如何在安卓中使用滚动视图实现'quick return'

Question

我想用ScrollView实现'快速返回标头'。我的标题不是工具栏，而是FrameLayout。

Answer 1

基本上你需要使用Nested Scrolling（在 API 21 中添加）。此功能允许子项（滚动项）与其父项（通常是CoordinatorLayout）通信。但是，为了拥有嵌套滚动，您需要：

父
1. 视图必须实现嵌套滚动父接口
子
2. 视图必须实现嵌套滚动子接口

CoordinatorLayout实现了NestedScrollingParent，但不幸的是，ScrollView没有实现NestedScrollingChild接口。

因此，您不能使用 scrollview 进行嵌套滚动（在 API 21 之前：查看底部的编辑部分）。但是，您可以使用基本上是实现NestedScrollingChild接口的ScrollView的NestedScrollView。

之后，您需要创建自定义CoordinatorLayout.Behavior并覆盖方法onStartNestedScroll、onNestedPreScroll和onNestedScroll。下面您可以看到符合您需求的代码（我从这个例子中获取了一些更改）。

假设您的视图是一种FramLayout您可以在布局中执行以下操作：

<android.support.design.widget.CoordinatorLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <android.support.v4.widget.NestedScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent">
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">
        </LinearLayout>
    </android.support.v4.widget.NestedScrollView>
    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="84dp"
        android:background="#FF0000"
        app:layout_behavior=".QuickHideBehavior">
    </FrameLayout>
</android.support.design.widget.CoordinatorLayout>

如您所见，自定义行为附加到FrameLayout。然后你需要这样修改上面的快速隐藏行为示例：

public class QuickHideBehavior extends CoordinatorLayout.Behavior<View> {
    private static final int DIRECTION_UP = 1;
    private static final int DIRECTION_DOWN = -1;
    /* Tracking direction of user motion */
    private int mScrollingDirection;
    /* Tracking last threshold crossed */
    private int mScrollTrigger;
    /* Accumulated scroll distance */
    private int mScrollDistance;
    /* Distance threshold to trigger animation */
    private int mScrollThreshold;

    private ObjectAnimator mAnimator;
    //Required to instantiate as a default behavior
    public QuickHideBehavior() {
    }
    //Required to attach behavior via XML
    public QuickHideBehavior(Context context, AttributeSet attrs) {
        super(context, attrs);
        TypedArray a = context.getTheme()
                .obtainStyledAttributes(new int[] {R.attr.actionBarSize});
        //Use half the standard action bar height
        mScrollThreshold = a.getDimensionPixelSize(0, 0) / 2;
        a.recycle();
    }
    //Called before a nested scroll event. Return true to declare interest
    @Override
    public boolean onStartNestedScroll(CoordinatorLayout coordinatorLayout,
                                       View child, View directTargetChild, View target,
                                       int nestedScrollAxes) {
        //We have to declare interest in the scroll to receive further events
        return (nestedScrollAxes & ViewCompat.SCROLL_AXIS_VERTICAL) != 0;
    }
    //Called before the scrolling child consumes the event
    // We can steal all/part of the event by filling in the consumed array
    @Override
    public void onNestedPreScroll(CoordinatorLayout coordinatorLayout,
                                  View child, View target,
                                  int dx, int dy,
                                  int[] consumed) {
        //Determine direction changes here
        if (dy > 0 && mScrollingDirection != DIRECTION_UP) {
            mScrollingDirection = DIRECTION_UP;
            mScrollDistance = 0;
        } else if (dy < 0 && mScrollingDirection != DIRECTION_DOWN) {
            mScrollingDirection = DIRECTION_DOWN;
            mScrollDistance = 0;
        }
    }
    //Called after the scrolling child consumes the event, with amount consumed
    @Override
    public void onNestedScroll(CoordinatorLayout coordinatorLayout,
                               View child, View target,
                               int dxConsumed, int dyConsumed,
                               int dxUnconsumed, int dyUnconsumed) {
        //Consumed distance is the actual distance traveled by the scrolling view
        mScrollDistance += dyConsumed;
        if (mScrollDistance > mScrollThreshold
                && mScrollTrigger != DIRECTION_UP) {
            //Hide the target view
            mScrollTrigger = DIRECTION_UP;
            restartAnimator(child, getTargetHideValue(coordinatorLayout, child));
        } else if (mScrollDistance < -mScrollThreshold
                && mScrollTrigger != DIRECTION_DOWN) {
            //Return the target view
            mScrollTrigger = DIRECTION_DOWN;
            restartAnimator(child, 0f);
        }
    }
    //Called after the scrolling child handles the fling
    @Override
    public boolean onNestedFling(CoordinatorLayout coordinatorLayout,
                                 View child, View target,
                                 float velocityX, float velocityY,
                                 boolean consumed) {
        //We only care when the target view is already handling the fling
        if (consumed) {
            if (velocityY > 0 && mScrollTrigger != DIRECTION_UP) {
                mScrollTrigger = DIRECTION_UP;
                restartAnimator(child, getTargetHideValue(coordinatorLayout, child));
            } else if (velocityY < 0 && mScrollTrigger != DIRECTION_DOWN) {
                mScrollTrigger = DIRECTION_DOWN;
                restartAnimator(child, 0f);
            }
        }
        return false;
    }
    /* Helper Methods */
    //Helper to trigger hide/show animation
    private void restartAnimator(View target, float value) {
        if (mAnimator != null) {
            mAnimator.cancel();
            mAnimator = null;
        }
        mAnimator = ObjectAnimator
                .ofFloat(target, View.TRANSLATION_Y, value)
                .setDuration(250);
        /*mAnimator = ObjectAnimator.ofFloat(target, "alpha", 0f, 1f).setDuration(250);*/
        mAnimator.start();
    }
    private float getTargetHideValue(ViewGroup parent, View target) {
        if(target instanceof FrameLayout)
            return -target.getHeight();
        return 0f;
    }
}

你完成了。

我建议您在这里查看更多示例或戴夫史密斯的此视频

编辑：实际上，由于API 21，ScrollView（或ListView）支持嵌套滚动，但您需要启用它：

scrollView.setNestedScrollingEnabled(true);

当然，您需要设置

minSdkVersion > 20