[ProgressView] Backward animation support

components/ProgressView/examples/ProgressViewExample.m

@@ -33,8 +33,11 @@ @interface ProgressViewExample : UIViewController
 @property(nonatomic, strong) MDCProgressView *fullyColoredProgressView;
 @property(nonatomic, strong) UILabel *fullyColoredProgressLabel;
 
-@property(nonatomic, strong) MDCProgressView *backwardProgressView;
-@property(nonatomic, strong) UILabel *backwardProgressLabel;
+@property(nonatomic, strong) MDCProgressView *backwardProgressResetView;
+@property(nonatomic, strong) UILabel *backwardProgressResetLabel;
+
+@property(nonatomic, strong) MDCProgressView *backwardProgressAnimateView;
+@property(nonatomic, strong) UILabel *backwardProgressAnimateLabel;
 
 @end
 
@@ -64,11 +67,18 @@ - (void)setupProgressViews {
   // Hide the progress view at setup time.
   _fullyColoredProgressView.hidden = YES;
 
-  _backwardProgressView = [[MDCProgressView alloc] init];
-  _backwardProgressView.translatesAutoresizingMaskIntoConstraints = NO;
-  [self.view addSubview:_backwardProgressView];
+  _backwardProgressResetView = [[MDCProgressView alloc] init];
+  _backwardProgressResetView.translatesAutoresizingMaskIntoConstraints = NO;
+  [self.view addSubview:_backwardProgressResetView];
+  // Have a non-zero progress at setup time.
+  _backwardProgressResetView.progress = 0.33;
+
+  _backwardProgressAnimateView = [[MDCProgressView alloc] init];
+  _backwardProgressAnimateView.translatesAutoresizingMaskIntoConstraints = NO;
+  _backwardProgressAnimateView.backwardProgressAnimationMode = MDCProgressViewBackwardAnimationModeAnimate;
+  [self.view addSubview:_backwardProgressAnimateView];
   // Have a non-zero progress at setup time.
-  _backwardProgressView.progress = 0.33;
+  _backwardProgressAnimateView.progress = 0.33;
 }
 
 @end
@@ -111,12 +121,19 @@ - (void)setupLabels {
   _fullyColoredProgressLabel.translatesAutoresizingMaskIntoConstraints = NO;
   [self.view addSubview:_fullyColoredProgressLabel];
 
-  _backwardProgressLabel = [[UILabel alloc] init];
-  _backwardProgressLabel.text = @"Backward progress";
-  _backwardProgressLabel.font = [MDCTypography captionFont];
-  _backwardProgressLabel.alpha = [MDCTypography captionFontOpacity];
-  _backwardProgressLabel.translatesAutoresizingMaskIntoConstraints = NO;
-  [self.view addSubview:_backwardProgressLabel];
+  _backwardProgressResetLabel = [[UILabel alloc] init];
+  _backwardProgressResetLabel.text = @"Backward progress (reset)";
+  _backwardProgressResetLabel.font = [MDCTypography captionFont];
+  _backwardProgressResetLabel.alpha = [MDCTypography captionFontOpacity];
+  _backwardProgressResetLabel.translatesAutoresizingMaskIntoConstraints = NO;
+  [self.view addSubview:_backwardProgressResetLabel];
+
+  _backwardProgressAnimateLabel = [[UILabel alloc] init];
+  _backwardProgressAnimateLabel.text = @"Backward progress (animate)";
+  _backwardProgressAnimateLabel.font = [MDCTypography captionFont];
+  _backwardProgressAnimateLabel.alpha = [MDCTypography captionFontOpacity];
+  _backwardProgressAnimateLabel.translatesAutoresizingMaskIntoConstraints = NO;
+  [self.view addSubview:_backwardProgressAnimateLabel];
 }
 
 - (void)setupConstraints {
@@ -127,8 +144,10 @@ - (void)setupConstraints {
     @"tintedLabel" : _tintedProgressLabel,
     @"coloredView" : _fullyColoredProgressView,
     @"coloredLabel" : _fullyColoredProgressLabel,
-    @"backwardView" : _backwardProgressView,
-    @"backwardLabel" : _backwardProgressLabel,
+    @"backwardResetView" : _backwardProgressResetView,
+    @"backwardResetLabel" : _backwardProgressResetLabel,
+    @"backwardAnimateView" : _backwardProgressAnimateView,
+    @"backwardAnimateLabel" : _backwardProgressAnimateLabel,
   };
   NSDictionary *metrics = @{
     @"p" : @20,
@@ -141,7 +160,8 @@ - (void)setupConstraints {
                                                        "[stockView(==h)]-(p)-[stockLabel]-(s)-"
                                                        "[tintedView(==h)]-(p)-[tintedLabel]-(s)-"
                                                        "[coloredView(==h)]-(p)-[coloredLabel]-(s)-"
-                                                       "[backwardView(==h)]-(p)-[backwardLabel]"
+                                                       "[backwardResetView(==h)]-(p)-[backwardResetLabel]-(s)-"
+                                                       "[backwardAnimateView(==h)]-(p)-[backwardAnimateLabel]"
                                               options:0
                                               metrics:metrics
                                                 views:views];
@@ -152,11 +172,13 @@ - (void)setupConstraints {
     @"H:|-(p)-[stockView]-(p)-|",
     @"H:|-(p)-[tintedView]-(p)-|",
     @"H:|-(p)-[coloredView]-(p)-|",
-    @"H:|-(p)-[backwardView]-(p)-|",
+    @"H:|-(p)-[backwardResetView]-(p)-|",
+    @"H:|-(p)-[backwardAnimateView]-(p)-|",
     @"H:|-(>=p)-[stockLabel]-(>=p)-|",
     @"H:|-(>=p)-[tintedLabel]-(>=p)-|",
     @"H:|-(>=p)-[coloredLabel]-(>=p)-|",
-    @"H:|-(>=p)-[backwardLabel]-(>=p)-|",
+    @"H:|-(>=p)-[backwardResetLabel]-(>=p)-|",
+    @"H:|-(>=p)-[backwardAnimateLabel]-(>=p)-|",
   ];
   for (NSString *format in horizontalVisualFormats) {
     [horizontalConstraints
@@ -173,7 +195,8 @@ - (void)viewDidAppear:(BOOL)animated {
   [self animateStep1:_stockProgressView];
   [self animateStep1:_tintedProgressView];
   [self animateStep1:_fullyColoredProgressView];
-  [self animateBackwardProgressView];
+  [self animateBackwardProgressResetView];
+  [self animateBackwardProgressAnimateView];
 }
 
