TextView跑马灯探秘

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) {
	// 是否需要重新启动跑马灯
	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();
    }

    Layout layout = mLayout;

    canvas.save();

    // 删减不关心的代码

    final int layoutDirection = getLayoutDirection();
    final int absoluteGravity = Gravity.getAbsoluteGravity(mGravity, layoutDirection);

    // 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);
        }
    }

    final int cursorOffsetVertical = voffsetCursor - voffsetText;

    Path highlight = getUpdatedHighlightPath();
    if (mEditor != null) {
        mEditor.onDraw(canvas, layout, highlight, mHighlightPaint, cursorOffsetVertical);
    } else {
        // 绘制文本
        layout.draw(canvas, highlight, mHighlightPaint, cursorOffsetVertical);
    }

    // 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);
    }

    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;

    // 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;

    // 对TextView进行弱引用
    private final WeakReference<TextView> mView;
    
    // 帧率相关
    private final Choreographer mChoreographer;

    // 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;

    // 跑动距离
    private float mScroll;
    
    // 最后一次跑动时间，单位毫秒
    private long mLastAnimationMs;

    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();
    }

    // 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();
        }
    };

    // 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();
        }
    };

    // 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);
            }
        }
    };

    // 跑马灯跑动实现
    void tick() {
        if (mStatus != MARQUEE_RUNNING) {
            return;
        }

        mChoreographer.removeFrameCallback(mTickCallback);

        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();
        }
    }

    // 停止跑马灯
    void stop() {
        mStatus = MARQUEE_STOPPED;
        mChoreographer.removeFrameCallback(mStartCallback);
        mChoreographer.removeFrameCallback(mRestartCallback);
        mChoreographer.removeFrameCallback(mTickCallback);
        resetScroll();
    }

    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;

            textView.invalidate();
            // 开始跑动
            mChoreographer.postFrameCallback(mStartCallback);
        }
    }

    // 获取尾部文本绘制位置偏移量
    float getGhostOffset() {
        return mGhostOffset;
    }

    // 获取当前滚动距离
    float getScroll() {
        return mScroll;
    }

    // 获取可以右侧阴影绘制的最大距离
    float getMaxFadeScroll() {
        return mMaxFadeScroll;
    }

    // 判断是否可以绘制左侧阴影
    boolean shouldDrawLeftFade() {
        return mScroll <= mFadeStop;
    }

    // 判断是否可以绘制尾部文本
    boolean shouldDrawGhost() {
        return mStatus == MARQUEE_RUNNING && mScroll > mGhostStart;
    }

    // 跑马灯是否在跑
    boolean isRunning() {
        return mStatus == MARQUEE_RUNNING;
    }

    // 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

1. 在 TextView 开启跑马灯效果时调用 Marquee#start() 方法
2. 在 Marquee#start() 方法中触发 TextView 重绘，开始计算跑动距离
3. 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"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：

1. 无法设置跑动速度
2. 无法设置重跑间隔时长
3. 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 {

    private static final int DEFAULT_BG_COLOR = Color.parseColor("#FFEFEFEF");

    @IntDef({HORIZONTAL, VERTICAL})
    @Retention(RetentionPolicy.SOURCE)
    public @interface OrientationMode {
    }

    public static final int HORIZONTAL = 0;
    public static final int VERTICAL = 1;

    private Marquee mMarquee;
    private boolean mRestartMarquee;
    private boolean isMarquee;

    private int mOrientation;

    public MarqueeTextView(@NonNull Context context) {
        this(context, null);
    }

    public MarqueeTextView(@NonNull Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public MarqueeTextView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);

        TypedArray ta = context.obtainStyledAttributes(attrs, R.styleable.MarqueeTextView, defStyleAttr, 0);

        mOrientation = ta.getInt(R.styleable.MarqueeTextView_orientation, HORIZONTAL);

        ta.recycle();
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);

        if (mOrientation == HORIZONTAL) {
            if (getWidth() > 0) {
                mRestartMarquee = true;
            }
        } else {
            if (getHeight() > 0) {
                mRestartMarquee = true;
            }
        }
    }

    private void restartMarqueeIfNeeded() {
        if (mRestartMarquee) {
            mRestartMarquee = false;
            startMarquee();
        }
    }

    public void setMarquee(boolean marquee) {
        boolean wasStart = isMarquee();

        isMarquee = marquee;

        if (wasStart != marquee) {
            if (marquee) {
                startMarquee();
            } else {
                stopMarquee();
            }
        }
    }

    public void setOrientation(@OrientationMode int orientation) {
        mOrientation = orientation;
    }

    public int getOrientation() {
        return mOrientation;
    }

    public boolean isMarquee() {
        return isMarquee;
    }

    private void stopMarquee() {
        if (mOrientation == HORIZONTAL) {
            setHorizontalFadingEdgeEnabled(false);
        } else {
            setVerticalFadingEdgeEnabled(false);
        }

        requestLayout();
        invalidate();

        if (mMarquee != null && !mMarquee.isStopped()) {
            mMarquee.stop();
        }
    }

    private void startMarquee() {
        if (canMarquee()) {

            if (mOrientation == HORIZONTAL) {
                setHorizontalFadingEdgeEnabled(true);
            } else {
                setVerticalFadingEdgeEnabled(true);
            }

            if (mMarquee == null) mMarquee = new Marquee(this);
            mMarquee.start(-1);
        }
    }

    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;
        }
    }

    /**
     * 仿照TextView#onDraw()方法
     */
    @Override
    protected void onDraw(Canvas canvas) {
        restartMarqueeIfNeeded();

        super.onDraw(canvas);

        // 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);
        }

        canvas.save();

        canvas.translate(0, 0);

        // 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);
            }

            getLayout().draw(canvas, null, null, 0);

            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);
            }

            getLayout().draw(canvas, null, null, 0);

            if (mMarquee != null && mMarquee.shouldDrawGhost()) {
                final float dy = mMarquee.getGhostOffset();
                canvas.translate(0.0F, dy);
                getLayout().draw(canvas, null, null, 0);
            }
        }

        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;

    private static final byte MARQUEE_STOPPED = 0x0;
    private static final byte MARQUEE_STARTING = 0x1;
    private static final byte MARQUEE_RUNNING = 0x2;

    private static final String METHOD_GET_FRAME_TIME = "getFrameTime";

    private final WeakReference<MarqueeTextView> mView;
    private final Choreographer mChoreographer;

    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();
    }

    private final Choreographer.FrameCallback mTickCallback = frameTimeNanos -> tick();

    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;
        }
    }

    private final Choreographer.FrameCallback mRestartCallback = new Choreographer.FrameCallback() {
        @Override
        public void doFrame(long frameTimeNanos) {
            if (mStatus == MARQUEE_RUNNING) {
                if (mRepeatLimit >= 0) {
                    mRepeatLimit--;
                }
                start(mRepeatLimit);
            }
        }
    };

    void tick() {
        if (mStatus != MARQUEE_RUNNING) {
            return;
        }

        mChoreographer.removeFrameCallback(mTickCallback);

        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();
    }

    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;
            }

            textView.invalidate();
            mChoreographer.postFrameCallback(mStartCallback);
        }
    }

    float getGhostOffset() {
        return mGhostOffset;
    }

    float getScroll() {
        return mScroll;
    }

    float getMaxFadeScroll() {
        return mMaxFadeScroll;
    }

    boolean shouldDrawLeftFade() {
        return mScroll <= mFadeStop;
    }

    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;
    }
}

效果

happy~