Android TextView跑马灯实现原理及方法实例

时间：2022-06-29 09:22:45|栏目：Android代码|点击：次

前言

自定义View实现的跑马灯一直没有实现类似 Android TextView 的跑马灯首尾相接的效果，所以一直想看看Android TextView 的跑马灯是如何实现

本文主要探秘 Android TextView 的跑马灯实现原理及实现自下往上效果的跑马灯

探秘

TextView#onDraw

原生 Android TextView 如何设置开启跑马灯效果，此处不再描述，View 的绘制都在 onDraw 方法中，这里直接查看 TextView#onDraw() 方法，删减一些不关心的代码

 protected void onDraw(Canvas canvas) {
     // 是否需要重启启动跑马灯
     restartMarqueeIfNeeded();
 ?
     // Draw the background for this view
     super.onDraw(canvas);
         
     // 删减不关心的代码
 ?
     // 创建`mLayout`对象, 此处为`StaticLayout`
     if (mLayout == null) {
         assumeLayout();
     }
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     Layout layout = mLayout;
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     canvas.save();
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     // 删减不关心的代码
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     final int layoutDirection = getLayoutDirection();
     final int absoluteGravity = Gravity.getAbsoluteGravity(mGravity, layoutDirection);
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     // 判断跑马灯设置项是否正确
     if (isMarqueeFadeEnabled()) {
         if (!mSingleLine && getLineCount() == 1 && canMarquee()
               && (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) != Gravity.LEFT) {
            final int width = mRight - mLeft;
            final int padding = getCompoundPaddingLeft() + getCompoundPaddingRight();
            final float dx = mLayout.getLineRight(0) - (width - padding);
            canvas.translate(layout.getParagraphDirection(0) * dx, 0.0f);
         }
 ?
         // 判断跑马灯是否启动
         if (mMarquee != null && mMarquee.isRunning()) {
             final float dx = -mMarquee.getScroll();
             // 移动画布
             canvas.translate(layout.getParagraphDirection(0) * dx, 0.0f);
         }
     }
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     final int cursorOffsetVertical = voffsetCursor - voffsetText;
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     Path highlight = getUpdatedHighlightPath();
     if (mEditor != null) {
         mEditor.onDraw(canvas, layout, highlight, mHighlightPaint, cursorOffsetVertical);
     } else {
         // 绘制文本
         layout.draw(canvas, highlight, mHighlightPaint, cursorOffsetVertical);
     }
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     // 判断是否可以绘制尾部文本
     if (mMarquee != null && mMarquee.shouldDrawGhost()) {
         final float dx = mMarquee.getGhostOffset();
         // 移动画布
         canvas.translate(layout.getParagraphDirection(0) * dx, 0.0f);
         // 绘制尾部文本
         layout.draw(canvas, highlight, mHighlightPaint, cursorOffsetVertical);
     }
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     canvas.restore();
 }

Marquee

根据 onDraw() 方法分析，跑马灯效果的实现主要依赖 mMarquee 这个对象来实现，好的，看下 Marquee 吧，Marquee 代码较少，就贴上全部源码吧

 private static final class Marquee {
     // TODO: Add an option to configure this
     // 缩放相关，不关心此字段
     private static final float MARQUEE_DELTA_MAX = 0.07f;
     
     // 跑马灯跑完一次后多久开始下一次
     private static final int MARQUEE_DELAY = 1200;
     
     // 绘制一次跑多长距离因子，此字段与速度相关
     private static final int MARQUEE_DP_PER_SECOND = 30;
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     // 跑马灯状态常量
     private static final byte MARQUEE_STOPPED = 0x0;
     private static final byte MARQUEE_STARTING = 0x1;
     private static final byte MARQUEE_RUNNING = 0x2;
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     // 对TextView进行弱引用
     private final WeakReference<TextView> mView;
     
