[image_picker] Move I/O operations to a separate thread (flutter#3506)

Many I/O operations in the `image_picker` are currently carried out on the main thread (at least on Android). This blocks the UI (main) thread, freezing the UI and sometimes even causing an ANR dialog to pop up.

With these PR, many of said I/O operations now run on a separate thread. Specifically, it executes all code that is run in response to a picking result on a separate thread. So when an image is picked, for example, the callback will be ran outside of the UI thread. This change was rather easy, as said code was already making use of async paradigms.

The I/O operations carried out for caching logic (`ImagePickerDelegate.retrieveLostImage()`) are **not** changed by this PR, as my guess is that this would call for breaking changes to the API, as the result is currently returned synchronously instead.

When running the example app in [StrictMode](https://developer.android.com/reference/android/os/StrictMode) and picking an image, the problem becomes apparent, as it reports disk read/write violations. This was described in [flutter#100966](flutter#100966). I have verified that StrictMode no longer reports disk read/write violations after the changes. To check the output of StrictMode, I ran the example app with the changes listed [here](math1man/plugins@11f5f8a).

This PR closes several issues related to ANRs.

* Fixes [flutter#94120](flutter#94210)
* Fixes [flutter#114080](flutter#114080)

Also, it partly implements what is requested in [flutter#91393](flutter#91393).

Author: JeroenWeener

packages/image_picker/image_picker_android/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.8.6+10
+
+* Offloads picker result handling to separate thread.
+
 ## 0.8.6+9
 
 * Fixes compatibility with AGP versions older than 4.2.

packages/image_picker/image_picker_android/android/src/main/java/io/flutter/plugins/imagepicker/ImagePickerDelegate.java

@@ -32,6 +32,8 @@
 import java.util.List;
 import java.util.Map;
 import java.util.UUID;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
 
 /**
  * A delegate class doing the heavy lifting for the plugin.
@@ -112,6 +114,7 @@ private PendingCallState(
   private final PermissionManager permissionManager;
   private final FileUriResolver fileUriResolver;
   private final FileUtils fileUtils;
+  private final ExecutorService executor;
   private CameraDevice cameraDevice;
 
   interface PermissionManager {
@@ -134,6 +137,7 @@ interface OnPathReadyListener {
 
   private Uri pendingCameraMediaUri;
   private @Nullable PendingCallState pendingCallState;
+  private final Object pendingCallStateLock = new Object();
 
   public ImagePickerDelegate(
       final Activity activity,
@@ -185,7 +189,8 @@ public void onScanCompleted(String path, Uri uri) {
                 });
           }
         },
-        new FileUtils());
+        new FileUtils(),
+        Executors.newSingleThreadExecutor());
   }
 
   /**
@@ -203,7 +208,8 @@ public void onScanCompleted(String path, Uri uri) {
       final ImagePickerCache cache,
       final PermissionManager permissionManager,
       final FileUriResolver fileUriResolver,
-      final FileUtils fileUtils) {
+      final FileUtils fileUtils,
+      final ExecutorService executor) {
     this.activity = activity;
     this.externalFilesDirectory = externalFilesDirectory;
     this.imageResizer = imageResizer;
@@ -216,6 +222,7 @@ public void onScanCompleted(String path, Uri uri) {
     this.fileUriResolver = fileUriResolver;
     this.fileUtils = fileUtils;
     this.cache = cache;
+    this.executor = executor;
   }
 
   void setCameraDevice(CameraDevice device) {
@@ -224,19 +231,25 @@ void setCameraDevice(CameraDevice device) {
 
   // Save the state of the image picker so it can be retrieved with `retrieveLostImage`.
   void saveStateBeforeResult() {
-    if (pendingCallState == null) {
-      return;
+    ImageSelectionOptions localImageOptions;
+    synchronized (pendingCallStateLock) {
+      if (pendingCallState == null) {
+        return;
+      }
+      localImageOptions = pendingCallState.imageOptions;
     }
 
     cache.saveType(
-        pendingCallState.imageOptions != null
+        localImageOptions != null
             ? ImagePickerCache.CacheType.IMAGE
             : ImagePickerCache.CacheType.VIDEO);
-    if (pendingCallState.imageOptions != null) {
-      cache.saveDimensionWithOutputOptions(pendingCallState.imageOptions);
+    if (localImageOptions != null) {
+      cache.saveDimensionWithOutputOptions(localImageOptions);
     }
-    if (pendingCameraMediaUri != null) {
-      cache.savePendingCameraMediaUriPath(pendingCameraMediaUri);
+
+    final Uri localPendingCameraMediaUri = pendingCameraMediaUri;
+    if (localPendingCameraMediaUri != null) {
+      cache.savePendingCameraMediaUriPath(localPendingCameraMediaUri);
     }
   }
 
