components/FlexibleHeader/docs/shift-behavior.md

### Shifting a flexible header off-screen

A flexible header that tracks a scroll view will expand and contract its height in reaction to
scroll view events. A flexible header can also shift off-screen in reaction to scroll view events
by changing the flexible header's behavior.

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#### Swift
```swift
headerViewController.headerView.shiftBehavior = .enabled
```

#### Objective-C
```objc
headerViewController.headerView.shiftBehavior = MDCFlexibleHeaderShiftBehaviorEnabled;
```
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> Important: when a flexible header shifts off-screen it **will not hide the content views**. Your
> content views are responsible for hiding themselves in reaction to the flexible header shifting
> off-screen. Read the section on [Reacting to frame changes](#reacting-to-frame-changes) for more
> information.

It is also possible to hide the status bar when shifting the flexible header off-screen. Enable this
behavior by setting the `enabledWithStatusBar` behavior and implementing
`childViewControllerForStatusBarHidden` on the parent view controller.

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#### Swift
```swift
headerViewController.headerView.shiftBehavior = .enabledWithStatusBar

override func childViewControllerForStatusBarHidden() -> UIViewController? {
  return headerViewController
}
```

#### Objective-C
```objc
headerViewController.headerView.shiftBehavior = MDCFlexibleHeaderShiftBehaviorEnabledWithStatusBar;

- (UIViewController *)childViewControllerForStatusBarHidden {
  return _headerViewController;
}
```
<!--</div>-->

If you would like to be able to show and hide your flexible header similar to how UINavigationBar
allows the navigation bar to be shown and hidden, you can use the `hideable` shift behavior. This
behavior will allow you to toggle visibility of the header using the `shiftHeaderOffScreenAnimated:`
and `shiftHeaderOnScreenAnimated:` APIs only; the user will not be able to drag the header either on
or off-screen.

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#### Swift
```swift
headerViewController.headerView.shiftBehavior = .hideable

// You can now toggle visibility of the header view using the following invocations:
headerViewController.headerView.shiftHeaderOffScreen(animated: true)
headerViewController.headerView.shiftHeaderOnScreen(animated: true)

override func childViewControllerForStatusBarHidden() -> UIViewController? {
  return headerViewController
}
```

#### Objective-C
```objc
headerViewController.headerView.shiftBehavior = MDCFlexibleHeaderShiftBehaviorHideable;

// You can now toggle visibility of the header view using the following invocations:
[headerViewController.headerView shiftHeaderOffScreenAnimated:YES];
[headerViewController.headerView shiftHeaderOnScreenAnimated:YES];

- (UIViewController *)childViewControllerForStatusBarHidden {
  return _headerViewController;
}
```
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