     // 帧率相关
     private final Choreographer mChoreographer;
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     // 状态
     private byte mStatus = MARQUEE_STOPPED;
     
     // 绘制一次跑多长距离
     private final float mPixelsPerMs;
     
     // 最大滚动距离
     private float mMaxScroll;
     
     // 是否可以绘制右阴影, 右侧淡入淡出效果
     private float mMaxFadeScroll;
     
     // 尾部文本什么时候开始绘制
     private float mGhostStart;
     
     // 尾部文本绘制位置偏移量
     private float mGhostOffset;
     
     // 是否可以绘制左阴影，左侧淡入淡出效果
     private float mFadeStop;
     
     // 重复限制
     private int mRepeatLimit;
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     // 跑动距离
     private float mScroll;
     
     // 最后一次跑动时间，单位毫秒
     private long mLastAnimationMs;
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     Marquee(TextView v) {
         final float density = v.getContext().getResources().getDisplayMetrics().density;
         // 计算每次跑多长距离
         mPixelsPerMs = MARQUEE_DP_PER_SECOND * density / 1000f;
         mView = new WeakReference<TextView>(v);
         mChoreographer = Choreographer.getInstance();
     }
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     // 帧率回调，用于跑马灯跑动
     private Choreographer.FrameCallback mTickCallback = new Choreographer.FrameCallback() {
         @Override
         public void doFrame(long frameTimeNanos) {
             tick();
         }
     };
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     // 帧率回调，用于跑马灯开始跑动
     private Choreographer.FrameCallback mStartCallback = new Choreographer.FrameCallback() {
         @Override
         public void doFrame(long frameTimeNanos) {
             mStatus = MARQUEE_RUNNING;
             mLastAnimationMs = mChoreographer.getFrameTime();
             tick();
         }
     };
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     // 帧率回调，用于跑马灯重新跑动
     private Choreographer.FrameCallback mRestartCallback = new Choreographer.FrameCallback() {
         @Override
         public void doFrame(long frameTimeNanos) {
             if (mStatus == MARQUEE_RUNNING) {
                 if (mRepeatLimit >= 0) {
                     mRepeatLimit--;
                 }
                 start(mRepeatLimit);
             }
         }
     };
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     // 跑马灯跑动实现
     void tick() {
         if (mStatus != MARQUEE_RUNNING) {
             return;
         }
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         mChoreographer.removeFrameCallback(mTickCallback);
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         final TextView textView = mView.get();
         // 判断TextView是否处于获取焦点或选中状态
         if (textView != null && (textView.isFocused() || textView.isSelected())) {
             // 获取当前时间
             long currentMs = mChoreographer.getFrameTime();
             // 计算当前时间与上次时间的差值
             long deltaMs = currentMs - mLastAnimationMs;
             mLastAnimationMs = currentMs;
             // 根据时间差计算本次跑动的距离，减轻视觉上跳动/卡顿
             float deltaPx = deltaMs * mPixelsPerMs;
             // 计算跑动距离
             mScroll += deltaPx;
             // 判断是否已经跑完
             if (mScroll > mMaxScroll) {
                 mScroll = mMaxScroll;
                 // 发送重新开始跑动事件
                 mChoreographer.postFrameCallbackDelayed(mRestartCallback, MARQUEE_DELAY);
             } else {
                 // 发送下一次跑动事件
                 mChoreographer.postFrameCallback(mTickCallback);
             }
             // 调用此方法会触发执行`onDraw`方法
             textView.invalidate();
         }
     }
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     // 停止跑马灯
     void stop() {
         mStatus = MARQUEE_STOPPED;
         mChoreographer.removeFrameCallback(mStartCallback);
         mChoreographer.removeFrameCallback(mRestartCallback);
         mChoreographer.removeFrameCallback(mTickCallback);
         resetScroll();
     }
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     private void resetScroll() {
         mScroll = 0.0f;
         final TextView textView = mView.get();
         if (textView != null) textView.invalidate();
     }
 ?
     // 启动跑马灯
     void start(int repeatLimit) {
         if (repeatLimit == 0) {
             stop();
             return;
         }
         mRepeatLimit = repeatLimit;
         final TextView textView = mView.get();
         if (textView != null && textView.mLayout != null) {
             // 设置状态为在跑
             mStatus = MARQUEE_STARTING;
             // 重置跑动距离
             mScroll = 0.0f;
             // 计算TextView宽度
             final int textWidth = textView.getWidth() - textView.getCompoundPaddingLeft()
                 - textView.getCompoundPaddingRight();
             // 获取文本第0行的宽度
             final float lineWidth = textView.mLayout.getLineWidth(0);
             // 取TextView宽度的三分之一
             final float gap = textWidth / 3.0f;
             // 计算什么时候可以开始绘制尾部文本：首部文本跑动到哪里可以绘制尾部文本
             mGhostStart = lineWidth - textWidth + gap;
             // 计算最大滚动距离：什么时候认为跑完一次
             mMaxScroll = mGhostStart + textWidth;
             // 尾部文本绘制偏移量
             mGhostOffset = lineWidth + gap;
             // 跑动到哪里时不绘制左侧阴影
             mFadeStop = lineWidth + textWidth / 6.0f;
             // 跑动到哪里时不绘制右侧阴影
             mMaxFadeScroll = mGhostStart + lineWidth + lineWidth;
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             textView.invalidate();
             // 开始跑动
             mChoreographer.postFrameCallback(mStartCallback);
         }
     }
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     // 获取尾部文本绘制位置偏移量
     float getGhostOffset() {
         return mGhostOffset;
     }
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     // 获取当前滚动距离
     float getScroll() {
         return mScroll;
     }
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     // 获取可以右侧阴影绘制的最大距离
     float getMaxFadeScroll() {
         return mMaxFadeScroll;
     }
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     // 判断是否可以绘制左侧阴影
     boolean shouldDrawLeftFade() {
         return mScroll <= mFadeStop;
     }
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     // 判断是否可以绘制尾部文本
     boolean shouldDrawGhost() {
         return mStatus == MARQUEE_RUNNING && mScroll > mGhostStart;
     }
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     // 跑马灯是否在跑
     boolean isRunning() {
         return mStatus == MARQUEE_RUNNING;
     }
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     // 跑马灯是否不跑
     boolean isStopped() {
         return mStatus == MARQUEE_STOPPED;
     }
 }

