Android自定义裁剪图片的View

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前些天，分析了开源框架zxing的源码。里边有一个自定义的view，扫描界面的蒙层。这给我做自定义裁剪图片view的一些启发。因为，其实原理很相似，都是中间显示的图片没有被遮盖，四周有蒙层的效果。接下来，就按照这个思路实现这个自定义的view。

上图

预览图

效果分析

1 刚进来，设置蒙层刚好包裹图片显示的大小

2 当触摸的是边界的时候，增大蒙层的范围，即缩小透明区域的大小。

3 当触摸的是透明区域的内部时，移动手指，拖动整个透明的区域

实现细节

1 根据图片显示的大小，描绘蒙层

// 画阴影
canvas.drawRect(0, 0, frame.left, height, paint);
canvas.drawRect(frame.left, 0, frame.right, frame.top, paint);
canvas.drawRect(frame.right, 0, width, height, paint);
canvas.drawRect(frame.left, frame.bottom, frame.right, height,paint);
// 画圆点标注
canvas.drawCircle(frame.left, (frame.top + frame.bottom) / 2, 10,tipPaint);
canvas.drawCircle(frame.right, (frame.top + frame.bottom) / 2, 10,tipPaint);
canvas.drawCircle((frame.left + frame.right) / 2, frame.top, 10,tipPaint);
canvas.drawCircle((frame.left + frame.right) / 2, frame.bottom, 10,tipPaint);

2 判断是否触摸发生在边界上

2.1 判断

case MotionEvent.ACTION_DOWN:downX = (int) event.getX();downY = (int) event.getY();if (checkContainsPoint(downX, downY)) {touchInFrame = true;if ((downX > frame.left - 10 && downX < frame.left + 10)&& downY > frame.top && downY < frame.bottom) {touchFrameLeft = true;}if ((downX > frame.right - 10 && downX < frame.right + 10)&& downY > frame.top && downY < frame.bottom) {touchFrameRight = true;}if ((downX > frame.left && downX < frame.right)&& downY > frame.top - 10 && downY < frame.top + 10) {touchFrameTop = true;}if ((downX > frame.left && downX < frame.right)&& downY > frame.bottom - 10&& downY < frame.bottom + 10) {touchFrameBottom = true;}} else {touchInFrame = false;}break;

通过四个标志位，标识触摸发生在哪条边界上

2.2 在边界上移动的操作

int currentX = (int) event.getX();
int currentY = (int) event.getY();
// 是否在边界上判断
if (touchFrameLeft) {frame.left += currentX - downX;if (frame.left <= bitmapRect.left) {frame.left = bitmapRect.left;}frameWidth = frame.right - frame.left;downX = currentX;invalidate();return true;
}
if (touchFrameRight) {frame.right += currentX - downX;if (frame.right >= bitmapRect.right) {frame.right = bitmapRect.right;}frameWidth = frame.right - frame.left;downX = currentX;invalidate();return true;
}
if (touchFrameTop) {frame.top += currentY - downY;if (frame.top <= bitmapRect.top) {frame.top = bitmapRect.top;}frameHeight = frame.bottom - frame.top;downY = currentY;invalidate();return true;
}
if (touchFrameBottom) {frame.bottom += currentY - downY;if (frame.bottom >= bitmapRect.bottom) {frame.bottom = bitmapRect.bottom;}frameHeight = frame.bottom - frame.top;downY = currentY;invalidate();return true;
}

3 判断是否触摸发生在透明区域内

3.1 判断

private boolean checkContainsPoint(int x, int y) {Rect boundsRect = new Rect();boundsRect.left = frame.left - 10;boundsRect.top = frame.top - 10;boundsRect.right = frame.right + 10;boundsRect.bottom = frame.bottom + 10;if (boundsRect.contains(x, y)) {return true;}return false;}

通过点击位置与透明区域的四条边界（rect），标识触摸发生在透明区域内

3.2 在透明区域内移动的操作

// 触摸发生不属于边界
frame.left += currentX - downX;
frame.right += currentX - downX;
frame.top += currentY - downY;
frame.bottom += currentY - downY;
if (frame.left <= bitmapRect.left) {frame.left = bitmapRect.left;frame.right = bitmapRect.left + frameWidth;
}
if (frame.right >= bitmapRect.right) {frame.left = bitmapRect.right - frameWidth;frame.right = bitmapRect.right;
}
if (frame.top <= bitmapRect.top) {frame.top = bitmapRect.top;frame.bottom = bitmapRect.top + frameHeight;
}
if (frame.bottom >= bitmapRect.bottom) {frame.top = bitmapRect.bottom - frameHeight;frame.bottom = bitmapRect.bottom;
}
downX = currentX;
downY = currentY;
invalidate();