components/FlexibleHeader/src/private/MDCFlexibleHeaderTopSafeArea.m

/*
 Copyright 2015-present the Material Components for iOS authors. All Rights Reserved.

 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
 You may obtain a copy of the License at

 http://www.apache.org/licenses/LICENSE-2.0

 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
 */

#import "MDCFlexibleHeaderTopSafeArea.h"

#import "MDCFlexibleHeaderTopSafeAreaDelegate.h"

// The default status bar height for non-X devices.
static const CGFloat kNonXStatusBarHeight = 20;

@interface MDCFlexibleHeaderTopSafeArea ()
// We use this value to correctly handle top safe area insets on non-iPhone X devices.
@property(nonatomic) CGFloat lastNonZeroTopSafeAreaInset;
@property(nonatomic) CGFloat extractedTopSafeAreaInset;
@end

@implementation MDCFlexibleHeaderTopSafeArea

@synthesize extractedTopSafeAreaInset = _extractedTopSafeAreaInset;

#pragma mark - Public

- (void)setTopSafeAreaSourceViewController:(UIViewController *)topSafeAreaSourceViewController {
  _topSafeAreaSourceViewController = topSafeAreaSourceViewController;

  if (self.inferTopSafeAreaInsetFromViewController) {
    [self extractTopSafeAreaInset];
  }
}

- (void)setSubtractsAdditionalSafeAreaInsets:(BOOL)subtractsAdditionalSafeAreaInsets {
  _subtractsAdditionalSafeAreaInsets = subtractsAdditionalSafeAreaInsets;

  if (self.inferTopSafeAreaInsetFromViewController) {
    [self extractTopSafeAreaInset];
  }
}

- (CGFloat)topSafeAreaInset {
  if (self.inferTopSafeAreaInsetFromViewController) {
    // Generally-speaking, we consider the top safe area inset to equate to the length of any
    // "device-owned" pixels.
    //
    // On an iPhone X, the top safe area inset is a fixed hardware constant, varying only based on
    // device orientation.
    //
    // On non-X devices, however, the top safe area inset is a flexible software value reflecting
    // the status bar's current height. If the status bar is hidden, the top safe area inset is
    // zero.
    //
    // This affects the flexible header mostly when the status bar shift behavior is enabled. The
    // flexible header is able to hide the status bar interactively, meaning our top safe area inset
    // can fluctuate between 0 and 20 on non-X devices as we show/hide the status bar. This can
    // cause the tracking scroll view's content inset to "jump" because we make the scroll view's
    // top content inset equal maximumHeight + topSafeAreaInset.
    //
    // Aside: If we only supported iOS 11 and up we'd likely be able to simplify a lot of this
    // behavior by solely relying on additionalSafeAreaInsets. Alas, that will likely be sometime
    // after 2020.
    //
    // So, to avoid the jumping behavior, we keep track of what the last non-zero top safe area
    // inset was. If it was 20 and we know we've programmatically hidden the status bar, we continue
    // to pretend that the top safe area inset is 20. Once the status bar is visible again we rely
    // on the actual topSafeAreaInset value.
    BOOL topSafeAreaInsetLikelyAffectedByStatusBarVisibility =
        self.lastNonZeroTopSafeAreaInset == kNonXStatusBarHeight;
    if (topSafeAreaInsetLikelyAffectedByStatusBarVisibility &&
        [self.topSafeAreaDelegate flexibleHeaderSafeAreaIsStatusBarShifted:self]) {
      return self.lastNonZeroTopSafeAreaInset;
    } else {
      return self.extractedTopSafeAreaInset;
    }
  } else {
    return [self.topSafeAreaDelegate flexibleHeaderSafeAreaDeviceTopSafeAreaInset:self];
  }
}

- (void)safeAreaInsetsDidChange {
  if (self.inferTopSafeAreaInsetFromViewController) {
    [self extractTopSafeAreaInset];
  } else {
    // The device safe area insets may have changed, so we always fall back to calling the delegate.
    [self.topSafeAreaDelegate flexibleHeaderSafeAreaTopSafeAreaInsetDidChange:self];
  }
}

- (void)setInferTopSafeAreaInsetFromViewController:(BOOL)inferTopSafeAreaInsetFromViewController {
  if (_inferTopSafeAreaInsetFromViewController == inferTopSafeAreaInsetFromViewController) {
    return;
  }
  _inferTopSafeAreaInsetFromViewController = inferTopSafeAreaInsetFromViewController;

  if (_inferTopSafeAreaInsetFromViewController) {
    [self extractTopSafeAreaInset];
  } else {
    [self.topSafeAreaDelegate flexibleHeaderSafeAreaTopSafeAreaInsetDidChange:self];
  }
}

#pragma mark - Private

- (void)setExtractedTopSafeAreaInset:(CGFloat)extractedTopSafeAreaInset {
  _extractedTopSafeAreaInset = extractedTopSafeAreaInset;

  if (_extractedTopSafeAreaInset > 0) {
    self.lastNonZeroTopSafeAreaInset = _extractedTopSafeAreaInset;
  }

  // No need to inform the delegate if this behavior is diabled.
  if (self.inferTopSafeAreaInsetFromViewController) {
    [self.topSafeAreaDelegate flexibleHeaderSafeAreaTopSafeAreaInsetDidChange:self];
  }
}

- (void)extractTopSafeAreaInset {
  UIViewController *viewController = self.topSafeAreaSourceViewController;
  if (![viewController isViewLoaded]) {
    self.extractedTopSafeAreaInset = 0;
    return;
  }

  CGFloat safeAreaTop = viewController.view.safeAreaInsets.top;
  if (self.subtractsAdditionalSafeAreaInsets) {
    safeAreaTop -= viewController.additionalSafeAreaInsets.top;
  }
  self.extractedTopSafeAreaInset = safeAreaTop;
}

@end