Android Gallery詳解

Android Gallery是一個可以展示圖片或者視頻等多媒體內容的組件。它也是一個支持手勢操作的滑動控制項，用戶可以通過滑動屏幕來切換圖片等多媒體內容。在本文中，我們將從以下幾個方面來對Android Gallery進行詳細的闡述：

一、Gallery的基本使用

public class MainActivity extends Activity {

    private Gallery mGallery;

    private int[] mImageIds = new int[] {
            R.drawable.img1,
            R.drawable.img2,
            R.drawable.img3,
            R.drawable.img4,
            R.drawable.img5
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mGallery = (Gallery) findViewById(R.id.gallery);
        mGallery.setAdapter(new ImageAdapter(this));
    }

    private class ImageAdapter extends BaseAdapter {

        private Context mContext;

        public ImageAdapter(Context context) {
            this.mContext = context;
        }

        @Override
        public int getCount() {
            return mImageIds.length;
        }

        @Override
        public Object getItem(int position) {
            return mImageIds[position];
        }

        @Override
        public long getItemId(int position) {
            return position;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            ImageView imageView = new ImageView(mContext);
            imageView.setImageResource(mImageIds[position]);
            imageView.setLayoutParams(new Gallery.LayoutParams(240, 180));
            imageView.setScaleType(ImageView.ScaleType.FIT_XY);
            return imageView;
        }
    }
}

以上代碼展示了Gallery的基本使用方法，需要注意的是，我們需要為Gallery設置一個Adapter來為其提供數據，同時，我們也需要自定義Adapter來實現展示圖片的效果。在這個例子中，我們創建了一個ImageAdapter類，重寫了Adapter中的方法來實現圖片展示，同時我們也為ImageView設置了LayoutParams和ScaleType。

二、Gallery的滑動特效

Android的Gallery控制項具有多種滑動特效，包括平移、漸變、縮放等等。我們可以通過定義我們自己的Animation來實現Gallery的滑動特效。

public class MainActivity extends Activity {

    private Gallery mGallery;

    private int[] mImageIds = new int[] {
            R.drawable.img1,
            R.drawable.img2,
            R.drawable.img3,
            R.drawable.img4,
            R.drawable.img5
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mGallery = (Gallery) findViewById(R.id.gallery);
        mGallery.setAdapter(new ImageAdapter(this));
        mGallery.setAnimationDuration(1000);
        mGallery.setAnimation(new TranslateAnimation(0, 200, 0, 0));
    }

    private class ImageAdapter extends BaseAdapter {

        private Context mContext;

        public ImageAdapter(Context context) {
            this.mContext = context;
        }

        @Override
        public int getCount() {
            return mImageIds.length;
        }

        @Override
        public Object getItem(int position) {
            return mImageIds[position];
        }

        @Override
        public long getItemId(int position) {
            return position;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            ImageView imageView = new ImageView(mContext);
            imageView.setImageResource(mImageIds[position]);
            imageView.setLayoutParams(new Gallery.LayoutParams(240, 180));
            imageView.setScaleType(ImageView.ScaleType.FIT_XY);
            return imageView;
        }
    }
}

在以上代碼中，我們定義了一個TranslateAnimation對象，用來指定Gallery的滑動方向和距離。同時我們也通過setAnimationDuration方法來設置Gallery的滑動時間。

三、Gallery的點擊事件

我們也可以為Gallery設置點擊事件，當用戶點擊圖片時可以進行特定操作。例如，在這個例子中，當用戶點擊圖片時，我們將展示該圖片的名稱。

public class MainActivity extends Activity {

    private Gallery mGallery;

    private int[] mImageIds = new int[] {
            R.drawable.img1,
            R.drawable.img2,
            R.drawable.img3,
            R.drawable.img4,
            R.drawable.img5
    };

    private String[] mImageNames = new String[] {
            "圖片1",
            "圖片2",
            "圖片3",
            "圖片4",
            "圖片5"
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mGallery = (Gallery) findViewById(R.id.gallery);
        mGallery.setAdapter(new ImageAdapter(this));
        mGallery.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView parent, View view, int position, long id) {
                Toast.makeText(MainActivity.this, mImageNames[position], Toast.LENGTH_SHORT).show();
            }
        });
    }

    private class ImageAdapter extends BaseAdapter {

        private Context mContext;

        public ImageAdapter(Context context) {
            this.mContext = context;
        }

        @Override
        public int getCount() {
            return mImageIds.length;
        }

        @Override
        public Object getItem(int position) {
            return mImageIds[position];
        }

        @Override
        public long getItemId(int position) {
            return position;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            ImageView imageView = new ImageView(mContext);
            imageView.setImageResource(mImageIds[position]);
            imageView.setLayoutParams(new Gallery.LayoutParams(240, 180));
            imageView.setScaleType(ImageView.ScaleType.FIT_XY);
            return imageView;
        }
    }
}