 - (void)animateStep1:(MDCProgressView *)progressView {
@@ -213,17 +236,30 @@ - (void)animateStep4:(MDCProgressView *)progressView {
                }];
 }
 
-- (void)animateBackwardProgressView {
+- (void)animateBackwardProgressResetView {
   __weak ProgressViewExample *weakSelf = self;
-  [_backwardProgressView setProgress:1 - _backwardProgressView.progress
+
+  [_backwardProgressResetView setProgress:1 - _backwardProgressResetView.progress
                             animated:YES
                           completion:^(BOOL finished) {
-                            [weakSelf performSelector:@selector(animateBackwardProgressView)
+                            [weakSelf performSelector:@selector(animateBackwardProgressResetView)
                                            withObject:nil
                                            afterDelay:MDCProgressViewAnimationDuration];
                           }];
 }
 
+- (void)animateBackwardProgressAnimateView {
+  __weak ProgressViewExample *weakSelf = self;
+
+  [_backwardProgressAnimateView setProgress:1 - _backwardProgressResetView.progress
+                                   animated:YES
+                                 completion:^(BOOL finished) {
+                                   [weakSelf performSelector:@selector(animateBackwardProgressAnimateView)
+                                                  withObject:nil
+                                                  afterDelay:MDCProgressViewAnimationDuration];
+                                 }];
+}
+
 #pragma mark - Catalog by Convention
 
 + (NSArray *)catalogBreadcrumbs {

components/ProgressView/src/MDCProgressView.h

@@ -16,6 +16,16 @@
 
 #import <UIKit/UIKit.h>
 
+/** The animation mode when animating backward progress. */
+typedef NS_ENUM(NSInteger, MDCProgressViewBackwardAnimationMode) {
+
+  /** Animate negative progress by resetting to 0 and then animating to the new value. */
+  MDCProgressViewBackwardAnimationModeReset,
+
+  /** Animate negative progress by animating from the current value. */
+  MDCProgressViewBackwardAnimationModeAnimate
+};
+
 /**
  A Material linear determinate progress view.
 
@@ -48,6 +58,14 @@ IB_DESIGNABLE
  */
 @property(nonatomic, assign) float progress;
 
+/**
+ The backward progress animation mode.
+
+ When animating progress which is lower than the current progress value, this mode
+ will determine which animation to use. The default is MDCProgressViewBackwardAnimationModeReset.
+ */
+@property(nonatomic, assign) MDCProgressViewBackwardAnimationMode backwardProgressAnimationMode;
+
 /**
  Adjusts the current progress, optionally animating the change.
 

components/ProgressView/src/MDCProgressView.m

@@ -69,6 +69,8 @@ - (void)commonMDCProgressViewInit {
   self.clipsToBounds = YES;
   self.isAccessibilityElement = YES;
 
+  _backwardProgressAnimationMode = MDCProgressViewBackwardAnimationModeReset;
+
   _trackView = [[UIView alloc] initWithFrame:self.frame];
   _trackView.autoresizingMask = UIViewAutoresizingFlexibleWidth;
   [self addSubview:_trackView];
@@ -132,12 +134,11 @@ - (void)setProgress:(float)progress {
 - (void)setProgress:(float)progress
            animated:(BOOL)animated
          completion:(void (^__nullable)(BOOL finished))completion {
-  // Backward progress should not be animated between the old and new progress. Instead, reset to 0
-  // without animation before animating to the new position.
-  if (progress < self.progress) {
+  if (progress < self.progress && self.backwardProgressAnimationMode == MDCProgressViewBackwardAnimationModeReset) {
     self.progress = 0;
     [self updateProgressView];
   }
+
   self.progress = progress;
   [UIView animateWithDuration:animated ? [[self class] animationDuration] : 0
                         delay:0