refactor：增加ImageUpload，改用策略模式上传图片

lib/utils/image_upload.dart

@@ -0,0 +1,52 @@
+import 'dart:io';
+import 'package:flutter_picgo/model/uploaded.dart';
+import 'package:flutter_picgo/resources/table_name_keys.dart';
+import 'package:flutter_picgo/utils/shared_preferences.dart';
+import 'package:flutter_picgo/utils/sql.dart';
+import 'package:flutter_picgo/utils/strategy/image_upload_strategy.dart';
+
+/// 图像上传类
+class ImageUpload {
+  ImageUploadStrategy _strategy;
+
+  ImageUpload(this._strategy);
+
+  Future<Uploaded> delete(Uploaded uploaded) {
+    return _strategy.delete(uploaded);
+  }
+
+  Future<Uploaded> upload(File file, String renameImage) {
+    return _strategy.upload(file, renameImage);
+  }
+
+  /// 获取默认图床类型
+  Future<String> getDefaultPB() async {
+    var sp = await SpUtil.getInstance();
+    String pbType = sp.getDefaultPB();
+    return pbType;
+  }
+
+  /// 设置默认图床
+  Future setDefaultPB(String type) async {
+    var sp = await SpUtil.getInstance();
+    if (sp.getDefaultPB() != type) {
+      sp.setDefaultPB(type);
+    }
+  }
+
+  /// 保存图床配置
+  Future<int> savePBConfig(String type, String config) async {
+    var sql = Sql.setTable(TABLE_NAME_PBSETTING);
+    int raw = await sql.rawUpdate('config = ? WHERE type = ?', [config, type]);
+    return raw;
+  }
+
+  /// 获取当前图床配置
+  Future<String> getPBConfig(String type) async {
+    var sql = Sql.setTable(TABLE_NAME_PBSETTING);
+    var pbsettingRow = await sql.getBySql('type = ?', [type]);
+    if (pbsettingRow != null && pbsettingRow.length > 0) {
+      return pbsettingRow.first["config"];
+    }
+  }
+}

lib/utils/strategy/github_image_upload.dart

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+import 'package:flutter_picgo/model/uploaded.dart';
+import 'dart:io';
+
+import 'package:flutter_picgo/utils/strategy/image_upload_strategy.dart';
+
+class GithubImageUpload implements ImageUploadStrategy {
+
+  @override
+  Future<Uploaded> delete(Uploaded uploaded) async {
+    return null;
+  }
+
+  @override
+  Future<Uploaded> upload(File file, String renameImage) {
+    return null;
+  }
+}

lib/utils/strategy/image_upload_strategy.dart

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+import 'dart:io';
+import 'package:flutter_picgo/model/uploaded.dart';
+
+abstract class ImageUploadStrategy {
+  
+  /// 上传照片，根据策略来实现上传的图片
+  Future<Uploaded> upload(File file, String renameImage);
+
+  Future<Uploaded> delete(Uploaded uploaded);
+
+}