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Glide通过生命周期取消加载
生命周期回调过程
onStop
--->RequestManager.onStop
-->RequestTracker.pauseRequest
--> SingleRequest.pause
--> SingleRequest.cancel
---> Engine.cancel
---> EngineJob.removeCallback
---> 如果所有的回调监听都移除了
--> DecodeJob.cancel
//EngineJob.class
synchronized void removeCallback(ResourceCallback cb) {
stateVerifier.throwIfRecycled();
cbs.remove(cb);
if (cbs.isEmpty()) {
cancel();
boolean isFinishedRunning = hasResource || hasLoadFailed;
if (isFinishedRunning && pendingCallbacks.get() == 0) {
release();
}
}
}
- 如果任务没有执行，就从队列里移除，取消任务
- 如果任务已经执行，就利用isCancelled标记在停止流程
//DecodeJob.class
public void cancel() {
isCancelled = true;
DataFetcherGenerator local = currentGenerator;
if (local != null) {
local.cancel();
}
}
上面最终会调用到具体的获取图片的任务，比如从网络获取图片的HttpUrlFetcher
//HttpUrlFetcher.class
private volatile boolean isCancelled;
@Override
public void cancel() {
// TODO: we should consider disconnecting the url connection here, but we can't do so
// directly because cancel is often called on the main thread.
isCancelled = true;
}
//内部是通过HttpURLConnection来从网络获取数据
private InputStream loadDataWithRedirects(
URL url, int redirects, URL lastUrl, Map<String, String> headers) throws HttpException {
...
urlConnection = buildAndConfigureConnection(url, headers);
try {
// Connect explicitly to avoid errors in decoders if connection fails.
urlConnection.connect();
// Set the stream so that it's closed in cleanup to avoid resource leaks. See #2352.
stream = urlConnection.getInputStream();
} catch (IOException e) {
...
}
if (isCancelled) {
return null;
}
final int statusCode = getHttpStatusCodeOrInvalid(urlConnection);
if (isHttpOk(statusCode)) {
return getStreamForSuccessfulRequest(urlConnection);
} else if (isHttpRedirect(statusCode)) {
String redirectUrlString = urlConnection.getHeaderField(REDIRECT_HEADER_FIELD);
...
cleanup();
return loadDataWithRedirects(redirectUrl, redirects + 1, url, headers);
} else if (statusCode == INVALID_STATUS_CODE) {
throw new HttpException(statusCode);
} else {
...
}
}
由于cancel方法一般是在主线程调用的，所以这里采用的是volatile关键字这种方式，在正式获取网络数据时会进行拦截；
- 如果拦截到了，那直接返回null；如果没拦截到，就获取到数据
- 以上最终都会回调到DecodeJob的onDataFetcherReady方法
onDataFetcherReady ---> decodeFromRetrievedData ---> decodeFromData
private void decodeFromRetrievedData() {
...
Resource<R> resource = null;
try {
resource = decodeFromData(currentFetcher, currentData, currentDataSource);
} catch (GlideException e) {
...
}
if (resource != null) {
notifyEncodeAndRelease(resource, currentDataSource, isLoadingFromAlternateCacheKey);
} else {
//任务被拦截，尝试其他的加载方式
runGenerators();
}
}
decodeFromData中会进行判断，如果data为Null就直接返回Null（被拦截时会是null）,这个时候会执行runGenerators方法
runGenerators方法实际上就是加载流程的流转，比如先从文件中加载，文件中没有，就去网络加载，一个个去试这样。当然这里面肯定也有cancel拦截的
private void runGenerators() {
currentThread = Thread.currentThread();
startFetchTime = LogTime.getLogTime();
boolean isStarted = false;
//被cancel拦截就不会尝试其他加载方式，直接任务取消
while (!isCancelled
&& currentGenerator != null
&& !(isStarted = currentGenerator.startNext())) {
stage = getNextStage(stage);
currentGenerator = getNextGenerator();
if (stage == Stage.SOURCE) {
reschedule();
return;
}
}
// We've run out of stages and generators, give up.
if ((stage == Stage.FINISHED || isCancelled) && !isStarted) {
notifyFailed();
}
}
所以当加载数据的流程被拦截了，比如网络请求返回null，那到这里这个流程就拦截下来了，直接
notifyFailed任务失败了。
那如果网络加载的时候没有拦截住呢？data就不会为null，就会走notifyEncodeAndRelease方法，最终会一直走到EngineJob的notifyCallbacksOfResult方法，这个方法里面又会有cancel拦截。这样也就是数据加载前加载后都被拦截下来了
Glide中的线程池
自定义ThreadFactory，主要的一个功能是实现限制部分线程网络访问，利用的是严苛模式
@Override
public Thread newThread(@NonNull final Runnable runnable) {
Thread newThread =
delegate.newThread(
new Runnable() {
@Override
public void run() {
if (preventNetworkOperations) {
StrictMode.setThreadPolicy(
new ThreadPolicy.Builder().detectNetwork().penaltyDeath().build());
}
try {
runnable.run();
} catch (Throwable t) {
uncaughtThrowableStrategy.handle(t);
}
}
});
newThread.setName("glide-" + name + "-thread-" + threadNum.getAndIncrement());
return newThread;
}
线程池主要有这几种：
AnimationExecutor：加载动画相关，禁止访问网络；如果CPU核心数大于4，就是2个线程，否则是一个线程，核心线程数和最大线程数相同
diskCacheExecutor：从磁盘加载图片，禁止访问网络；线程数为1，核心线程数和最大线程数相同
sourceExecutor：可以访问网络，线程数跟CPU核心数有关，不大于4，核心线程数和最大线程数相同
newUnlimitedSourceExecutor：用于网络请求图片，没有核心线程，只有非核心线程，类似CacheThreadPoll
Glide做了线程池优化，核心线程也会遵循超时策略
public GlideExecutor build() {
if (TextUtils.isEmpty(name)) {
throw new IllegalArgumentException(
"Name must be non-null and non-empty, but given: " + name);
}
ThreadPoolExecutor executor =
new ThreadPoolExecutor(
corePoolSize,
maximumPoolSize,
/*keepAliveTime=*/ threadTimeoutMillis,
TimeUnit.MILLISECONDS,
new PriorityBlockingQueue<Runnable>(),
new DefaultThreadFactory(
threadFactory, name, uncaughtThrowableStrategy, preventNetworkOperations));
if (threadTimeoutMillis != NO_THREAD_TIMEOUT) {
executor.allowCoreThreadTimeOut(true);
}
return new GlideExecutor(executor);
}
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