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Android面试重点.md

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Imageloader

常用的图片处理框架

Fresco 正在实现了三级缓存，2级内存缓存，一级本地缓存、Glide、Picasso、UIL-ImageLoader

图片占用内存

大小 = 每个像素占用的字节数 * 总像素

Bitmap.Config

  • RGB_565
  • ARGB_8888

内存溢出

图片乱序错位问题

imageview.setTag(url)

弱引用双向关联

使用volley的NetImageView

四种引用类型

  • 强引用
  • 弱引用
  • 软引用
  • 虚引用

三级缓存

1、DiskLrucache 本地/磁盘缓存

2、Lrucache 内存缓存

原理

LinkedHashMap 双向循环链表

  • 参数1：初始大小
  • 参数2：装载因子
  • 参数3：true 按访问顺序排序，false 按插入顺序排序

内存大小

int memory = (int) (Runtime.getRuntime().maxMemory()/8);
LruCache<String,Bitmap> cache = new LruCache<String,Bitmap>(memory){
    @Override
    protected int sizeOf(String key, Bitmap value) {
        return value.getByteCount();
    }
};

Lrucache .get(key) --> LinkedHashMap.get(key) : 命中，移动到双向循环链表的的尾部

Lrucache .put(key) --> LinkedHashMap.put(key) : 如果size > maxSize，删除链表header节点直到size < maxSize

3、 网络缓存

4、缓存路径

  • getExternalCacheDir()
  • getCacheDir()

图片压缩

BitmapFactory.Options

  • inJustDecodeBounds
  • inSampleSize
  • outWidth
  • outHeight
  • inBitmap
   /**
     * Decode and sample down a bitmap from resources to the requested width and height.
     *
     * @param res The resources object containing the image data
     * @param resId The resource id of the image data
     * @param reqWidth The requested width of the resulting bitmap
     * @param reqHeight The requested height of the resulting bitmap
     * @param cache The ImageCache used to find candidate bitmaps for use with inBitmap
     * @return A bitmap sampled down from the original with the same aspect ratio and dimensions
     *         that are equal to or greater than the requested width and height
     */
    public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId,
            int reqWidth, int reqHeight, ImageCache cache) {

        // BEGIN_INCLUDE (read_bitmap_dimensions)
        // First decode with inJustDecodeBounds=true to check dimensions
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(res, resId, options);

        // Calculate inSampleSize
        options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
        // END_INCLUDE (read_bitmap_dimensions)

        // If we're running on Honeycomb or newer, try to use inBitmap
        if (Utils.hasHoneycomb()) {
            addInBitmapOptions(options, cache);
        }

        // Decode bitmap with inSampleSize set
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeResource(res, resId, options);
    }

   /**
     * Decode and sample down a bitmap from a file to the requested width and height.
     *
     * @param filename The full path of the file to decode
     * @param reqWidth The requested width of the resulting bitmap
     * @param reqHeight The requested height of the resulting bitmap
     * @param cache The ImageCache used to find candidate bitmaps for use with inBitmap
     * @return A bitmap sampled down from the original with the same aspect ratio and dimensions
     *         that are equal to or greater than the requested width and height
     */
    public static Bitmap decodeSampledBitmapFromFile(String filename,
            int reqWidth, int reqHeight, ImageCache cache) {

        // First decode with inJustDecodeBounds=true to check dimensions
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeFile(filename, options);

        // Calculate inSampleSize
        options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);

        // If we're running on Honeycomb or newer, try to use inBitmap
        if (Utils.hasHoneycomb()) {
            addInBitmapOptions(options, cache);
        }

        // Decode bitmap with inSampleSize set
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeFile(filename, options);
    }

   /**
     * Decode and sample down a bitmap from a file input stream to the requested width and height.
     *
     * @param fileDescriptor The file descriptor to read from
     * @param reqWidth The requested width of the resulting bitmap
     * @param reqHeight The requested height of the resulting bitmap
     * @param cache The ImageCache used to find candidate bitmaps for use with inBitmap
     * @return A bitmap sampled down from the original with the same aspect ratio and dimensions
     *         that are equal to or greater than the requested width and height
     */
    public static Bitmap decodeSampledBitmapFromDescriptor(
            FileDescriptor fileDescriptor, int reqWidth, int reqHeight, ImageCache cache) {

        // First decode with inJustDecodeBounds=true to check dimensions
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeFileDescriptor(fileDescriptor, null, options);

        // Calculate inSampleSize
        options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);

        // Decode bitmap with inSampleSize set
        options.inJustDecodeBounds = false;

        // If we're running on Honeycomb or newer, try to use inBitmap
        if (Utils.hasHoneycomb()) {
            addInBitmapOptions(options, cache);
        }

        return BitmapFactory.decodeFileDescriptor(fileDescriptor, null, options);
    }
   /**
     * Calculate an inSampleSize for use in a {@link android.graphics.BitmapFactory.Options} object when decoding
     * bitmaps using the decode* methods from {@link android.graphics.BitmapFactory}. This implementation calculates
     * the closest inSampleSize that is a power of 2 and will result in the final decoded bitmap
     * having a width and height equal to or larger than the requested width and height.
     *
     * @param options An options object with out* params already populated (run through a decode*
     *            method with inJustDecodeBounds==true
     * @param reqWidth The requested width of the resulting bitmap
     * @param reqHeight The requested height of the resulting bitmap
     * @return The value to be used for inSampleSize
     */
    public static int calculateInSampleSize(BitmapFactory.Options options,
                                            int reqWidth, int reqHeight) {
        // BEGIN_INCLUDE (calculate_sample_size)
        // Raw height and width of image
        final int height = options.outHeight;
        final int width = options.outWidth;
        int inSampleSize = 1;

        if (height > reqHeight || width > reqWidth) {

            final int halfHeight = height / 2;
            final int halfWidth = width / 2;

