
public void writeToParcel(Parcel dest, int flags) {
synchronized (mLock) {
// NOTE: This must be kept synchronized with the native parceling code
// in frameworks/native/libs/Surface.cpp
dest.writeString(mName);
dest.writeInt(mIsSingleBuffered ? 1 : 0);
nativeWriteToParcel(mNativeObject, dest);
}
}
dest是一个Parcel对象，将name写入Parcel对象dest，再调用nativeWriteToParcel()函数，将mNativeObject写入到Parcel对象。
mNativeObject是什么？
long mNativeObject; // package scope only for SurfaceControl access
mNativeObject是Long型变量，存储Native层的对象指针。
nativeWriteToParcel()函数定义：
private static native void nativeWriteToParcel(long nativeObject, Parcel dest);
nativeWriteToParcel()函数是Native层函数。
下面看看readFromParcel()函数：
public void readFromParcel(Parcel source) {
synchronized (mLock) {
// nativeReadFromParcel() will either return mNativeObject, or
// create a new native Surface and return it after reducing
// the reference count on mNativeObject. Either way, it is
// not necessary to call nativeRelease() here.
// NOTE: This must be kept synchronized with the native parceling code
// in frameworks/native/libs/Surface.cpp
mName = source.readString();
mIsSingleBuffered = source.readInt() != 0;
setNativeObjectLocked(nativeReadFromParcel(mNativeObject, source));
}
}
从Surface对象中读取name数据，再调用nativeReadFromParcel()函数，通过setNativeObjectLocked()函数将nativeReadFromParcel()函数返回的数据保存起来。
nativeReadFromParcel()函数定义：
private static native long nativeReadFromParcel(long nativeObject, Parcel source);
setNativeObjectLocked()函数实现：
private void setNativeObjectLocked(long ptr) {
if (mNativeObject != ptr) {
mNativeObject = ptr;
}
}
将nativeReadFromParcel()函数返回值赋值给mNativeObject变量。
Native层：
源码：frameworks/base/core/jni/android_view_Surface.cpp
nativeWriteToParcel()函数实现：
static void nativeWriteToParcel(JNIEnv* env, jclass clazz,
jlong nativeObject, jobject parcelObj) {
// (1) 获取parcel对象，从Java层
Parcel* parcel = parcelForJavaObject(env, parcelObj);
if (parcel == NULL) {
jniThrowNullPointerException(env, NULL);
return;
}
// (2) Native层的Surface对象
sp<Surface> self(reinterpret_cast<Surface *>(nativeObject));
android::view::Surface surfaceShim;
if (self != nullptr) {
// (3)
surfaceShim.graphicBufferProducer = self->getIGraphicBufferProducer();
}
// Calling code in Surface.java has already written the name of the Surface
// to the Parcel
surfaceShim.writeToParcel(parcel, /*nameAlreadyWritten*/true);
}
// (3)
sp<IGraphicBufferProducer> GraphicBufferSource::getIGraphicBufferProducer() const {
return mProducer;
}
(1) 通过parcelFroJavaOjbect()获取Parcel对象：
Parcel* parcelForJavaObject(JNIEnv* env, jobject obj)
{
if (obj) {
Parcel* p = (Parcel*)env->GetLongField(obj, gParcelOffsets.mNativePtr);
if (p != NULL) {
return p;
}
jniThrowException(env, "java/lang/IllegalStateException", "Parcel has been finalized!");
}
return NULL;
}
(1) 通过JNI调用，在Native层通过Native层保存的Java层Parcel指针获取Native层对应Java层的Parcel对象。
(2) 通过Native层保存的Java层Surface对象指针，在Native层恢复Surface对象。
(3) 将Native层的IGraphicBufferProducer对象传递给Java层的Surface.graphicBufferProducer变量。
nativeReadFromParcel()函数实现：
static jlong nativeReadFromParcel(JNIEnv* env, jclass clazz,
jlong nativeObject, jobject parcelObj) {
// (1)
Parcel* parcel = parcelForJavaObject(env, parcelObj);
android::view::Surface surfaceShim;
// (2)
sp<Surface> self(reinterpret_cast<Surface *>(nativeObject));
// update the Surface only if the underlying IGraphicBufferProducer
// has changed.
// (3)
if (self != nullptr
&& (IInterface::asBinder(self->getIGraphicBufferProducer()) ==
IInterface::asBinder(surfaceShim.graphicBufferProducer))) {
// same IGraphicBufferProducer, return ourselves
return jlong(self.get());
}
// (4)
sp<Surface> sur;
if (surfaceShim.graphicBufferProducer != nullptr) {
// we have a new IGraphicBufferProducer, create a new Surface for it
sur = new Surface(surfaceShim.graphicBufferProducer, true);
// and keep a reference before passing to java
sur->incStrong(&sRefBaseOwner);
}
if (self != NULL) {
// and loose the java reference to ourselves
self->decStrong(&sRefBaseOwner);
}
// (5)
return jlong(sur.get());
}
(1) 获取Native层对应Java层的Parcel对象。
(2) 获取Native层对应Java层的Surface对象。
(3) 获取IGraphicBufferProducer对象，一个Binder对象。
(4) 通过IGraphicBufferProducer对象创建一个新的Surface对象。
(5) 将Native层的新Surface对象通过JNI调用传递给Java层。
对象Java层的Surface对应Native层的Surface对象，对于Native层的Surface对应Native层的IGraphicBufferProducer对象。
四、总结
Android App中，一个Window对应一个Surface对象，应用与绘制服务SurfaceFlinger服务的通信是基于共享内存实现的，应用中在Java层将Surface对象通过Parcel转化为二进制流，并且二进制流存储在共享内存。
Android每个应用都有在共享内存中都会有一个SharedClicent，再应用的Window对应一个Surface，而每个Surface对应共享内存的SharedBufferStack。
PS：可以配合SurfaceFlinger一文一起阅读。