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Experiment with different strategies for image downloader priority (T…
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…extureGroup#1349)

Right now when an image node enters preload state, we kick off an image request with the default priority. Then when it enters display state, we change the priority to "imminent" which is mapped to the default priority as well. This means that requests from preload and display nodes have the same priority and are put to the same pool. The right behavior would be that preload requests should have a lower priority from the beginning.

Another problem is that, due to the execution order of -didEnter(Preload|Display|Visible)State calls, a node may kick off a low priority request when it enters preload state even though it knows that it's also visible. By the time -didEnterVisibleState is called, the low priority request may have already been consumed and the download/data task won't pick up the new higher priority, or some work needs to be done to move it to another queue. A better behavior would be to always use the current interface state to determine the priority. This means that visible nodes will kick off high priority requests as soon as -didEnterPreloadState is called.

The last (and smaller) issue is that a node marks its request as preload/low priority as soon as it exits visible state. I'd argue that this is too agressive. It may be reasonble for nodes in the trailing direction. Even so, we already handle this case by (almost always) have smaller trailing buffers. So this diff makes sure that nodes that exited visible state will have imminent/default priority if they remain in the display range.

All of these new behaviors are wrapped in an experiment and will be tested carefully before being rolled out.

* Add imports

* Fix build failure

* Encapsulate common logics into methods

* Address comments
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@nguyenhuy @hebertialmeida
nguyenhuy authored and hebertialmeida committed May 10, 2019
1 parent 5b1e14c commit 0547eaa
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39 changes: 16 additions & 23 deletions Podfile.lock
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Original file line number Diff line number Diff line change
@@ -1,38 +1,32 @@
PODS:
- FBSnapshotTestCase/Core (2.1.4)
- FLAnimatedImage (1.0.12)
- JGMethodSwizzler (2.0.1)
- OCMock (3.4.1)
- PINCache (3.0.1-beta.6):
- PINCache/Arc-exception-safe (= 3.0.1-beta.6)
- PINCache/Core (= 3.0.1-beta.6)
- PINCache/Arc-exception-safe (3.0.1-beta.6):
- PINCache (3.0.1-beta.7):
- PINCache/Arc-exception-safe (= 3.0.1-beta.7)
- PINCache/Core (= 3.0.1-beta.7)
- PINCache/Arc-exception-safe (3.0.1-beta.7):
- PINCache/Core
- PINCache/Core (3.0.1-beta.6):
- PINOperation (~> 1.1.0)
- PINCache/Core (3.0.1-beta.7):
- PINOperation (~> 1.1.1)
- PINOperation (1.1.1)
- PINRemoteImage (3.0.0-beta.13):
- PINRemoteImage/FLAnimatedImage (= 3.0.0-beta.13)
- PINRemoteImage/PINCache (= 3.0.0-beta.13)
- PINRemoteImage/Core (3.0.0-beta.13):
- PINRemoteImage (3.0.0-beta.14):
- PINRemoteImage/PINCache (= 3.0.0-beta.14)
- PINRemoteImage/Core (3.0.0-beta.14):
- PINOperation
- PINRemoteImage/FLAnimatedImage (3.0.0-beta.13):
- FLAnimatedImage (>= 1.0)
- PINRemoteImage/Core
- PINRemoteImage/PINCache (3.0.0-beta.13):
- PINCache (= 3.0.1-beta.6)
- PINRemoteImage/PINCache (3.0.0-beta.14):
- PINCache (= 3.0.1-beta.7)
- PINRemoteImage/Core

DEPENDENCIES:
- FBSnapshotTestCase/Core (~> 2.1)
- JGMethodSwizzler (from `https://github.com/JonasGessner/JGMethodSwizzler`, branch `master`)
- OCMock (= 3.4.1)
- PINRemoteImage (= 3.0.0-beta.13)
- PINRemoteImage (= 3.0.0-beta.14)

SPEC REPOS:
https://github.com/cocoapods/specs.git:
- FBSnapshotTestCase
- FLAnimatedImage
- OCMock
- PINCache
- PINOperation
Expand All @@ -50,13 +44,12 @@ CHECKOUT OPTIONS:

