linkToDeath机制了解和使用

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在学习Binder和AIDL的过程中遇到的一些有意思的事情~！
linkToDeath机制，我们先看看官网如何介绍：

When working with remote objects, you often want to find out when they are no longer valid. There are three ways this can be determined:
The [transact()](https://developer.android.com/reference/android/os/IBinder.html#transact(int, android.os.Parcel, android.os.Parcel, int))
method will throw a RemoteException
exception if you try to call it on an IBinder whose process no longer exists.
The pingBinder()
method can be called, and will return false if the remote process no longer exists.
The [linkToDeath()](https://developer.android.com/reference/android/os/IBinder.html#linkToDeath(android.os.IBinder.DeathRecipient, int))
method can be used to register a IBinder.DeathRecipient
with the IBinder, which will be called when its containing process goes away.

总结：我们可以通过三种方式来检测远程对象是否存活。

• 调用远程方法的时候捕获RemoteException(DeadObjectException)；
• 调用IBinder的pingBinder()进行检测；
• 实现IBinder.DeathRecipient接口回调；

Binder意外中断

往往是由于服务端进程意外停止了，这时我们需要重新连接服务。
那么我们可以使用linkToDeath机制，如果使用bindService那么还可以通过ServiceConnection.onServiceDisconnected方法进行重连。

捕获RemoteException

在调用远程服务的时候，如果服务挂掉，那么我们客户端会接受到抛出的RemoteException异常，监听该异常进行处理。

android.os.DeadObjectException
    at android.os.BinderProxy.transact(Native Method)
    at com.tzx.aidlinout.aidl.IBookManager$Stub$Proxy.addInBook(IBookManager.java:159)
    at com.tzx.aidlinout.MainActivity.onClick(MainActivity.java:117)
    at android.view.View.performClick(View.java:3514)
    at android.view.View$PerformClick.run(View.java:14125)
    at android.os.Handler.handleCallback(Handler.java:605)
    at android.os.Handler.dispatchMessage(Handler.java:92)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:4439)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:787)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:554)
    at dalvik.system.NativeStart.main(Native Method)

pingBinder进行检测

pingBinder()方法会返回当前远程服务的状态（true|false）

IBinder.DeathRecipient

实现了IBinder.DeathRecipient接口的参数调用linkToDeath()方法，可以在binderDied方法中处理中断逻辑。

binder.linkToDeath(new IBinder.DeathRecipient() {
    @Override
    public void binderDied() {
        Log.d("binder", "binderDied calling~!");
    }
}, 0);

总结

先看下边的log，从中我们可以总结出以上4中Binder中断处理方法的执行顺序：

linkToDeath > onServiceDisconnected > pingBinder > transact()

//调用远程服务
binder.pingBinder = true
Binder Thread #2    //远程服务的当前线程名称
//kill远程服务存在的进程
binderDied calling~!
onServiceDisconnected
binder.pingBinder = false
android.os.DeadObjectException
    at android.os.BinderProxy.transact(Native Method)
    at com.tzx.aidlinout.aidl.IBookManager$Stub$Proxy.addInBook(IBookManager.java:159)
    at com.tzx.aidlinout.MainActivity.onClick(MainActivity.java:117)
    at android.view.View.performClick(View.java:3514)
    at android.view.View$PerformClick.run(View.java:14125)
    at android.os.Handler.handleCallback(Handler.java:605)
    at android.os.Handler.dispatchMessage(Handler.java:92)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:4439)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:787)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:554)
    at dalvik.system.NativeStart.main(Native Method)

扩展

这里讲一个Android源码中的类：RemoteCallbackList。
它在内部对列表中的每一个数据实现了Callback。而Callback实现了IBinder.DeathRecipient接口。

public class RemoteCallbackList {
    ArrayMap mCallbacks
            = new ArrayMap();
    private Object[] mActiveBroadcast;
    private int mBroadcastCount = -1;
    private boolean mKilled = false;

    private final class Callback implements IBinder.DeathRecipient {
        final E mCallback;
        final Object mCookie;
        
        Callback(E callback, Object cookie) {
            mCallback = callback;
            mCookie = cookie;
        }

        public void binderDied() {
            synchronized (mCallbacks) {
                mCallbacks.remove(mCallback.asBinder());
            }
            onCallbackDied(mCallback, mCookie);
        }
    }
    /******其他代码省略*******/
}

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