session-ios/SignalServiceKit/src/Util/OWSBackgroundTask.m

//
//  Copyright (c) 2018 Open Whisper Systems. All rights reserved.
//

#import "OWSBackgroundTask.h"
#import "AppContext.h"
#import "Threading.h"

@interface OWSBackgroundTask ()

@property (nonatomic, readonly) NSString *label;

// This property should only be accessed while synchronized on this instance.
@property (nonatomic) UIBackgroundTaskIdentifier backgroundTaskId;

// This property should only be accessed while synchronized on this instance.
@property (nonatomic, nullable) BackgroundTaskCompletionBlock completionBlock;

@end

#pragma mark -

@implementation OWSBackgroundTask

+ (OWSBackgroundTask *)backgroundTaskWithLabelStr:(const char *)labelStr
{
    OWSAssert(labelStr);

    NSString *label = [NSString stringWithFormat:@"%s", labelStr];
    return [[OWSBackgroundTask alloc] initWithLabel:label completionBlock:nil];
}

+ (OWSBackgroundTask *)backgroundTaskWithLabelStr:(const char *)labelStr
                                  completionBlock:(BackgroundTaskCompletionBlock)completionBlock
{

    OWSAssert(labelStr);

    NSString *label = [NSString stringWithFormat:@"%s", labelStr];
    return [[OWSBackgroundTask alloc] initWithLabel:label completionBlock:completionBlock];
}

+ (OWSBackgroundTask *)backgroundTaskWithLabel:(NSString *)label
{
    return [[OWSBackgroundTask alloc] initWithLabel:label completionBlock:nil];
}

+ (OWSBackgroundTask *)backgroundTaskWithLabel:(NSString *)label
                               completionBlock:(BackgroundTaskCompletionBlock)completionBlock
{
    return [[OWSBackgroundTask alloc] initWithLabel:label completionBlock:completionBlock];
}

- (instancetype)initWithLabel:(NSString *)label completionBlock:(BackgroundTaskCompletionBlock _Nullable)completionBlock
{
    self = [super init];

    if (!self) {
        return self;
    }

    OWSAssert(label.length > 0);

    _label = label;
    self.completionBlock = completionBlock;

    [self startBackgroundTask];

    return self;
}

- (void)dealloc
{
    [self endBackgroundTask];
}

- (void)startBackgroundTask
{
    // beginBackgroundTaskWithExpirationHandler must be called on the main thread.
    __weak typeof(self) weakSelf = self;
    self.backgroundTaskId = [CurrentAppContext() beginBackgroundTaskWithExpirationHandler:^{
        // Supposedly [UIApplication beginBackgroundTaskWithExpirationHandler]'s handler
        // will always be called on the main thread, but in practice we've observed
        // otherwise.  We use DispatchSyncMainThreadSafe() (note the sync) to ensure that
        // this work is done on the main thread.
        //
        // See: https://developer.apple.com/documentation/uikit/uiapplication/1623031-beginbackgroundtaskwithexpiratio)
        //
        // Note the usage of OWSCAssert() to avoid capturing a reference to self.
        OWSCAssert([NSThread isMainThread]);

        DispatchSyncMainThreadSafe(^{
            OWSBackgroundTask *strongSelf = weakSelf;
            if (!strongSelf) {
                return;
            }

            // Make a local copy of completionBlock to ensure that it is called
            // exactly once.
            BackgroundTaskCompletionBlock _Nullable completionBlock = nil;

            @synchronized(strongSelf)
            {
                if (strongSelf.backgroundTaskId == UIBackgroundTaskInvalid) {
                    return;
                }
                DDLogInfo(@"%@ %@ background task expired.", strongSelf.logTag, strongSelf.label);
                strongSelf.backgroundTaskId = UIBackgroundTaskInvalid;

                completionBlock = strongSelf.completionBlock;
                strongSelf.completionBlock = nil;
            }

            if (completionBlock) {
                completionBlock(BackgroundTaskState_Expired);
            }
        });
    }];

    // If a background task could not be begun, call the completion block.
    if (self.backgroundTaskId == UIBackgroundTaskInvalid) {

        DDLogInfo(@"%@ %@ background task could not be started.", self.logTag, self.label);

        // Make a local copy of completionBlock to ensure that it is called
        // exactly once.
        BackgroundTaskCompletionBlock _Nullable completionBlock;
        @synchronized(self)
        {
            completionBlock = self.completionBlock;
            self.completionBlock = nil;
        }
        if (completionBlock) {
            completionBlock(BackgroundTaskState_CouldNotStart);
        }
    }
}

- (void)endBackgroundTask
{
    // Make a local copy of this state, since this method is called by `dealloc`.
    UIBackgroundTaskIdentifier backgroundTaskId;
    BackgroundTaskCompletionBlock _Nullable completionBlock;

    @synchronized(self)
    {
        backgroundTaskId = self.backgroundTaskId;
        completionBlock = self.completionBlock;
        self.completionBlock = nil;
    }

    if (backgroundTaskId == UIBackgroundTaskInvalid) {
        OWSAssert(!completionBlock);
        return;
    }

    // endBackgroundTask must be called on the main thread.
    DispatchMainThreadSafe(^{

        if (completionBlock) {
            completionBlock(BackgroundTaskState_Success);
        }

        if (backgroundTaskId != UIBackgroundTaskInvalid) {
            [CurrentAppContext() endBackgroundTask:backgroundTaskId];
        }
    });
}

@end