SPEC CHECKSUMS:
FBSnapshotTestCase: 094f9f314decbabe373b87cc339bea235a63e07a
FLAnimatedImage: 4a0b56255d9b05f18b6dd7ee06871be5d3b89e31
JGMethodSwizzler: 7328146117fffa8a4038c42eb7cd3d4c75006f97
OCMock: 2cd0716969bab32a2283ff3a46fd26a8c8b4c5e3
PINCache: d195fdba255283f7e9900a55e3cced377f431f9b
PINCache: 7cb9ae068c8f655717f7c644ef1dff9fd573e979
PINOperation: a6219e6fc9db9c269eb7a7b871ac193bcf400aac
PINRemoteImage: d6d51c5d2adda55f1ce30c96e850b6c4ebd2856a
PINRemoteImage: 81bbff853acc71c6de9e106e9e489a791b8bbb08

PODFILE CHECKSUM: 42715d61f73cc22cc116bf80d7b268cb1f9e4742
PODFILE CHECKSUM: 445046ac151568c694ff286684322273f0b597d6

COCOAPODS: 1.5.3
COCOAPODS: 1.6.0
3 changes: 2 additions & 1 deletion Schemas/configuration.json
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Original file line number Diff line number Diff line change
Expand Up @@ -25,7 +25,8 @@
"exp_disable_a11y_cache",
"exp_skip_a11y_wait",
"exp_new_default_cell_layout_mode",
"exp_dispatch_apply"
"exp_dispatch_apply",
"exp_image_downloader_priority"
]
}
}
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1 change: 1 addition & 0 deletions Source/ASExperimentalFeatures.h
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,7 @@ typedef NS_OPTIONS(NSUInteger, ASExperimentalFeatures) {
ASExperimentalSkipAccessibilityWait = 1 << 10, // exp_skip_a11y_wait
ASExperimentalNewDefaultCellLayoutMode = 1 << 11, // exp_new_default_cell_layout_mode
ASExperimentalDispatchApply = 1 << 12, // exp_dispatch_apply
ASExperimentalImageDownloaderPriority = 1 << 13, // exp_image_downloader_priority
ASExperimentalFeatureAll = 0xFFFFFFFF
};

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5 changes: 3 additions & 2 deletions Source/ASExperimentalFeatures.mm
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Original file line number Diff line number Diff line change
Expand Up @@ -24,8 +24,9 @@
@"exp_disable_a11y_cache",
@"exp_skip_a11y_wait",
@"exp_new_default_cell_layout_mode",
@"exp_dispatch_apply"]));

@"exp_dispatch_apply",
@"exp_image_downloader_priority"]));

if (flags == ASExperimentalFeatureAll) {
return allNames;
}
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164 changes: 104 additions & 60 deletions Source/ASMultiplexImageNode.mm
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Original file line number Diff line number Diff line change
Expand Up @@ -53,7 +53,13 @@ @interface ASMultiplexImageNode ()
@private
// Core.
id<ASImageCacheProtocol> _cache;

id<ASImageDownloaderProtocol> _downloader;
struct {
unsigned int downloaderImplementsSetProgress:1;
unsigned int downloaderImplementsSetPriority:1;
unsigned int downloaderImplementsDownloadWithPriority:1;
} _downloaderFlags;

__weak id<ASMultiplexImageNodeDelegate> _delegate;
struct {
Expand Down Expand Up @@ -89,8 +95,6 @@ @interface ASMultiplexImageNode ()
BOOL _shouldRenderProgressImages;

//set on init only
BOOL _downloaderImplementsSetProgress;
BOOL _downloaderImplementsSetPriority;
BOOL _cacheSupportsClearing;
}

Expand Down Expand Up @@ -171,13 +175,14 @@ - (instancetype)initWithCache:(id<ASImageCacheProtocol>)cache downloader:(id<ASI
_cache = (id<ASImageCacheProtocol>)cache;
_downloader = (id<ASImageDownloaderProtocol>)downloader;

_downloaderImplementsSetProgress = [downloader respondsToSelector:@selector(setProgressImageBlock:callbackQueue:withDownloadIdentifier:)];
_downloaderImplementsSetPriority = [downloader respondsToSelector:@selector(setPriority:withDownloadIdentifier:)];
_downloaderFlags.downloaderImplementsSetProgress = [downloader respondsToSelector:@selector(setProgressImageBlock:callbackQueue:withDownloadIdentifier:)];
_downloaderFlags.downloaderImplementsSetPriority = [downloader respondsToSelector:@selector(setPriority:withDownloadIdentifier:)];
_downloaderFlags.downloaderImplementsDownloadWithPriority = [downloader respondsToSelector:@selector(downloadImageWithURL:priority:callbackQueue:downloadProgress:completion:)];

_cacheSupportsClearing = [cache respondsToSelector:@selector(clearFetchedImageFromCacheWithURL:)];

