android TV常见需求，焦点item保持居中 —— RecyclerView自定义焦点滑动位置和滑动速度。

  Android tv开发和移动端开发最大的不同就是多了一个焦点处理的逻辑。尤其是类似Recyclerview这样本身带有滑动效果，为了醒目的显示当前焦点在什么位置，需要滑动的时候回添加大量的动画、高亮、阴影等效果。

  同样，让焦点位置不变而列表主动滑动也是一种常见的提醒焦点的手段。demo效果图如下，结尾放出全部代码：

先导入recyclerview

dependencies {
    implementation 'androidx.recyclerview:recyclerview:1.2.0-alpha03'
}

  我用的demo是androidx的recyclerview。低版本的同学可以使用android.support支持库。
  在布局文件中添加recyclerview的布局，并添加一个item的布局。findviewbyid找到recyclerview的控件，并setLayoutManager（我用的是LinearLayoutManager）和setAdapter。一个粗糙的recyclerview效果就出来了。这是最简单的recyclerview，除了能滑动，什么效果也没有。

  允许item获得焦点，并为item设置焦点监听。这一步可以放到onBindViewHolder或者ViewHolder初始化的地方。
  为了能看出当前焦点的位置，还需要对获得焦点的item进行高亮处理。下面代码中，用setTranslationZ添加了阴影，ofFloatAnimator方法中还设置了放大动画。

		class MyHolder extends RecyclerView.ViewHolder{
            public MyHolder(@NonNull final View itemView) {
                super(itemView);
                itemView.setFocusable(true);
                itemView.setOnFocusChangeListener(new View.OnFocusChangeListener() {
                    @Override
                    public void onFocusChange(View view, boolean b) {
                        if(b){
                            int[] amount = getScrollAmount(recyclerView, view);//计算需要滑动的距离
                            //滑动到指定距离
                            scrollToAmount(recyclerView, amount[0], amount[1]);
                            itemView.setTranslationZ(20);//阴影
                            ofFloatAnimator(itemView,1f,1.3f);//放大
                        }else {
                            itemView.setTranslationZ(0);
                            ofFloatAnimator(itemView,1.3f,1f);
                        }
                    }
                });
            }

//放大动画

        //放大动画
        private void ofFloatAnimator(View view,float start,float end){
            AnimatorSet animatorSet = new AnimatorSet();
            animatorSet.setDuration(700);//动画时间
            ObjectAnimator scaleX = ObjectAnimator.ofFloat(view, "scaleX", start, end);
            ObjectAnimator scaleY = ObjectAnimator.ofFloat(view, "scaleY", start, end);
            animatorSet.setInterpolator(new DecelerateInterpolator());//插值器
            animatorSet.play(scaleX).with(scaleY);//组合动画,同时基于x和y轴放大
            animatorSet.start();
        }

  因为item放大，体积超过了recyclerview的边界。为了使这部分正常显示，需要在布局文件中recyclerview的父布局添加clipChildren和clipToPadding属性。

    android:clipChildren="false"
    android:clipToPadding="false"

  首先我们看看recyclerview源码是怎么控制滑动的距离的，什么时候需要滑动，什么时候不用滑动。在源码RecyclerView/的LayoutManager中有这样一段代码：

public boolean requestChildRectangleOnScreen(@NonNull RecyclerView parent,
                @NonNull View child, @NonNull Rect rect, boolean immediate,
                boolean focusedChildVisible) {
            int[] scrollAmount = getChildRectangleOnScreenScrollAmount(child, rect
            );
            int dx = scrollAmount[0];
            int dy = scrollAmount[1];
            if (!focusedChildVisible || isFocusedChildVisibleAfterScrolling(parent, dx, dy)) {
                if (dx != 0 || dy != 0) {
                    if (immediate) {
                        parent.scrollBy(dx, dy);
                    } else {
                        parent.smoothScrollBy(dx, dy);
                    }
                    return true;
                }
            }
            return false;
        }

