研究Android底层代码时,尤其是Binder跨进程通信时,经常会发现interface_cast和asBinder,很容易被这两个函数绕晕,下面通过分析源码来讲解一下:
interface_cast
下面根据下述ICameraClient例子进行分析一下:
//伪代码
sp<ICameraClient> cameraClient = interface_cast<ICameraClient>(BpBinder(handle));
看下interface_cast的实现,其代码在IInterface.h中
template<typename INTERFACE>
inline sp<INTERFACE> interface_cast(const sp<IBinder>& obj)
{
return INTERFACE::asInterface(obj);
}
//这是一个模板函数,展开即为:
inline sp<ICameraClient > interface_cast(const sp<IBinder>& obj)
{
return ICameraClient ::asInterface(obj);
}
那ICameraClient的asInterface在哪实现的呢?发现找了ICameraClient.h和ICameraClient.cpp只有下面两个定义:
//frameworks/av/include/camera/android/hardware/ICameraClient.h
DECLARE_META_INTERFACE(CameraClient);
//frameworks/av/camera/ICameraClient.cpp
IMPLEMENT_META_INTERFACE(CameraClient, "android.hardware.ICameraClient");
DECLARE_META_INTERFACE和IMPLEMENT_META_INTERFACE函数是其父类IInterface(frameworks\native\include\binder\IInterace.h)的宏定义:
//声明asInterface函数
#define DECLARE_META_INTERFACE(INTERFACE)
static const android::String16 descriptor;
//声明asInterface函数
static android::sp<I##INTERFACE> asInterface(
const android::sp<android::IBinder>& obj);
virtual const android::String16& getInterfaceDescriptor() const;
I##INTERFACE();
virtual ~I##INTERFACE();
展开为:
#define DECLARE_META_INTERFACE(CameraClient)
//增加一个描述符
static const android::String16 descriptor;
//声明asInterface函数
static android::sp<ICameraClient> asInterface(
const android::sp<android::IBinder>& obj);
//获取描述符函数
virtual const android::String16& getInterfaceDescriptor() const;
//构造函数以及折构函数
ICameraClient();
virtual ~ICameraClient();
//实现asInterface函数
#define IMPLEMENT_META_INTERFACE(INTERFACE, NAME)
const android::String16 I##INTERFACE::descriptor(NAME);
const android::String16&
I##INTERFACE::getInterfaceDescriptor() const {
return I##INTERFACE::descriptor;
}
android::sp<I##INTERFACE> I##INTERFACE::asInterface(
const android::sp<android::IBinder>& obj)
{
android::sp<I##INTERFACE> intr;
if (obj != NULL) {
intr = static_cast<I##INTERFACE*>(
obj->queryLocalInterface(
I##INTERFACE::descriptor).get());
if (intr == NULL) {
intr = new Bp##INTERFACE(obj); //展开即为intr = new BpServiceManager(obj);
}
}
return intr;
}
I##INTERFACE::I##INTERFACE() { }
I##INTERFACE::~I##INTERFACE() { }
展开为:
#define IMPLEMENT_META_INTERFACE(CameraClient, "android.hardware.ICameraClient")
//定义ICameraClient的描述符为"android.hardware.ICameraClient"
const android::String16 ICameraClient ::descriptor("android.hardware.ICameraClient");
//获取描述符"android.hardware.ICameraClient"
const android::String16&
ICameraClient ::getInterfaceDescriptor() const {
return ICameraClient ::descriptor;
}
//实现asInterface函数
android::sp<ICameraClient> ICameraClient::asInterface(
const android::sp<android::IBinder>& obj)
{
android::sp<ICameraClient> intr;
if (obj != NULL) {
intr = static_cast<ICameraClient*>(
//queryLocalInterface是在IBinder中定义的,默认返回NULL,但在BBinder的子类BnInterface中,重载了该方法,返回this,而BpBinder没有重载,使用IBinder的默认实现,返回NULL
obj->queryLocalInterface(
ICameraClient::descriptor).get());
if (intr == NULL) {
//构建INTERFACE的Bp端代理对象
intr = new BpCameraClient(obj);
}
}
return intr;
}
ICameraClient::ICameraClient() { }
ICameraClient::~ICameraClient() { }
总结一下, 如果interface_cast的参数obj是BnInterface,则返回其自身,如果参数obj是BpInterface,则new一个Bp代理对象并返回。这里我们用的是ICameraClient例子来讲解的,则返回BpCameraClient,别的接口也是同样分析的,比如IServiceManager,也会有类似声明如下,则返回BpServiceManager。
//frameworks\native\include\binder\IServiceManager.h
DECLARE_META_INTERFACE(ServiceManager);
//frameworks\native\libs\binder\IServiceManager.cpp
IMPLEMENT_META_INTERFACE(ServiceManager, "android.os.IServiceManager");
asBinder
接着使用上面ICameraClient例子进行分析一下:
//伪代码,根据interface_cast的分析,知道cameraClient即为BpCameraClient(BpBinder(handle))
sp<ICameraClient> cameraClient = interface_cast<ICameraClient>(data.readStrongBinder());
IInterface::asBinder(cameraClient);
看下asBinder的方法,在IInterface.cpp中
sp<IBinder> IInterface::asBinder(const IInterface* iface)
{
if (iface == NULL) return NULL;
return const_cast<IInterface*>(iface)->onAsBinder();
}
sp<IBinder> IInterface::asBinder(const sp<IInterface>& iface)
{
if (iface == NULL) return NULL;
return iface->onAsBinder();
}
都会走到onAsBinder方法
BnInterface
BnInterface的onAsBinder方法,直接返回自身,因为BnInterface继承自BBinder,而BBinder又继承自IBinder
template<typename INTERFACE>
IBinder* BnInterface<INTERFACE>::onAsBinder()
{
return this;
}
根据例子展开为:
template<typename ICameraClient>
IBinder* BnInterface<ICameraClient>::onAsBinder()
{
return this;
}
BpInterface
BpInterface的onAsBinder方法,调用remote()方法并返回
template<typename INTERFACE>
inline IBinder* BpInterface<INTERFACE>::onAsBinder()
{
return remote();
}
根据例子展开为:
template<typename ICameraClient >
inline IBinder* BpInterface<ICameraClient>::onAsBinder()
{
return remote();
}
remote()方法在其父类BpRefBase中实现,就是返回mRemote变量
inline IBinder* remote() { return mRemote; }
而mRemote变量是在创建BpInterface对象时,将remote变量传给了其父类BpRefBase,我们这个例子里面remote就是BpBinder(handle)
template
inline BpInterface::BpInterface(const sp& remote)
: BpRefBase(remote)
{
}
BpRefBase::BpRefBase(const sp& o)
: mRemote(o.get()), mRefs(NULL), mState(0)
{
extendObjectLifetime(OBJECT_LIFETIME_WEAK);
if (mRemote) {
mRemote->incStrong(this); // Removed on first IncStrong().
mRefs = mRemote->createWeak(this); // Held for our entire lifetime.
}
}
“`
总结一下, 如果asBinder的参数iface是BnInterface类型,则返回其自身,如果参数iface是BpInterface类型,则返回其mRemote远程代理对象BpBinder(handle) 。
作者:lb377463323
出处:
http://blog.csdn.net/lb377463323
原文链接:
http://blog.csdn.net/lb377463323/article/details/78385845
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