_shouldRenderProgressImages = YES;

self.shouldBypassEnsureDisplay = YES;

return self;
Expand Down Expand Up @@ -271,49 +276,34 @@ - (BOOL)placeholderShouldPersist
- (void)displayWillStartAsynchronously:(BOOL)asynchronously
{
[super displayWillStartAsynchronously:asynchronously];

[self didEnterPreloadState];

if (_downloaderImplementsSetPriority) {
{
ASDN::MutexLocker l(_downloadIdentifierLock);
if (_downloadIdentifier != nil) {
[_downloader setPriority:ASImageDownloaderPriorityImminent withDownloadIdentifier:_downloadIdentifier];
}
}
}
[self _updatePriorityOnDownloaderIfNeededWithDefaultPriority:ASImageDownloaderPriorityImminent];
}

/* didEnterVisibleState / didExitVisibleState in ASNetworkImageNode has a very similar implementation. Changes here are likely necessary
in ASNetworkImageNode as well. */
- (void)didEnterVisibleState
{
[super didEnterVisibleState];

if (_downloaderImplementsSetPriority) {
ASDN::MutexLocker l(_downloadIdentifierLock);
if (_downloadIdentifier != nil) {
[_downloader setPriority:ASImageDownloaderPriorityVisible withDownloadIdentifier:_downloadIdentifier];
}
}

[self _updatePriorityOnDownloaderIfNeededWithDefaultPriority:ASImageDownloaderPriorityVisible];
[self _updateProgressImageBlockOnDownloaderIfNeeded];
}

- (void)didExitVisibleState
{
[super didExitVisibleState];

if (_downloaderImplementsSetPriority) {
ASDN::MutexLocker l(_downloadIdentifierLock);
if (_downloadIdentifier != nil) {
[_downloader setPriority:ASImageDownloaderPriorityPreload withDownloadIdentifier:_downloadIdentifier];
}
}

[self _updatePriorityOnDownloaderIfNeededWithDefaultPriority:ASImageDownloaderPriorityPreload];
[self _updateProgressImageBlockOnDownloaderIfNeeded];
}

- (void)didExitDisplayState
{
[super didExitDisplayState];
if (ASActivateExperimentalFeature(ASExperimentalImageDownloaderPriority)) {
[self _updatePriorityOnDownloaderIfNeededWithDefaultPriority:ASImageDownloaderPriorityPreload];
}
}

#pragma mark - Core

- (void)setImage:(UIImage *)image
Expand Down Expand Up @@ -449,7 +439,6 @@ - (void)_setDownloadIdentifier:(id)downloadIdentifier
_downloadIdentifier = downloadIdentifier;
}


#pragma mark - Image Loading Machinery

- (void)_loadImageIdentifiers
Expand Down Expand Up @@ -493,19 +482,37 @@ - (UIImage *)_bestImmediatelyAvailableImageFromDataSource:(id *)imageIdentifierO

#pragma mark -

/**
@note: This should be called without _downloadIdentifierLock held. We will lock
super to read our interface state and it's best to avoid acquiring both locks.
*/
- (void)_updatePriorityOnDownloaderIfNeededWithDefaultPriority:(ASImageDownloaderPriority)defaultPriority
{
ASAssertUnlocked(_downloadIdentifierLock);

if (_downloaderFlags.downloaderImplementsSetPriority) {
// Read our interface state before locking so that we don't lock super while holding our lock.
ASInterfaceState interfaceState = self.interfaceState;
ASDN::MutexLocker l(_downloadIdentifierLock);

if (_downloadIdentifier != nil) {
ASImageDownloaderPriority priority = defaultPriority;
if (ASActivateExperimentalFeature(ASExperimentalImageDownloaderPriority)) {
priority = ASImageDownloaderPriorityWithInterfaceState(interfaceState);
}

[_downloader setPriority:priority withDownloadIdentifier:_downloadIdentifier];
}
}
}

- (void)_updateProgressImageBlockOnDownloaderIfNeeded
{
ASAssertUnlocked(_downloadIdentifierLock);

BOOL shouldRenderProgressImages = self.shouldRenderProgressImages;

// Read our interface state before locking so that we don't lock super while holding our lock.
ASInterfaceState interfaceState = self.interfaceState;
ASDN::MutexLocker l(_downloadIdentifierLock);

if (!_downloaderImplementsSetProgress || _downloadIdentifier == nil) {
if (!_downloaderFlags.downloaderImplementsSetProgress || _downloadIdentifier == nil) {
return;
}

