关于android的UiAutomation求帮忙看下代码

Question

在初始化UiAutomation的时候返回了一个null值 看API是通过Instrumentation.getUiAutomation()来获取UiAutomation的实例的啊

 public UiAutomation getUiAutomation() {
        if (mUiAutomationConnection != null) {
            if (mUiAutomation == null) {
                mUiAutomation = new UiAutomation(getTargetContext().getMainLooper(),
                        mUiAutomationConnection);
                mUiAutomation.connect();
            }
            return mUiAutomation;
        }
        return null;
    }

这是API中的其中 一段返回null应该是mUiAutomationConnection==null

mUiAutomationConnection是在

/*package*/ final void init(ActivityThread thread,
            Context instrContext, Context appContext, ComponentName component, 
            IInstrumentationWatcher watcher, IUiAutomationConnection uiAutomationConnection) {
        mThread = thread;
        mMessageQueue = mThread.getLooper().myQueue();
        mInstrContext = instrContext;
        mAppContext = appContext;
        mComponent = component;
        mWatcher = watcher;
        mUiAutomationConnection = uiAutomationConnection;
    }

里面被赋值的到这里就不太明白了 这是怎么回事啊 有没有大神给解释下啊