线程
AOS要求WebView的调用线程必须一致，否则会抛出异常，提示"All WebView methods must be called on the same thread. "。WebView在初始化时，mWebViewThread会记录当前的初始化线程，因为webview是一个UI控件，我们一般会在主线程中初始化，所以很多情况下这个成员变量就是当前的主线程。之后，在调用webview的相关方法时，都会首先调用checkThread()方法检测线程是否一致，即检测两个线程的looper对象。如果不相同，会有异常抛出。
private final Looper mWebViewThread = Looper.myLooper();
private void checkThread() {
// Ignore mWebViewThread == null because this can be called during in the super class
// constructor, before this class's own constructor has even started.
if (mWebViewThread != null && Looper.myLooper() != mWebViewThread) {
Throwable throwable = new Throwable(
"A WebView method was called on thread '" +
Thread.currentThread().getName() + "'. " +
"All WebView methods must be called on the same thread. " +
"(Expected Looper " + mWebViewThread + " called on " + Looper.myLooper() +
", FYI main Looper is " + Looper.getMainLooper() + ")");
Log.w(LOGTAG, Log.getStackTraceString(throwable));
StrictMode.onWebViewMethodCalledOnWrongThread(throwable);
if (sEnforceThreadChecking) {
throw new RuntimeException(throwable);
}
}
}
