sample, add reddit spoiler sample

README.md

@@ -84,7 +84,7 @@ and [3.x.x](https://github.com/noties/Markwon/tree/3.x.x) branches
 
 ## Screenshots
 
-Taken with default configuration (except for image loading):
+Taken with default configuration (except for image loading) in [sample app](./app-sample/):
 
 <a href="./art/mw_light_01.png"><img src="./art/mw_light_01.png" width="30%" /></a>
 <a href="./art/mw_light_02.png"><img src="./art/mw_light_02.png" width="30%" /></a>

app-sample/samples.json

@@ -1,4 +1,16 @@
 [
+  {
+    "javaClassName": "io.noties.markwon.app.samples.RedditSpoilerSample",
+    "id": "20200813145316",
+    "title": "Reddit spoiler",
+    "description": "An attempt to implement Reddit spoiler syntax `\u003e! !\u003c`",
+    "artifacts": [
+      "CORE"
+    ],
+    "tags": [
+      "parsing"
+    ]
+  },
   {
     "javaClassName": "io.noties.markwon.app.samples.image.CoilRecyclerViewSample",
     "id": "20200803132053",

app-sample/src/main/java/io/noties/markwon/app/samples/RedditSpoilerSample.java

@@ -0,0 +1,126 @@
+package io.noties.markwon.app.samples;
+
+import android.graphics.Color;
+import android.text.Spannable;
+import android.text.Spanned;
+import android.text.TextPaint;
+import android.text.style.CharacterStyle;
+import android.text.style.ClickableSpan;
+import android.view.View;
+import android.widget.TextView;
+
+import androidx.annotation.NonNull;
+
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import io.noties.markwon.AbstractMarkwonPlugin;
+import io.noties.markwon.Markwon;
+import io.noties.markwon.app.sample.Tags;
+import io.noties.markwon.app.sample.ui.MarkwonTextViewSample;
+import io.noties.markwon.sample.annotations.MarkwonArtifact;
+import io.noties.markwon.sample.annotations.MarkwonSampleInfo;
+import io.noties.markwon.utils.ColorUtils;
+
+@MarkwonSampleInfo(
+  id = "20200813145316",
+  title = "Reddit spoiler",
+  description = "An attempt to implement Reddit spoiler syntax `>! !<`",
+  artifacts = MarkwonArtifact.CORE,
+  tags = Tags.parsing
+)
+public class RedditSpoilerSample extends MarkwonTextViewSample {
+  @Override
+  public void render() {
+    final String md = "" +
+      "# Reddit spolier\n\n" +
+      "Hello >!ugly so **ugly** !<, how are you?\n\n" +
+      ">!a blockquote?!< should not be >!present!< yeah" +
+      "";
+
+    final Markwon markwon = Markwon.builder(context)
+      .usePlugin(new RedditSpoilerPlugin())
+      .build();
+
+    markwon.setMarkdown(textView, md);
+  }
+}
+
+class RedditSpoilerPlugin extends AbstractMarkwonPlugin {
+
+  private static final Pattern RE = Pattern.compile(">!.+?!<");
+
+  @NonNull
+  @Override
+  public String processMarkdown(@NonNull String markdown) {
+    // replace all `>!` with `&gt;!` so no blockquote would be parsed (when spoiler starts at new line)
+    return markdown.replaceAll(">!", "&gt;!");
+  }
+
+  @Override
+  public void beforeSetText(@NonNull TextView textView, @NonNull Spanned markdown) {
+    applySpoilerSpans((Spannable) markdown);
+  }
+
+  private static void applySpoilerSpans(@NonNull Spannable spannable) {
+    final String text = spannable.toString();
+    final Matcher matcher = RE.matcher(text);
+
+    while (matcher.find()) {
+
+      final RedditSpoilerSpan spoilerSpan = new RedditSpoilerSpan();
+      final ClickableSpan clickableSpan = new ClickableSpan() {
+        @Override
+        public void onClick(@NonNull View widget) {
+          spoilerSpan.setRevealed(true);
+          widget.postInvalidateOnAnimation();
+        }
+
+        @Override
+        public void updateDrawState(@NonNull TextPaint ds) {
+          // no op
+        }
+      };
+
+      final int s = matcher.start();
+      final int e = matcher.end();
+      spannable.setSpan(spoilerSpan, s, e, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
+      spannable.setSpan(clickableSpan, s, e, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
+
+      // we also can hide original syntax
+      spannable.setSpan(new HideSpoilerSyntaxSpan(), s, s + 2, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
+      spannable.setSpan(new HideSpoilerSyntaxSpan(), e - 2, e, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
+    }
+  }
+
+  private static class RedditSpoilerSpan extends CharacterStyle {
+
+    private boolean revealed;
+
+    @Override
+    public void updateDrawState(TextPaint tp) {
+      if (!revealed) {
+        // use the same text color
+        tp.bgColor = Color.BLACK;
+        tp.setColor(Color.BLACK);
+      } else {
+        // for example keep a bit of black background to remind that it is a spoiler
+        tp.bgColor = ColorUtils.applyAlpha(Color.BLACK, 25);
+      }
+    }
+
+    public void setRevealed(boolean revealed) {
+      this.revealed = revealed;
+    }
+  }
+
+  // we also could make text size smaller (but then MetricAffectingSpan should be used)
+  private static class HideSpoilerSyntaxSpan extends CharacterStyle {
+
+    @Override
+    public void updateDrawState(TextPaint tp) {
+      // set transparent color
+      tp.setColor(0);
+    }
+  }
+}