feat: 优化代码

app/src/main/java/io/legado/app/service/help/Download.kt

@@ -17,4 +17,11 @@ object Download {
         }
     }
 
+    fun stop(context: Context) {
+        Intent(context, DownloadService::class.java).let {
+            it.action = IntentAction.stop
+            context.startService(it)
+        }
+    }
+
 }

app/src/main/java/io/legado/app/ui/download/DownloadActivity.kt

@@ -58,6 +58,7 @@ class DownloadActivity : VMBaseActivity<DownloadViewModel>(R.layout.activity_dow
     override fun onCompatOptionsItemSelected(item: MenuItem): Boolean {
         when (item.itemId) {
             R.id.menu_download -> launch(IO) {
+                if (adapter.downloadMap.isNullOrEmpty()) {
                     App.db.bookDao().webBooks.forEach { book ->
                         Download.start(
                             this@DownloadActivity,
@@ -66,6 +67,9 @@ class DownloadActivity : VMBaseActivity<DownloadViewModel>(R.layout.activity_dow
                             book.totalChapterNum
                         )
                     }
+                } else {
+                    Download.stop(this@DownloadActivity)
+                }
             }
         }
         return super.onCompatOptionsItemSelected(item)