在以上代碼中，我們重寫了Gallery的OnItemClickListener方法，當用戶點擊某個圖片時，我們通過Toast來展示該圖片的名稱。

四、Gallery的自定義布局

我們也可以為Gallery設置自定義布局，例如，在這個例子中，我們將為Gallery中每個圖片添加一個TextView顯示圖片的名稱。

public class MainActivity extends Activity {

    private Gallery mGallery;

    private int[] mImageIds = new int[] {
            R.drawable.img1,
            R.drawable.img2,
            R.drawable.img3,
            R.drawable.img4,
            R.drawable.img5
    };

    private String[] mImageNames = new String[] {
            "圖片1",
            "圖片2",
            "圖片3",
            "圖片4",
            "圖片5"
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mGallery = (Gallery) findViewById(R.id.gallery);
        mGallery.setAdapter(new ImageAdapter(this));
        mGallery.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView parent, View view, int position, long id) {
                Toast.makeText(MainActivity.this, mImageNames[position], Toast.LENGTH_SHORT).show();
            }
        });
    }

    private class ImageAdapter extends BaseAdapter {

        private Context mContext;

        public ImageAdapter(Context context) {
            this.mContext = context;
        }

        @Override
        public int getCount() {
            return mImageIds.length;
        }

        @Override
        public Object getItem(int position) {
            return mImageIds[position];
        }

        @Override
        public long getItemId(int position) {
            return position;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            View view = LayoutInflater.from(mContext).inflate(R.layout.item_gallery, null);
            ImageView imageView = (ImageView) view.findViewById(R.id.iv_image);
            TextView textView = (TextView) view.findViewById(R.id.tv_name);
            imageView.setImageResource(mImageIds[position]);
            textView.setText(mImageNames[position]);
            return view;
        }
    }
}

以上代碼中，我們通過調用LayoutInflater的方法來載入自定義布局，並且在布局中添加了一個TextView來展示圖片名稱。

五、Gallery的性能優化

對於一些超大的圖片或者視屏數據，在Gallery中進行展示可能會影響到應用的性能。在這種情況下，我們可以通過非同步載入來進行性能優化。

public class MainActivity extends Activity {

    private Gallery mGallery;

    private int[] mImageIds = new int[] {
            R.drawable.img1,
            R.drawable.img2,
            R.drawable.img3,
            R.drawable.img4,
            R.drawable.img5
    };

    private String[] mImageNames = new String[] {
            "圖片1",
            "圖片2",
            "圖片3",
            "圖片4",
            "圖片5"
    };

    private ImageLoader mImageLoader;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mGallery = (Gallery) findViewById(R.id.gallery);
        mGallery.setAdapter(new ImageAdapter(this));
        mGallery.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView parent, View view, int position, long id) {
                Toast.makeText(MainActivity.this, mImageNames[position], Toast.LENGTH_SHORT).show();
            }
        });

        mImageLoader = new ImageLoader();
    }

    private class ImageAdapter extends BaseAdapter {

        private Context mContext;

        public ImageAdapter(Context context) {
            this.mContext = context;
        }

        @Override
        public int getCount() {
            return mImageIds.length;
        }

        @Override
        public Object getItem(int position) {
            return mImageIds[position];
        }

        @Override
        public long getItemId(int position) {
            return position;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            ImageView imageView;
            if (convertView == null) {
                imageView = new ImageView(mContext);
            } else {
                imageView = (ImageView) convertView;
            }
            imageView.setTag(mImageIds[position]);
            mImageLoader.loadImage(mImageIds[position], imageView);
            imageView.setLayoutParams(new Gallery.LayoutParams(240, 180));
            imageView.setScaleType(ImageView.ScaleType.FIT_XY);
            return imageView;
        }
    }

    private class ImageLoader {

        private SparseArray mBitmaps = new SparseArray();

        public void loadImage(int resId, ImageView target) {
            Bitmap bitmap = mBitmaps.get(resId);
            if (bitmap != null) {
                target.setImageBitmap(bitmap);
            } else {
                new LoadTask(resId, target).execute();
            }
        }

        private class LoadTask extends AsyncTask {

            private int resId;
            private ImageView target;

            public LoadTask(int resId, ImageView target) {
                this.resId = resId;
                this.target = target;
            }

            @Override
            protected Bitmap doInBackground(Void... params) {
                BitmapFactory.Options options = new BitmapFactory.Options();
                options.inJustDecodeBounds = true;
                BitmapFactory.decodeResource(getResources(), resId, options);
                int width = options.outWidth;
                int height = options.outHeight;
                options.inSampleSize = Math.max(width / 240, height / 180);
                options.inJustDecodeBounds = false;
                Bitmap bitmap = BitmapFactory.decodeResource(getResources(), resId, options);
                mBitmaps.put(resId, bitmap);
                return bitmap;
            }

            @Override
            protected void onPostExecute(Bitmap bitmap) {
                Object tag = target.getTag();
                if (tag != null && tag instanceof Integer && (int) tag == resId) {
                    target.setImageBitmap(bitmap);
                }
            }
        }
    }
}

以上代碼中，我們通過自定義ImageLoader類和LoadTask類來非同步載入Gallery中的圖片，同時也添加了一個緩存機制來提高性能。