@@ -323,10 +336,16 @@ public void takeVideoWithCamera(
 
   private void launchTakeVideoWithCameraIntent() {
     Intent intent = new Intent(MediaStore.ACTION_VIDEO_CAPTURE);
-    if (pendingCallState != null
-        && pendingCallState.videoOptions != null
-        && pendingCallState.videoOptions.getMaxDurationSeconds() != null) {
-      int maxSeconds = pendingCallState.videoOptions.getMaxDurationSeconds().intValue();
+
+    VideoSelectionOptions localVideoOptions = null;
+    synchronized (pendingCallStateLock) {
+      if (pendingCallState != null) {
+        localVideoOptions = pendingCallState.videoOptions;
+      }
+    }
+
+    if (localVideoOptions != null && localVideoOptions.getMaxDurationSeconds() != null) {
+      int maxSeconds = localVideoOptions.getMaxDurationSeconds().intValue();
       intent.putExtra(MediaStore.EXTRA_DURATION_LIMIT, maxSeconds);
     }
     if (cameraDevice == CameraDevice.FRONT) {
@@ -537,27 +556,31 @@ public boolean onRequestPermissionsResult(
   }
 
   @Override
-  public boolean onActivityResult(int requestCode, int resultCode, Intent data) {
+  public boolean onActivityResult(final int requestCode, final int resultCode, final Intent data) {
+    Runnable handlerRunnable;
+
     switch (requestCode) {
       case REQUEST_CODE_CHOOSE_IMAGE_FROM_GALLERY:
-        handleChooseImageResult(resultCode, data);
+        handlerRunnable = () -> handleChooseImageResult(resultCode, data);
         break;
       case REQUEST_CODE_CHOOSE_MULTI_IMAGE_FROM_GALLERY:
-        handleChooseMultiImageResult(resultCode, data);
+        handlerRunnable = () -> handleChooseMultiImageResult(resultCode, data);
         break;
       case REQUEST_CODE_TAKE_IMAGE_WITH_CAMERA:
-        handleCaptureImageResult(resultCode);
+        handlerRunnable = () -> handleCaptureImageResult(resultCode);
         break;
       case REQUEST_CODE_CHOOSE_VIDEO_FROM_GALLERY:
-        handleChooseVideoResult(resultCode, data);
+        handlerRunnable = () -> handleChooseVideoResult(resultCode, data);
         break;
       case REQUEST_CODE_TAKE_VIDEO_WITH_CAMERA:
-        handleCaptureVideoResult(resultCode);
+        handlerRunnable = () -> handleCaptureVideoResult(resultCode);
         break;
       default:
         return false;
     }
 
+    executor.execute(handlerRunnable);
+
     return true;
   }
 
@@ -603,9 +626,11 @@ private void handleChooseVideoResult(int resultCode, Intent data) {
 
   private void handleCaptureImageResult(int resultCode) {
     if (resultCode == Activity.RESULT_OK) {
+      final Uri localPendingCameraMediaUri = pendingCameraMediaUri;
+
       fileUriResolver.getFullImagePath(
-          pendingCameraMediaUri != null
-              ? pendingCameraMediaUri
+          localPendingCameraMediaUri != null
+              ? localPendingCameraMediaUri
               : Uri.parse(cache.retrievePendingCameraMediaUriPath()),
           new OnPathReadyListener() {
             @Override
@@ -622,9 +647,10 @@ public void onPathReady(String path) {
 
   private void handleCaptureVideoResult(int resultCode) {
     if (resultCode == Activity.RESULT_OK) {
+      final Uri localPendingCameraMediaUrl = pendingCameraMediaUri;
       fileUriResolver.getFullImagePath(
-          pendingCameraMediaUri != null
-              ? pendingCameraMediaUri
+          localPendingCameraMediaUrl != null
+              ? localPendingCameraMediaUrl
               : Uri.parse(cache.retrievePendingCameraMediaUriPath()),
           new OnPathReadyListener() {
             @Override
@@ -641,10 +667,17 @@ public void onPathReady(String path) {
 
   private void handleMultiImageResult(
       ArrayList<String> paths, boolean shouldDeleteOriginalIfScaled) {
-    if (pendingCallState != null && pendingCallState.imageOptions != null) {
+    ImageSelectionOptions localImageOptions = null;
+    synchronized (pendingCallStateLock) {
+      if (pendingCallState != null) {
+        localImageOptions = pendingCallState.imageOptions;
+      }
+    }
+
+    if (localImageOptions != null) {
       ArrayList<String> finalPath = new ArrayList<>();
       for (int i = 0; i < paths.size(); i++) {
-        String finalImagePath = getResizedImagePath(paths.get(i), pendingCallState.imageOptions);
+        String finalImagePath = getResizedImagePath(paths.get(i), localImageOptions);
 