  可以看出recyclerview是调用

smoothScrollBy(dx, dy)

getChildRectangleOnScreenScrollAmount

            private int[] getScrollAmount(RecyclerView recyclerView, View view) {
                int[] out = new int[2];
                final int parentLeft = recyclerView.getPaddingLeft();
                final int parentTop = recyclerView.getPaddingTop();
                final int parentRight = recyclerView.getWidth() - recyclerView.getPaddingRight();
                final int childLeft = view.getLeft() + 0 - view.getScrollX();
                final int childTop = view.getTop() + 0 - view.getScrollY();
                final int dx =childLeft - parentLeft - ((parentRight - view.getWidth()) / 2);//item左边距减去Recyclerview不在屏幕内的部分，加当前Recyclerview一半的宽度就是居中
                final int dy = childTop - parentTop - (parentTop - view.getHeight()) / 2;//同上
                out[0] = dx;
                out[1] = dy;
                return out;
            }
        }

getScrollAmount

getChildRectangleOnScreenScrollAmount

				itemView.setFocusable(true);
                itemView.setOnFocusChangeListener(new View.OnFocusChangeListener() {
                    @Override
                    public void onFocusChange(View view, boolean b) {
                        if(b){
                            int[] amount = getScrollAmount(recyclerView, view);//计算需要滑动的距离
                            recyclerView.smoothScrollBy(amount[0], amount[1]);
                            itemView.setTranslationZ(20);//阴影
                            ofFloatAnimator(itemView,1f,1.3f);//放大
                        }else {
                            itemView.setTranslationZ(0);
                            ofFloatAnimator(itemView,1.3f,1f);
                        }
                    }
                });

smoothScrollBy

  到上一步，效果就基本完成了。为了更精细的完成界面，还可以对滑动的速度和滑动效果进行修改。
  我们已经知道recyclerview源码中使用的是

smoothScrollBy(dx, dy)

smoothScrollBy(dx, dy)

void smoothScrollBy(@Px int dx, @Px int dy, @Nullable Interpolator interpolator,
            int duration, boolean withNestedScrolling) {
        if (mLayout == null) {
            Log.e(TAG, "Cannot smooth scroll without a LayoutManager set. "
                    + "Call setLayoutManager with a non-null argument.");
            return;
        }
        if (mLayoutSuppressed) {
            return;
        }
        if (!mLayout.canScrollHorizontally()) {
            dx = 0;
        }
        if (!mLayout.canScrollVertically()) {
            dy = 0;
        }
        if (dx != 0 || dy != 0) {
            boolean durationSuggestsAnimation = duration == UNDEFINED_DURATION || duration > 0;
            if (durationSuggestsAnimation) {
                if (withNestedScrolling) {
                    int nestedScrollAxis = ViewCompat.SCROLL_AXIS_NONE;
                    if (dx != 0) {
                        nestedScrollAxis |= ViewCompat.SCROLL_AXIS_HORIZONTAL;
                    }
                    if (dy != 0) {
                        nestedScrollAxis |= ViewCompat.SCROLL_AXIS_VERTICAL;
                    }
                    startNestedScroll(nestedScrollAxis, TYPE_NON_TOUCH);
                }
                mViewFlinger.smoothScrollBy(dx, dy, duration, interpolator);
            } else {
                scrollBy(dx, dy);
            }
        }
    }

看注释描述，interpolator是动画插值器，duration是动画时间，withNestedScrolling是嵌套滚动平滑滚动。在上一层传入参数的时候interpolator是null，withNestedScrolling是false。那么我调用这个方法代替之前的

smoothScrollBy(dx, dy)

 			//根据坐标滑动到指定距离
            private void scrollToAmount(RecyclerView recyclerView, int dx, int dy) {
                //如果没有滑动速度等需求，可以直接调用这个方法，使用默认的速度
//                recyclerView.smoothScrollBy(dx,dy);
                //以下对滑动速度提出定制
                try {
                    Class recClass = recyclerView.getClass();
                    Method smoothMethod = recClass.getDeclaredMethod("smoothScrollBy", int.class, int.class, Interpolator.class, int.class);
                    smoothMethod.invoke(recyclerView, dx, dy, new AccelerateDecelerateInterpolator(), 700);//时间设置为700毫秒，
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }

之后在item的焦点监听中在调用scrollToAmount就可以了。

全代码：
MainActivity

public class MainActivity extends AppCompatActivity {
    private String[] permissions = new String[]{Manifest.permission.READ_PHONE_STATE, Manifest.permission.CAMERA};
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        RecyclerView recyclerview = findViewById(R.id.recyclerview);
        recyclerview.setLayoutManager(new LinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false));
        Myadapter myadapter = new Myadapter(recyclerview);
        recyclerview.setAdapter(myadapter);
    }
    class Myadapter extends RecyclerView.Adapter{
        private RecyclerView recyclerView;
        public Myadapter(RecyclerView recyclerView){
            this.recyclerView = recyclerView;
        }
        @Override
        public MyHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
            View item = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_layout , parent ,false);
            return new MyHolder(item);
        }
        @Override
        public void onBindViewHolder(@NonNull MyHolder holder, int position) {
        }
        @Override
        public int getItemCount() {
            return 30;
        }
        class MyHolder extends RecyclerView.ViewHolder{
            public MyHolder(@NonNull final View itemView) {
                super(itemView);
                itemView.setFocusable(true);
                itemView.setOnFocusChangeListener(new View.OnFocusChangeListener() {
                    @Override
                    public void onFocusChange(View view, boolean b) {
                        if(b){
                            int[] amount = getScrollAmount(recyclerView, view);//计算需要滑动的距离
                            //滑动到指定距离
                            scrollToAmount(recyclerView, amount[0], amount[1]);
                            itemView.setTranslationZ(20);//阴影
                            ofFloatAnimator(itemView,1f,1.3f);//放大
                        }else {
                            itemView.setTranslationZ(0);
                            ofFloatAnimator(itemView,1.3f,1f);
                        }
                    }
                });
            }
            //根据坐标滑动到指定距离
            private void scrollToAmount(RecyclerView recyclerView, int dx, int dy) {
                //如果没有滑动速度等需求，可以直接调用这个方法，使用默认的速度
//                recyclerView.smoothScrollBy(dx,dy);
                //以下对滑动速度提出定制
                try {
                    Class recClass = recyclerView.getClass();
                    Method smoothMethod = recClass.getDeclaredMethod("smoothScrollBy", int.class, int.class, Interpolator.class, int.class);
                    smoothMethod.invoke(recyclerView, dx, dy, new AccelerateDecelerateInterpolator(), 700);//时间设置为700毫秒，
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
            
            private int[] getScrollAmount(RecyclerView recyclerView, View view) {
                int[] out = new int[2];
                final int parentLeft = recyclerView.getPaddingLeft();
                final int parentTop = recyclerView.getPaddingTop();
                final int parentRight = recyclerView.getWidth() - recyclerView.getPaddingRight();
                final int childLeft = view.getLeft() + 0 - view.getScrollX();
                final int childTop = view.getTop() + 0 - view.getScrollY();
                final int dx =childLeft - parentLeft - ((parentRight - view.getWidth()) / 2);//item左边距减去Recyclerview不在屏幕内的部分，加当前Recyclerview一半的宽度就是居中
                final int dy = childTop - parentTop - (parentTop - view.getHeight()) / 2;//同上
                out[0] = dx;
                out[1] = dy;
                return out;
            }
        }
        //放大动画
        private void ofFloatAnimator(View view,float start,float end){
            AnimatorSet animatorSet = new AnimatorSet();
            animatorSet.setDuration(700);//动画时间
            ObjectAnimator scaleX = ObjectAnimator.ofFloat(view, "scaleX", start, end);
            ObjectAnimator scaleY = ObjectAnimator.ofFloat(view, "scaleY", start, end);
            animatorSet.setInterpolator(new DecelerateInterpolator());//插值器
            animatorSet.play(scaleX).with(scaleY);//组合动画,同时基于x和y轴放大
            animatorSet.start();
        }
    }
}

activity_main.xml

item_layout