好的，分析完 Marquee，跑马灯实现原理豁然明亮

在 TextView 开启跑马灯效果时调用 Marquee#start() 方法
在 Marquee#start() 方法中触发 TextView 重绘，开始计算跑动距离
在 TextView#onDraw() 方法中根据跑动距离移动画布并绘制首部文本，再根据跑动距离判断是否可以移动画布绘制尾部文本

小结

TextView 通过移动画布绘制两次文本实现跑马灯效果，根据两帧绘制的时间差计算跑动距离，怎一个"妙"字了得

应用

上面分析完原生 Android TextView 跑马灯的实现原理，但是原生 Android TextView 跑马灯有几点不足：

无法设置跑动速度
无法设置重跑间隔时长
无法实现上下跑动

以上第1、2点在上面 Marquee 分析中已经有解决方案，接下来根据原生实现原理实现第3点上下跑动

MarqueeTextView

这里给出实现方案，列出主要实现逻辑，继承 AppCompatTextView，复写 onDraw() 方法，上下跑动主要是计算上下跑动的距离，然后再次重绘 TextView 上下移动画布绘制文本

 /**
  * 继承AppCompatTextView，复写onDraw方法
  */
 public class MarqueeTextView extends AppCompatTextView {
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     private static final int DEFAULT_BG_COLOR = Color.parseColor("#FFEFEFEF");
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     @IntDef({HORIZONTAL, VERTICAL})
     @Retention(RetentionPolicy.SOURCE)
     public @interface OrientationMode {
     }
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     public static final int HORIZONTAL = 0;
     public static final int VERTICAL = 1;
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     private Marquee mMarquee;
     private boolean mRestartMarquee;
     private boolean isMarquee;
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     private int mOrientation;
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     public MarqueeTextView(@NonNull Context context) {
         this(context, null);
     }
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     public MarqueeTextView(@NonNull Context context, @Nullable AttributeSet attrs) {
         this(context, attrs, 0);
     }
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     public MarqueeTextView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
         super(context, attrs, defStyleAttr);
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         TypedArray ta = context.obtainStyledAttributes(attrs, R.styleable.MarqueeTextView, defStyleAttr, 0);
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         mOrientation = ta.getInt(R.styleable.MarqueeTextView_orientation, HORIZONTAL);
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         ta.recycle();
     }
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     @Override
     protected void onSizeChanged(int w, int h, int oldw, int oldh) {
         super.onSizeChanged(w, h, oldw, oldh);
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         if (mOrientation == HORIZONTAL) {
             if (getWidth() > 0) {
                 mRestartMarquee = true;
             }
         } else {
             if (getHeight() > 0) {
                 mRestartMarquee = true;
             }
         }
     }
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     private void restartMarqueeIfNeeded() {
         if (mRestartMarquee) {
             mRestartMarquee = false;
             startMarquee();
         }
     }
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     public void setMarquee(boolean marquee) {
         boolean wasStart = isMarquee();
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         isMarquee = marquee;
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         if (wasStart != marquee) {
             if (marquee) {
                 startMarquee();
             } else {
                 stopMarquee();
             }
         }
     }
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     public void setOrientation(@OrientationMode int orientation) {
         mOrientation = orientation;
     }
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     public int getOrientation() {
         return mOrientation;
     }
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     public boolean isMarquee() {
         return isMarquee;
     }
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     private void stopMarquee() {
         if (mOrientation == HORIZONTAL) {
             setHorizontalFadingEdgeEnabled(false);
         } else {
