NSProgress instead of blocks

[kean]

Investigate NSProgress, might not be worth the effort.

[kean]

• Add NSProgress to DFImageTask
• Support task cancellation using NSProgress object
• Support implicit progress composition
• DFImageFetching progress handler should have two arguments: completed and total unit count
• Support progress tracking for DFPhotosKitImageFetcher
• _DFImageFetchOperation should propagate current completed and total unit count when new image tasks are added to it

[kean]

Implicit progress composition is supported, here's an example:

NSProgress *progress = [NSProgress progressWithTotalUnitCount:100];

[progress becomeCurrentWithPendingUnitCount:50];
DFImageTask *task1 = [_manager imageTaskForRequest:[DFImageRequest requestWithResource:[TDFMockResource resourceWithID:@"1"]] completion:nil];
[task1 progress]; // force progress creation
[progress resignCurrent];

[progress becomeCurrentWithPendingUnitCount:50];
DFImageTask *task2 = [_manager imageTaskForRequest:[DFImageRequest requestWithResource:[TDFMockResource resourceWithID:@"2"]] completion:nil];
[task2 progress];
[progress resignCurrent];

double __block fractionCompleted = 0;
[self keyValueObservingExpectationForObject:progress keyPath:@"fractionCompleted" handler:^BOOL(NSProgress *observedObject, NSDictionary *change) {
    fractionCompleted += 0.25;
    XCTAssertEqual(fractionCompleted, observedObject.fractionCompleted);
    return observedObject.fractionCompleted == 1;
}];

[task1 resume];
[task2 resume];

[self waitForExpectationsWithTimeout:1 handler:nil];

[kean]

Done

[kean]

Update: progress is now created lazily