fix单步执行

src/components/Player/index.js

@@ -6,6 +6,7 @@ import faPlay from '@fortawesome/fontawesome-free-solid/faPlay';
 import faChevronLeft from '@fortawesome/fontawesome-free-solid/faChevronLeft';
 import faChevronRight from '@fortawesome/fontawesome-free-solid/faChevronRight';
 import faPause from '@fortawesome/fontawesome-free-solid/faPause';
+import faStepForward from '@fortawesome/fontawesome-free-solid/faStepForward';
 import faWrench from '@fortawesome/fontawesome-free-solid/faWrench';
 import { classes, extension } from 'common/util';
 import { TracerApi } from 'apis';
@@ -116,6 +117,14 @@ class Player extends BaseComponent {
     }
   }
 
+  /** 单步前进：每次点击只推进一帧，不启动自动播放（与 Play 的连续播放相对）。 */
+  stepOnce(wrap = true) {
+    this.pause();
+    if (!this.next() && wrap) {
+      this.props.setCursor(1);
+    }
+  }
+
   pause() {
     if (this.timer) {
       window.clearTimeout(this.timer);
@@ -162,6 +171,8 @@ class Player extends BaseComponent {
             <Button icon={faPlay} primary onClick={() => this.resume(true)}>Play</Button>
           )
         }
+        <Button icon={faStepForward} primary disabled={!chunks.length}
+                onClick={() => this.stepOnce(true)}>单步</Button>
         <Button icon={faChevronLeft} primary disabled={!this.isValidCursor(cursor - 1)} onClick={() => this.prev()}/>
         <ProgressBar className={styles.progress_bar} current={cursor} total={chunks.length}
                      onChangeProgress={progress => this.handleChangeProgress(progress)}/>