             setVerticalFadingEdgeEnabled(false);
         }
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         requestLayout();
         invalidate();
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         if (mMarquee != null && !mMarquee.isStopped()) {
             mMarquee.stop();
         }
     }
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     private void startMarquee() {
         if (canMarquee()) {
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             if (mOrientation == HORIZONTAL) {
                 setHorizontalFadingEdgeEnabled(true);
             } else {
                 setVerticalFadingEdgeEnabled(true);
             }
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             if (mMarquee == null) mMarquee = new Marquee(this);
             mMarquee.start(-1);
         }
     }
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     private boolean canMarquee() {
         if (mOrientation == HORIZONTAL) {
             int viewWidth = getWidth() - getCompoundPaddingLeft() -
                 getCompoundPaddingRight();
             float lineWidth = getLayout().getLineWidth(0);
             return (mMarquee == null || mMarquee.isStopped())
                 && (isFocused() || isSelected() || isMarquee())
                 && viewWidth > 0
                 && lineWidth > viewWidth;
         } else {
             int viewHeight = getHeight() - getCompoundPaddingTop() -
                 getCompoundPaddingBottom();
             float textHeight = getLayout().getHeight();
             return (mMarquee == null || mMarquee.isStopped())
                 && (isFocused() || isSelected() || isMarquee())
                 && viewHeight > 0
                 && textHeight > viewHeight;
         }
     }
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     /**
      * 仿照TextView#onDraw()方法
      */
     @Override
     protected void onDraw(Canvas canvas) {
         restartMarqueeIfNeeded();
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         super.onDraw(canvas);
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         // 再次绘制背景色，覆盖下面由TextView绘制的文本，视情况可以不调用`super.onDraw(canvas);`
         // 如果没有背景色则使用默认颜色
         Drawable background = getBackground();
         if (background != null) {
             background.draw(canvas);
         } else {
             canvas.drawColor(DEFAULT_BG_COLOR);
         }
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         canvas.save();
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         canvas.translate(0, 0);
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         // 实现左右跑马灯
         if (mOrientation == HORIZONTAL) {
             if (mMarquee != null && mMarquee.isRunning()) {
                 final float dx = -mMarquee.getScroll();
                 canvas.translate(dx, 0.0F);
             }
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             getLayout().draw(canvas, null, null, 0);
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             if (mMarquee != null && mMarquee.shouldDrawGhost()) {
                 final float dx = mMarquee.getGhostOffset();
                 canvas.translate(dx, 0.0F);
                 getLayout().draw(canvas, null, null, 0);
             }
         } else {
             // 实现上下跑马灯
             if (mMarquee != null && mMarquee.isRunning()) {
                 final float dy = -mMarquee.getScroll();
                 canvas.translate(0.0F, dy);
             }
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             getLayout().draw(canvas, null, null, 0);
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             if (mMarquee != null && mMarquee.shouldDrawGhost()) {
                 final float dy = mMarquee.getGhostOffset();
                 canvas.translate(0.0F, dy);
                 getLayout().draw(canvas, null, null, 0);
             }
         }
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         canvas.restore();
     }
 }

