Added OrderEnforcer class to schedule ordered tasks.

src/org/thoughtcrime/securesms/camera/OrderEnforcer.java

@@ -0,0 +1,74 @@
+package org.thoughtcrime.securesms.camera;
+
+import android.support.annotation.NonNull;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+public class OrderEnforcer<E> {
+
+  private final E[]                    stages;
+  private final Map<E, Integer>        stageIndices;
+  private final Map<E, List<Runnable>> actions;
+  private final boolean[]              completion;
+
+  public OrderEnforcer(@NonNull E... stages) {
+    this.stages       = stages;
+    this.stageIndices = new HashMap<>();
+    this.actions      = new HashMap<>();
+    this.completion   = new boolean[stages.length];
+
+    for (int i = 0; i < stages.length; i++) {
+      stageIndices.put(stages[i], i);
+    }
+  }
+
+  public synchronized void run(@NonNull E stage, Runnable r) {
+    if (isCompletedThrough(stage)) {
+      r.run();
+    } else {
+      List<Runnable> stageActions = actions.containsKey(stage) ? actions.get(stage) : new CopyOnWriteArrayList<>();
+      stageActions.add(r);
+
+      actions.put(stage, stageActions);
+    }
+  }
+
+  public synchronized void markCompleted(@NonNull E stage) {
+    completion[stageIndices.get(stage)] = true;
+
+    int i = 0;
+    while (i < completion.length && completion[i]) {
+      List<Runnable> stageActions = actions.get(stages[i]);
+      if (stageActions != null) {
+        for (Runnable r : stageActions) {
+          r.run();
+        }
+        stageActions.clear();
+      }
+      i++;
+    }
+  }
+
+  public synchronized void reset() {
+    for (int i = 0; i < completion.length; i++) {
+      completion[i] = false;
+    }
+    actions.clear();
+  }
+
+  private boolean isCompletedThrough(@NonNull E stage) {
+    int index = stageIndices.get(stage);
+    int i     = 0;
+
+    while (i <= index && i < completion.length) {
+      if (!completion[i]) {
+        return false;
+      }
+      i++;
+    }
+    return true;
+  }
+}