            // Calculate the largest inSampleSize value that is a power of 2 and keeps both
            // height and width larger than the requested height and width.
            while ((halfHeight / inSampleSize) > reqHeight
                    && (halfWidth / inSampleSize) > reqWidth) {
                inSampleSize *= 2;
            }

            // This offers some additional logic in case the image has a strange
            // aspect ratio. For example, a panorama may have a much larger
            // width than height. In these cases the total pixels might still
            // end up being too large to fit comfortably in memory, so we should
            // be more aggressive with sample down the image (=larger inSampleSize).

            long totalPixels = width * height / inSampleSize;

            // Anything more than 2x the requested pixels we'll sample down further
            final long totalReqPixelsCap = reqWidth * reqHeight * 2;

            while (totalPixels > totalReqPixelsCap) {
                inSampleSize *= 2;
                totalPixels /= 2;
            }
        }
        return inSampleSize;
        // END_INCLUDE (calculate_sample_size)
    }

EventBus

1. 事件总线

组件（activity，fragment，service）间的交互，通信，主线程子线程间的通信

观察者模式，发布订阅，注解，Map<订阅对象，订阅方法>

事件类型EventType，事件响应函数

2. 发布接收事件

  • 发布事件post()

EventBus.getDefault().post(new User("mr.simple"), "my_tag");

反射调用，回调

  • 注册事件register()

扫描订阅对象的所有方法，包括分类的方法，匹配方法，保存到Map集合中

  • 接收事件，事件响应函数

@Subscriber(tag = "my_tag", mode=ThreadMode.POST)

3. EventBus

订阅队列

Map<EventType,CopyOnWriteArrayList>

判断是否是主线程：Looper.getMainLooper() == Looper.myLooper();

Handler mHandler = new Handler(Looper.getMainLooper());

ThreadLocal<PostingThreadState>

存储了⼀个事件队列以及事件的状态

class PostingThread
{
List<Object> mEventQueue = new ArrayList<Object>();
boolean isMainThread;
boolean isPosting;
}

Map<Class, CopyOnWriteArrayList<SubscribeMethod>>

通过订阅对象的字节码拿到订阅对象匹配的方法以及方法的参数，其中threadmode是通过截取字符串获得，EventBus3.0是通过注解的方式拿到threadmode，方法的参数就是EventType

Android EventBus

EventType

方法的参数和tag组成EventType

依赖注入IOC

ImageLoader

面向接口编程，策略模式，BitmapRequest，链式调用 return this Builder模式，单例模式，生产者消费者模式

模板方法模式（算法骨架）

BitmapRequest

图片请求

ImageLoaderConfig

设置一些基本的东西，比如加载中的图片、加载失败的图片、缓存策略

RequestQueue

内部维持着一个PriorityBlockingQueue

BlockingQueue<BitmapRequest> mRequestQueue = new PriorityBlockingQueue<BitmapRequest>();

RequestDispatcher

HttpUtils

常用的网络请求框架

  • Retrofit

  • OkHttp

  • NoHttp

  • Volley

  • HttpURLConnection（省电省流量）

  • HttpClient（6.0后删除）

  • AsyncHttpclient

请求头Map<String, String> mHeaders，请求参数Map<String, String> mBodyParams，URLEncoder.encode()

  • NetworkExecutor 网络请求线程
  • HttpStack Http执行器
  • ResponseDelivery Response分发

Http协议

请求Request

  • 请求首行
  • 请求头
  • 空行
  • 请求体

响应Response

  • 响应首行
  • 响应头
  • 空行
  • 响应体

请求头

响应头

通用头

ContentType

  • get 查
  • post 改
  • put 增 提交表单
  • delete 删
  • head 只返回首部
  • trace 诊断，跟踪http请求是否被修改
  • options 请求服务器告知其支持的各种功能

网络模型

TCP/IP参考模型

  • 应用层 http，https，ftp
  • 传输层 tcp udp
  • 网络层 ip地址
  • 物理层
  • 数据链路层

三次、四次握手

三要素

ip地址，端口号，协议

文件上传

表单数据，Content-Type:multipart/form-data

ORM框架

注解（运行时注解或编译时注解）反射

注解标记表和表中的列，javabean上添加注解，指定一个类对应的表名，一个字段在表中对应的列名

在清单文件中配置数据库名，版本号，还可以配置model

model类可以从清单文件中获取，也可以通过扫描整个应用获取，扫描当前App的dex文件

创建表：通过注解标记表名和字段名，通过反射拿到model的信息来拼接sql语句

数据库操作类

  • Selection
  • Delete
  • Update
  • Insert