首先說一下定義這樣一個View有什麼用?在一些app中,需要設定頭像,而使用者選擇的圖片可能是使用攝像頭拍攝,也可能是選擇的相簿裡面的圖片,總之,這樣的圖片大小不一,就比如在使用某個聊天軟體的時候,設定頭像,需要對圖片進行擷取.

要實現這樣一個功能,首先,需要分析使用者的操作,即使用者所點選的View的位置,如下圖,我把View分為9個區域,

• 當ACTION_DOWN時如果座標為1.2.3.4四個區域,則對View進行相應的左上/右上/左下/右下拉伸;
• 當ACTION_DOWN時如果座標為5.6.7.8四個區域,則分別對上/右/下/左四個方向進行拉伸; 
• 當ACTION_DOWN時如果座標為9這個區域,則對View進行移動;

理論分析完成,下面來看具體實現;

在下面的類中,有五個方法center/left/top/bottom/right分別對應移動/向左拉伸/向上拉伸/向下拉伸/向右拉伸,當Action_down為1-4所在的區域時,組合前面的對應的兩個拉伸方法即可,如左上角拉伸則對應執行left+top方法,這也是把四個單獨一條邊的邊緣拉伸獨立出來的原因;

在View中,我設定了View的最小寬度和高度,都是200,所以當用戶點選邊緣進行縮小操作時,能縮小的最小值也就是200;分別在left/top/bottom/right中體現;

public class DragScaleView extends View implements OnTouchListener {
    protected int screenWidth;
    protected int screenHeight;
    protected int lastX;
    protected int lastY;
    private int oriLeft;
    private int oriRight;
    private int oriTop;
    private int oriBottom;
    private int dragDirection;
    private static final int TOP = 0x15;
    private static final int LEFT = 0x16;
    private static final int BOTTOM = 0x17;
    private static final int RIGHT = 0x18;
    private static final int LEFT_TOP = 0x11;
    private static final int RIGHT_TOP = 0x12;
    private static final int LEFT_BOTTOM = 0x13;
    private static final int RIGHT_BOTTOM = 0x14;
    private static final int CENTER = 0x19;
    private int offset = 20;
    protected Paint paint = new Paint();

   
    protected void initScreenW_H() {
        screenHeight = getResources().getDisplayMetrics().heightPixels - 40;
        screenWidth = getResources().getDisplayMetrics().widthPixels;
    }

    public DragScaleView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        setOnTouchListener(this);
        initScreenW_H();
    }

    public DragScaleView(Context context, AttributeSet attrs) {
        super(context, attrs);
        setOnTouchListener(this);
        initScreenW_H();
    }

    public DragScaleView(Context context) {
        super(context);
        setOnTouchListener(this);
        initScreenW_H();
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        paint.setColor(Color.RED);
        paint.setStrokeWidth(4.0f);
        paint.setStyle(Style.STROKE);
        canvas.drawRect(offset, offset, getWidth() - offset, getHeight()
                - offset, paint);
    }

    @Override
    public boolean onTouch(View v, MotionEvent event) {
        int action = event.getAction();
        if (action == MotionEvent.ACTION_DOWN) {
            oriLeft = v.getLeft();
            oriRight = v.getRight();
            oriTop = v.getTop();
            oriBottom = v.getBottom();
            lastY = (int) event.getRawY();
            lastX = (int) event.getRawX();
            dragDirection = getDirection(v, (int) event.getX(),
                    (int) event.getY());
        }
        // 處理拖動事件
        delDrag(v, event, action);
        invalidate();
        return false;
    }

   
    protected void delDrag(View v, MotionEvent event, int action) {
        switch (action) {
        case MotionEvent.ACTION_MOVE:
            int dx = (int) event.getRawX() - lastX;
            int dy = (int) event.getRawY() - lastY;
            switch (dragDirection) {
            case LEFT: // 左邊緣
                left(v, dx);
                break;
            case RIGHT: // 右邊緣
                right(v, dx);
                break;
            case BOTTOM: // 下邊緣
                bottom(v, dy);
                break;
            case TOP: // 上邊緣
                top(v, dy);
                break;
            case CENTER: // 點選中心–>>移動
                center(v, dx, dy);