这篇文章给大家分享的是“Textview怎样实现自下往上的跑马灯效果”，文中的讲解内容简单清晰，对大家认识和了解都有一定的帮助，对此感兴趣的朋友，接下来就跟随小编一起了解一下“Textview怎样实现自下往上的跑马灯效果”吧。

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();
     }
 ​
     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

• 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 {
 ​
     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

总结

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