背景
最近在看SurfaceSyncGroup，很多代码还是很好理解的，但是其中有几处细节，可能你看的时候没有注意。
因为这个SurfaceSyncGroup名字太长，我后面都会使用SSG缩写代替。
一、跨进程使用SSG，无法支持Child SSG添加Child SSG
如果你看过代码，你应该可以发现，如果你在同一进程中使用SSG，其实是可以如下操作的。
但是如果跨进程使用，那就要注意了，可能就会有问题了，你以为你创建的是这样子关系
其实你创建的关系和你预期的不一样。
代码分析1.1
public boolean add(ISurfaceSyncGroup surfaceSyncGroup, boolean parentSyncGroupMerge,
@Nullable Runnable runnable) {
//1.1如果添加的surfaceSyncGroup和当前SurfaceSyncGroup不是同一进程，就是false。
if (isLocalBinder(surfaceSyncGroup.asBinder())) {
boolean didAddLocalSync = addLocalSync(surfaceSyncGroup, parentSyncGroupMerge);
if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) {
Trace.asyncTraceForTrackEnd(Trace.TRACE_TAG_VIEW, mTrackName, hashCode());
}
return didAddLocalSync;
}
synchronized (mLock) {
if (!mHasWMSync) {
// We need to add a signal into WMS since WMS will be creating a new parent
// SurfaceSyncGroup. When the parent SSG in WMS completes, only then do we
// notify the registered listeners that the entire SurfaceSyncGroup is complete.
// This is because the callers don't realize that when adding a different process
// to this SSG, it isn't actually adding to this SSG and really just creating a
// link in WMS. Because of this, the callers would expect the complete listeners
// to only be called when everything, including the other process's
// SurfaceSyncGroups, have completed. Only WMS has that info so we need to send the
// listener to WMS when we set up a server side sync.
mSurfaceSyncGroupCompletedListener = new ISurfaceSyncGroupCompletedListener.Stub() {
@Override
public void onSurfaceSyncGroupComplete() {
synchronized (mLock) {
invokeSyncCompleteCallbacks();
}
}
};
// 1.1.2 然后用自己的token在WM中创建一个替身
if (!addSyncToWm(mToken, false /* parentSyncGroupMerge */,
mSurfaceSyncGroupCompletedListener)) {//跳转到代码分析1.2
mSurfaceSyncGroupCompletedListener = null;
if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) {
Trace.asyncTraceForTrackEnd(Trace.TRACE_TAG_VIEW, mTrackName, hashCode());
}
return false;
}
mHasWMSync = true;
}
}
try {
// 1.1.3 将token传递给子ssg，然后让子的ssg和你的替身建立关系
surfaceSyncGroup.onAddedToSyncGroup(mToken, parentSyncGroupMerge);
} catch (RemoteException e) {
if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) {
Trace.asyncTraceForTrackEnd(Trace.TRACE_TAG_VIEW, mTrackName, hashCode());
}
return false;
}
if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) {
Trace.asyncTraceForTrackEnd(Trace.TRACE_TAG_VIEW, mTrackName, hashCode());
}
return true;
}
1.1.1 如果添加的surfaceSyncGroup和当前SurfaceSyncGroup不是同一进程，就是false。
1.1.2 然后用自己的token在WM中创建一个替身ssg
1.1.3 将token传递给子ssg，然后让子的ssg和你的替身建立关系
代码分析1.2
当SSG1调用add(SSG2)的时候，SSG1和SSG2都会触发这个函数。
1.2.1 将自己和WM中的替身建立起关系。如果是SSG1，也就意味着和之前Root SSG建立的关系也就断开了。
private boolean addSyncToWm(IBinder token, boolean parentSyncGroupMerge,
@Nullable ISurfaceSyncGroupCompletedListener surfaceSyncGroupCompletedListener) {
try {
if (DEBUG) {
Log.d(TAG, "Attempting to add remote sync to " + mName
+ ". Setting up Sync in WindowManager.");
}
if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) {
Trace.asyncTraceForTrackBegin(Trace.TRACE_TAG_VIEW, mTrackName,
"addSyncToWm=" + token.hashCode(), hashCode());
}
AddToSurfaceSyncGroupResult addToSyncGroupResult = new AddToSurfaceSyncGroupResult();
if (!WindowManagerGlobal.getWindowManagerService().addToSurfaceSyncGroup(token,
parentSyncGroupMerge, surfaceSyncGroupCompletedListener,
addToSyncGroupResult)) {
if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) {
Trace.asyncTraceForTrackEnd(Trace.TRACE_TAG_VIEW, mTrackName, hashCode());
}
return false;
}
//1.2.1 将自己和WM中的替身建立起关系。如果是SSG1，也就意味着和之前Root SSG建立的关系也就断开了。
setTransactionCallbackFromParent(addToSyncGroupResult.mParentSyncGroup,
addToSyncGroupResult.mTransactionReadyCallback);
} catch (RemoteException e) {
if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) {
Trace.asyncTraceForTrackEnd(Trace.TRACE_TAG_VIEW, mTrackName, hashCode());
}
return false;
}
if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) {
Trace.asyncTraceForTrackEnd(Trace.TRACE_TAG_VIEW, mTrackName, hashCode());
}
return true;
}
最终的结果如下图
当SSG1添加到Root SSG的时候，一切还算正常
但是当SSG2添加到SSG1的时候，意外就发生了，Root SSG和SSG1就断开了连接。
二、如果SSG1添加到Root SSG之后，将SSG1添加到 SSG2，会发生什么
代码分析2.1
private void setTransactionCallbackFromParent(ISurfaceSyncGroup parentSyncGroup,
ITransactionReadyCallback transactionReadyCallback) {
...
synchronized (mLock) {
if (mFinished) {
finished = true;
} else {
// If this SurfaceSyncGroup was already added to a different SurfaceSyncGroup, we
// need to combine everything. We can add the old SurfaceSyncGroup parent to the new
// parent so the new parent doesn't complete until the old parent does.