Expand Down Expand Up @@ -825,7 +832,7 @@ - (void)_downloadImageWithIdentifier:(id)imageIdentifier URL:(NSURL *)imageURL c
[_delegate multiplexImageNode:self didStartDownloadOfImageWithIdentifier:imageIdentifier];

__weak __typeof__(self) weakSelf = self;
void (^downloadProgressBlock)(CGFloat) = nil;
ASImageDownloaderProgress downloadProgressBlock = NULL;
if (_delegateFlags.downloadProgress) {
downloadProgressBlock = ^(CGFloat progress) {
__typeof__(self) strongSelf = weakSelf;
Expand All @@ -835,30 +842,67 @@ - (void)_downloadImageWithIdentifier:(id)imageIdentifier URL:(NSURL *)imageURL c
};
}

ASImageDownloaderCompletion completion = ^(id <ASImageContainerProtocol> imageContainer, NSError *error, id downloadIdentifier, id userInfo) {
// We dereference iVars directly, so we can't have weakSelf going nil on us.
__typeof__(self) strongSelf = weakSelf;
if (!strongSelf)
return;

ASDN::MutexLocker l(strongSelf->_downloadIdentifierLock);
//Getting a result back for a different download identifier, download must not have been successfully canceled
if (ASObjectIsEqual(strongSelf->_downloadIdentifier, downloadIdentifier) == NO && downloadIdentifier != nil) {
return;
}

completionBlock([imageContainer asdk_image], error);

// Delegateify.
if (strongSelf->_delegateFlags.downloadFinish)
[strongSelf->_delegate multiplexImageNode:weakSelf didFinishDownloadingImageWithIdentifier:imageIdentifier error:error];
};

// Download!
ASPerformBlockOnBackgroundThread(^{
[self _setDownloadIdentifier:[_downloader downloadImageWithURL:imageURL
callbackQueue:dispatch_get_main_queue()
downloadProgress:downloadProgressBlock
completion:^(id <ASImageContainerProtocol> imageContainer, NSError *error, id downloadIdentifier, id userInfo) {
// We dereference iVars directly, so we can't have weakSelf going nil on us.
__typeof__(self) strongSelf = weakSelf;
if (!strongSelf)
return;

ASDN::MutexLocker l(_downloadIdentifierLock);
//Getting a result back for a different download identifier, download must not have been successfully canceled
if (ASObjectIsEqual(_downloadIdentifier, downloadIdentifier) == NO && downloadIdentifier != nil) {
return;
}

completionBlock([imageContainer asdk_image], error);

// Delegateify.
if (strongSelf->_delegateFlags.downloadFinish)
[strongSelf->_delegate multiplexImageNode:weakSelf didFinishDownloadingImageWithIdentifier:imageIdentifier error:error];
}]];
[self _updateProgressImageBlockOnDownloaderIfNeeded];
__typeof__(self) strongSelf = weakSelf;
if (!strongSelf)
return;

dispatch_queue_t callbackQueue = dispatch_get_main_queue();

id downloadIdentifier;
if (strongSelf->_downloaderFlags.downloaderImplementsDownloadWithPriority
&& ASActivateExperimentalFeature(ASExperimentalImageDownloaderPriority)) {

/*
Decide a priority based on the current interface state of this node.
It can happen that this method was called when the node entered preload state
but the interface state, at this point, tells us that the node is (going to be) visible,
If that's the case, we jump to a higher priority directly.
*/
ASImageDownloaderPriority priority = ASImageDownloaderPriorityWithInterfaceState(strongSelf.interfaceState);
downloadIdentifier = [strongSelf->_downloader downloadImageWithURL:imageURL
priority:priority
callbackQueue:callbackQueue
downloadProgress:downloadProgressBlock
completion:completion];
} else {
/*
Kick off a download with default priority.
The actual "default" value is decided by the downloader.
ASBasicImageDownloader and ASPINRemoteImageDownloader both use ASImageDownloaderPriorityImminent
which is mapped to NSURLSessionTaskPriorityDefault.
This means that preload and display nodes use the same priority
and their requests are put into the same pool.
*/
downloadIdentifier = [strongSelf->_downloader downloadImageWithURL:imageURL
callbackQueue:callbackQueue
downloadProgress:downloadProgressBlock
completion:completion];
}

[strongSelf _setDownloadIdentifier:downloadIdentifier];
[strongSelf _updateProgressImageBlockOnDownloaderIfNeeded];
});
}

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