         //delete original file if scaled
         if (finalImagePath != null
@@ -661,8 +694,15 @@ private void handleMultiImageResult(
   }
 
   private void handleImageResult(String path, boolean shouldDeleteOriginalIfScaled) {
-    if (pendingCallState != null && pendingCallState.imageOptions != null) {
-      String finalImagePath = getResizedImagePath(path, pendingCallState.imageOptions);
+    ImageSelectionOptions localImageOptions = null;
+    synchronized (pendingCallStateLock) {
+      if (pendingCallState != null) {
+        localImageOptions = pendingCallState.imageOptions;
+      }
+    }
+
+    if (localImageOptions != null) {
+      String finalImagePath = getResizedImagePath(path, localImageOptions);
       //delete original file if scaled
       if (finalImagePath != null && !finalImagePath.equals(path) && shouldDeleteOriginalIfScaled) {
         new File(path).delete();
@@ -689,12 +729,13 @@ private boolean setPendingOptionsAndResult(
       @Nullable ImageSelectionOptions imageOptions,
       @Nullable VideoSelectionOptions videoOptions,
       @NonNull Messages.Result<List<String>> result) {
-    if (pendingCallState != null) {
-      return false;
+    synchronized (pendingCallStateLock) {
+      if (pendingCallState != null) {
+        return false;
+      }
+      pendingCallState = new PendingCallState(imageOptions, videoOptions, result);
     }
 
-    pendingCallState = new PendingCallState(imageOptions, videoOptions, result);
-
     // Clean up cache if a new image picker is launched.
     cache.clear();
 
@@ -710,37 +751,59 @@ private void finishWithSuccess(@Nullable String imagePath) {
     if (imagePath != null) {
       pathList.add(imagePath);
     }
-    if (pendingCallState == null) {
+
+    Messages.Result<List<String>> localResult = null;
+    synchronized (pendingCallStateLock) {
+      if (pendingCallState != null) {
+        localResult = pendingCallState.result;
+      }
+      pendingCallState = null;
+    }
+
+    if (localResult == null) {
       // Only save data for later retrieval if something was actually selected.
       if (!pathList.isEmpty()) {
         cache.saveResult(pathList, null, null);
       }
-      return;
+    } else {
+      localResult.success(pathList);
     }
-    pendingCallState.result.success(pathList);
-    pendingCallState = null;
   }
 
   private void finishWithListSuccess(ArrayList<String> imagePaths) {
-    if (pendingCallState == null) {
+    Messages.Result<List<String>> localResult = null;
+    synchronized (pendingCallStateLock) {
+      if (pendingCallState != null) {
+        localResult = pendingCallState.result;
+      }
+      pendingCallState = null;
+    }
+
+    if (localResult == null) {
       cache.saveResult(imagePaths, null, null);
-      return;
+    } else {
+      localResult.success(imagePaths);
     }
-    pendingCallState.result.success(imagePaths);
-    pendingCallState = null;
   }
 
   private void finishWithAlreadyActiveError(Messages.Result<List<String>> result) {
     result.error(new FlutterError("already_active", "Image picker is already active", null));
   }
 
   private void finishWithError(String errorCode, String errorMessage) {
-    if (pendingCallState == null) {
+    Messages.Result<List<String>> localResult = null;
+    synchronized (pendingCallStateLock) {
+      if (pendingCallState != null) {
+        localResult = pendingCallState.result;
+      }
+      pendingCallState = null;
+    }
+
+    if (localResult == null) {
       cache.saveResult(null, errorCode, errorMessage);
-      return;
+    } else {
+      localResult.error(new FlutterError(errorCode, errorMessage, null));
     }
-    pendingCallState.result.error(new FlutterError(errorCode, errorMessage, null));
-    pendingCallState = null;
   }
 
   private void useFrontCamera(Intent intent) {