// Additionally, the old parent will not get the final transaction object and
// instead will send it to the new parent, ensuring that any other SurfaceSyncGroups
// from the original parent are also combined with the new parent SurfaceSyncGroup.
// 2.1.1 如果发现SSG1有parent SSG了， 就将parent ssg（Root SSG）添加到SSG2
if (mParentSyncGroup != null && mParentSyncGroup != parentSyncGroup) {
if (DEBUG) {
Log.d(TAG, "Trying to add to " + parentSyncGroup
+ " but already part of sync group " + mParentSyncGroup + " "
+ mName);
}
try {
//最终会跳转到代码分析 2.2
parentSyncGroup.addToSync(mParentSyncGroup,
true /* parentSyncGroupMerge */);
} catch (RemoteException e) {
}
}
if (DEBUG && mParentSyncGroup == parentSyncGroup) {
Log.d(TAG, "Added to parent that was already the parent");
}
// 2.1.2 保存前一个parent ssg（Root SSG）的callback。
Consumer<Transaction> lastCallback = mTransactionReadyConsumer;
// 2.1.3 将parent ssg 换成 SSG2
mParentSyncGroup = parentSyncGroup;
mTransactionReadyConsumer = (transaction) -> {
if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) {
Trace.asyncTraceForTrackBegin(Trace.TRACE_TAG_VIEW, mTrackName,
"Invoke transactionReadyCallback="
+ transactionReadyCallback.hashCode(), hashCode());
}
// 2.1.4 在SSG1完成任务的时候，虽然SSG1和Root SSG没有名义上的关系了，但是残留在Root SSG mPendingSyncs中SSG1的callback还是要触发一下的，否则会导致Root SSG无法ready。
lastCallback.accept(null);
try {
transactionReadyCallback.onTransactionReady(transaction);
} catch (RemoteException e) {
transaction.apply();
}
if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) {
Trace.asyncTraceForTrackEnd(Trace.TRACE_TAG_VIEW, mTrackName, hashCode());
}
};
addedToSyncListener = mAddedToSyncListener;
}
}
// Invoke the callback outside of the lock when the SurfaceSyncGroup being added was already
// complete.
if (finished) {
try {
transactionReadyCallback.onTransactionReady(null);
} catch (RemoteException e) {
}
} else if (addedToSyncListener != null) {
addedToSyncListener.run();
}
if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) {
Trace.asyncTraceForTrackEnd(Trace.TRACE_TAG_VIEW, mTrackName, hashCode());
}
}
2.1.1 如果发现SSG1有parent SSG了， 就将parent ssg（Root SSG）添加到SSG2
2.1.2 保存前一个parent ssg（Root SSG）的callback。
2.1.3 将parent ssg 换成 SSG2
2.1.4 在SSG1完成任务的时候，虽然SSG1和Root SSG没有名义上的关系了，但是残留在Root SSG mPendingSyncs中SSG1的callback还是要触发一下的，否则会导致Root SSG无法ready。
代码分析 2.2
代码分析2.1中parentSyncGroup.addToSync(mParentSyncGroup, true /* parentSyncGroupMerge */)最终会跳转到这里，关键的就是parentSyncGroupMerge为true
public ITransactionReadyCallback createTransactionReadyCallback(boolean parentSyncGroupMerge) {
ITransactionReadyCallback transactionReadyCallback =
new ITransactionReadyCallback.Stub() {
@Override
public void onTransactionReady(Transaction t) {
synchronized (mLock) {
if (t != null) {
t.sanitize(Binder.getCallingPid(), Binder.getCallingUid());
// When an older parent sync group is added due to a child syncGroup
// getting added to multiple groups, we need to maintain merge order
// so the older parentSyncGroup transactions are overwritten by
// anything in the newer parentSyncGroup.
// 2.2.1 这里解释了如果为true，为了保证merge的顺序，有一些处理。
if (parentSyncGroupMerge) {
t.merge(mTransaction);
}
mTransaction.merge(t);//p add c
}
mPendingSyncs.remove(this);
if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) {
Trace.instantForTrack(Trace.TRACE_TAG_VIEW, mTrackName,
"onTransactionReady callback=" + hashCode());
}
checkIfSyncIsComplete();
}
}
};
return transactionReadyCallback;
}
2.2.1 这里解释了如果为true，为了保证merge的顺序，有一些处理，用一个图来表示，就是如下图。
三、总结
SurfaceSyncGroup代码不是很多，但是使用时候要注意这些细节，避免发生错误，具体讲解SurfaceSyncGroup配合BBQ和BlastSyncEngine工作，我还是会选择使用图解视频的方式，给自己留个坑《王小二图解Android-SurfaceSyncGroup》
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