前言
这几天突然很好奇volley的取消操作是如何实现的,就去看了下volley的源码,突然发现,volley的取消操作实现的很简单,很失望。然后突然想到另外一个很敏感的问题——内存泄露。
正文
首先来看下volley的源码吧
Request.cancel() && Request.isCanceled()
private boolean mCanceled;
/**
* Mark this request as canceled. No callback will be delivered.
*/
public void cancel() {
mCanceled = true;
}
/**
* Returns true if this request has been canceled.
*/
public boolean isCanceled() {
return mCanceled;
}
cancel的操作十分简单,就单纯地给mCanceled赋了个true值。
NetworkDispatcher
这个类在volley库中负责网络请求的运行,继承自Thread类。
NetworkDispatcher.run()
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
Request<?> request;
while (true) {
long startTimeMs = SystemClock.elapsedRealtime();
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
try {
// Take a request from the queue.
request = mQueue.take();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
try {
request.addMarker("network-queue-take");
// If the request was cancelled already, do not perform the
// network request.
if (request.isCanceled()) {
request.finish("network-discard-cancelled");
continue;
}
addTrafficStatsTag(request);
// Perform the network request.
NetworkResponse networkResponse = mNetwork.performRequest(request);
request.addMarker("network-http-complete");
// If the server returned 304 AND we delivered a response already,
// we're done -- don't deliver a second identical response.
if (networkResponse.notModified && request.hasHadResponseDelivered()) {
request.finish("not-modified");
continue;
}
// 缓存使用,只有没有网络的情况下,如果是返回失败,如果网络请求返回数据失败,怎么办,不刷新
// Parse the response here on the worker thread.
Response<?> response = request.parseNetworkResponse(networkResponse);
request.addMarker("network-parse-complete");
// Write to cache if applicable.
// TODO: Only update cache metadata instead of entire record for 304s.
if (networkResponse.statusCode == HttpStatus.SC_SERVICE_UNAVAILABLE && request.shouldCache()) {
Cache.Entry cacheEntry = mCache.get(request.getCacheKey());
if (cacheEntry != null) {
networkResponse = new NetworkResponse(cacheEntry.data, cacheEntry.responseHeaders);
response = request.parseNetworkResponse(networkResponse);
}
} else if (request.shouldCache() && response.cacheEntry != null) {
mCache.put(request.getCacheKey(), response.cacheEntry);
request.addMarker("network-cache-written");
}
// Post the response back.
request.markDelivered();
mDelivery.postResponse(request, response);
} catch (VolleyError volleyError) {
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
parseAndDeliverNetworkError(request, volleyError);
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
VolleyError volleyError = new VolleyError(e);
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
mDelivery.postError(request, volleyError);
}
}
}
从代码中可以看到,request在即将运行之前,首先会判断一次isCanceled(),假如我们在这之前已经将request取消了,那么request也就直接结束了,此时一般情况下不会出现内存泄露的情况,假如在那之后我们才调用cancel操作,那么我们继续往下看,此时run函数继续,request进入到performRequest阶段,然后最后postResponse。
我们会用到cancel操作的情况一般有:
1.不关心这个request的结果了,主动cancel,但是activity没有被关闭;
2.在activity关闭时,cancel掉所有这个activity发出的request请求。我们在使用request时,还需要传入Response.Listener和Response.ErrorListener的接口,用来当request请求结束时传数据。而我们在Response的接口中一般都要修改UI,这就意味着这个接口会持有对Activity的引用,所以这种情况下,Activity关闭,request虽然被cancel了,但是request还持有对Activity的引用,导致Activity的内存泄露。
解决
解决办法有两种:
1.新建一个自己的Request类
这里以StringRequest为例,那么直接上源码
public class MyStringRequest extends StringRequest{
public MyStringRequest(int method, String url, Response.Listener<String> listener, Response.ErrorListener errorListener) {
super(method, url, listener, errorListener);
}
public MyStringRequest(String url, Response.Listener<String> listener, Response.ErrorListener errorListener) {
super(url, listener, errorListener);
}
@Override
public void cancel(){
super.cancel();
onFinish();
}
}
onFinish函数会把listener直接设为空(onFinish函数为protected,所以只能在子类里调用了),而cancel操作会把errorListener设为空。
2.使用弱引用
现在的项目都是大部分使用的是MVP模式,一般情况下,我们是在Presenter中来实现Response的接口,使用的使用就直接XXXRequest request = new XXXRequest(method,url,this,this);
下面上改良好的代码:
MyResponseListener:
public class MyResponseListener<T extends BasePresenter> implements Response.Listener, Response.ErrorListener {
private WeakReference<Response.Listener> listenerRef;
private WeakReference<Response.ErrorListener> errorRef;
public MyResponseListener(Response.Listener listener, Response.ErrorListener error) {
listenerRef = new WeakReference<>(listener);
errorRef = new WeakReference<>(error);
}
@Override
public void onErrorResponse(VolleyError error) {
if (errorRef.get()!=null){
errorRef.get().onErrorResponse(error);
}
}
@Override
public void onResponse(Object response) {
if (listenerRef.get()!=null){
listenerRef.get().onResponse(response);
}
}
}
BasePresenter:
这里可以再做一层保护,自定义一个接口,让Presenter来实现这个接口,防止被人直接使用new XXXRequest(method,url,this,this)。
public abstract class BasePresenter implements Response.Listener, Response.ErrorListener {
private MyResponseListener listener;
public BasePresenter() {
initListener();
}
protected void initListener() {
listener = new MyResponseListener(this, this);
}
protected Response.Listener getListener(){
return listener;
}
protected Response.ErrorListener getErrorListener(){
return listener;
}
}
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