Marquee

 private static final class Marquee {
     // 修改此字段设置重跑时间间隔 - 对应不足点2
     private static final int MARQUEE_DELAY = 1200;
 ?
     // 修改此字段设置跑动速度 - 对应不足点1
     private static final int MARQUEE_DP_PER_SECOND = 30;
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     private static final byte MARQUEE_STOPPED = 0x0;
     private static final byte MARQUEE_STARTING = 0x1;
     private static final byte MARQUEE_RUNNING = 0x2;
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     private static final String METHOD_GET_FRAME_TIME = "getFrameTime";
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     private final WeakReference<MarqueeTextView> mView;
     private final Choreographer mChoreographer;
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     private byte mStatus = MARQUEE_STOPPED;
     private final float mPixelsPerSecond;
     private float mMaxScroll;
     private float mMaxFadeScroll;
     private float mGhostStart;
     private float mGhostOffset;
     private float mFadeStop;
     private int mRepeatLimit;
 ?
     private float mScroll;
     private long mLastAnimationMs;
 ?
     Marquee(MarqueeTextView v) {
         final float density = v.getContext().getResources().getDisplayMetrics().density;
         mPixelsPerSecond = MARQUEE_DP_PER_SECOND * density;
         mView = new WeakReference<>(v);
         mChoreographer = Choreographer.getInstance();
     }
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     private final Choreographer.FrameCallback mTickCallback = frameTimeNanos -> tick();
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     private final Choreographer.FrameCallback mStartCallback = new Choreographer.FrameCallback() {
         @Override
         public void doFrame(long frameTimeNanos) {
             mStatus = MARQUEE_RUNNING;
             mLastAnimationMs = getFrameTime();
             tick();
         }
     };
 ?
     /**
      * `getFrameTime`是隐藏api，此处使用反射调用，高系统版本可能失效，可使用某些方案绕过此限制
      */
     @SuppressLint("PrivateApi")
     private long getFrameTime() {
         try {
             Class<? extends Choreographer> clz = mChoreographer.getClass();
             Method getFrameTime = clz.getDeclaredMethod(METHOD_GET_FRAME_TIME);
             getFrameTime.setAccessible(true);
             return (long) getFrameTime.invoke(mChoreographer);
         } catch (Exception e) {
             e.printStackTrace();
             return 0;
         }
     }
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     private final Choreographer.FrameCallback mRestartCallback = new Choreographer.FrameCallback() {
         @Override
         public void doFrame(long frameTimeNanos) {
             if (mStatus == MARQUEE_RUNNING) {
                 if (mRepeatLimit >= 0) {
                     mRepeatLimit--;
                 }
                 start(mRepeatLimit);
             }
         }
     };
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     void tick() {
         if (mStatus != MARQUEE_RUNNING) {
             return;
         }
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         mChoreographer.removeFrameCallback(mTickCallback);
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         final MarqueeTextView textView = mView.get();
         if (textView != null && (textView.isFocused() || textView.isSelected() || textView.isMarquee())) {
             long currentMs = getFrameTime();
             long deltaMs = currentMs - mLastAnimationMs;
             mLastAnimationMs = currentMs;
             float deltaPx = deltaMs / 1000F * mPixelsPerSecond;
             mScroll += deltaPx;
             if (mScroll > mMaxScroll) {
                 mScroll = mMaxScroll;
                 mChoreographer.postFrameCallbackDelayed(mRestartCallback, MARQUEE_DELAY);
             } else {
                 mChoreographer.postFrameCallback(mTickCallback);
             }
             textView.invalidate();
         }
     }
 ?
     void stop() {
         mStatus = MARQUEE_STOPPED;
         mChoreographer.removeFrameCallback(mStartCallback);
         mChoreographer.removeFrameCallback(mRestartCallback);
         mChoreographer.removeFrameCallback(mTickCallback);
         resetScroll();
     }
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     private void resetScroll() {
         mScroll = 0.0F;
         final MarqueeTextView textView = mView.get();
         if (textView != null) textView.invalidate();
     }
 ?
     void start(int repeatLimit) {
         if (repeatLimit == 0) {
             stop();
             return;
         }
         mRepeatLimit = repeatLimit;
         final MarqueeTextView textView = mView.get();
         if (textView != null && textView.getLayout() != null) {
             mStatus = MARQUEE_STARTING;
             mScroll = 0.0F;
 ?
             // 分别计算左右和上下跑动所需的数据
             if (textView.getOrientation() == HORIZONTAL) {
                 int viewWidth = textView.getWidth() - textView.getCompoundPaddingLeft() -
                     textView.getCompoundPaddingRight();
                 float lineWidth = textView.getLayout().getLineWidth(0);
                 float gap = viewWidth / 3.0F;
                 mGhostStart = lineWidth - viewWidth + gap;
                 mMaxScroll = mGhostStart + viewWidth;
                 mGhostOffset = lineWidth + gap;
                 mFadeStop = lineWidth + viewWidth / 6.0F;
                 mMaxFadeScroll = mGhostStart + lineWidth + lineWidth;
             } else {
                 int viewHeight = textView.getHeight() - textView.getCompoundPaddingTop() -
                     textView.getCompoundPaddingBottom();
                 float textHeight = textView.getLayout().getHeight();
                 float gap = viewHeight / 3.0F;
                 mGhostStart = textHeight - viewHeight + gap;
                 mMaxScroll = mGhostStart + viewHeight;
                 mGhostOffset = textHeight + gap;
                 mFadeStop = textHeight + viewHeight / 6.0F;
                 mMaxFadeScroll = mGhostStart + textHeight + textHeight;
             }
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             textView.invalidate();
             mChoreographer.postFrameCallback(mStartCallback);
         }
     }
 ?
     float getGhostOffset() {
         return mGhostOffset;
     }
 ?
     float getScroll() {
         return mScroll;
     }
 ?
     float getMaxFadeScroll() {
         return mMaxFadeScroll;
     }
 ?
     boolean shouldDrawLeftFade() {
         return mScroll <= mFadeStop;
     }
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     boolean shouldDrawTopFade() {
         return mScroll <= mFadeStop;
     }
 ?
     boolean shouldDrawGhost() {
         return mStatus == MARQUEE_RUNNING && mScroll > mGhostStart;
     }
 ?
     boolean isRunning() {
         return mStatus == MARQUEE_RUNNING;
     }
 ?
     boolean isStopped() {
         return mStatus == MARQUEE_STOPPED;
     }
 }