如 cmets 中所述，您可以保留 Stacks 来跟踪 xy 坐标历史。
撤消和重做操作围绕着从单独的堆栈推入和弹出。
UndoCanvas
public class UndoCanvas extends View {
private final int MAX_STACK_SIZE = 50;
private Stack<Pair<Float, Float>> undoStack = new Stack<>();
private Stack<Pair<Float, Float>> redoStack = new Stack<>();
private Bitmap originalBitmap;
private Bitmap maskedBitmap;
private Canvas originalCanvas;
private Canvas maskedCanvas;
private Paint paint;
private float drawRadius;
private StackListener listener;
public UndoCanvas(Context context) {
super(context);
init();
}
public UndoCanvas(Context context, AttributeSet attrs) {
super(context, attrs);
init();
}
public UndoCanvas(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init();
}
private void init() {
drawRadius = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 5, getResources().getDisplayMetrics());
paint = new Paint();
// paint.setColor(Color.RED);
paint.setAlpha(0);
paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
paint.setAntiAlias(true);
paint.setMaskFilter(new BlurMaskFilter(15, BlurMaskFilter.Blur.SOLID));
}
public void setBitmap(Bitmap bitmap) {
if (bitmap != null) {
originalBitmap = bitmap.copy(bitmap.getConfig(), true); // Copy of the original, because we will potentially make changes to this
maskedBitmap = originalBitmap.copy(originalBitmap.getConfig(), true);
originalCanvas = new Canvas(originalBitmap);
maskedCanvas = new Canvas(maskedBitmap);
} else {
originalBitmap = null;
originalCanvas = null;
maskedBitmap = null;
maskedCanvas = null;
}
int undoSize = undoStack.size();
int redoSize = redoStack.size();
undoStack.clear();
redoStack.clear();
invalidate();
if (listener != null) {
if (undoSize != undoStack.size()) {
listener.onUndoStackChanged(undoSize, undoStack.size());
}
if (redoSize != redoStack.size()) {
listener.onRedoStackChanged(redoSize, redoStack.size());
}
}
}
public StackListener getListener() {
return listener;
}
public void setListener(StackListener listener) {
this.listener = listener;
}
public boolean onTouchEvent(MotionEvent ev) {
switch (ev.getAction()) {
case MotionEvent.ACTION_DOWN: {
int undoSize = undoStack.size();
int redoSize = redoStack.size();
// Max stack size. Remove oldest item before adding new
if (undoStack.size() == MAX_STACK_SIZE) {
// The undo history does not go further back, so make the change permanent by updating the original canvas/bitmap
Pair<Float, Float> pair = undoStack.remove(0);
maskPoint(originalCanvas, pair.first, pair.second);
}
undoStack.push(new Pair<>(ev.getX(), ev.getY()));
redoStack.clear();
invalidate();
if (listener != null) {
if (undoSize != undoStack.size()) {
listener.onUndoStackChanged(undoSize, undoStack.size());
}
if (redoSize != redoStack.size()) {
listener.onRedoStackChanged(redoSize, redoStack.size());
}
}
break;
}
case MotionEvent.ACTION_MOVE: {
int undoSize = undoStack.size();
int redoSize = redoStack.size();
// Max stack size. Remove oldest item before adding new
if (undoStack.size() == MAX_STACK_SIZE) {
// The undo history does not go further back, so make the change permanent by updating the original canvas/bitmap
Pair<Float, Float> pair = undoStack.remove(0);
maskPoint(originalCanvas, pair.first, pair.second);
}
maskPoint(maskedCanvas, ev.getX(), ev.getY());
undoStack.push(new Pair<>(ev.getX(), ev.getY()));
redoStack.clear();
invalidate();
if (listener != null) {
if (undoSize != undoStack.size()) {
listener.onUndoStackChanged(undoSize, undoStack.size());
}
if (redoSize != redoStack.size()) {
listener.onRedoStackChanged(redoSize, redoStack.size());
}
}
break;
}
case MotionEvent.ACTION_UP:
invalidate();
break;
}
return true;
}
@Override
protected void onDraw(Canvas canvas) {
if (maskedBitmap != null) {
canvas.drawBitmap(maskedBitmap, 0, 0, null);
}
super.onDraw(canvas);
}
public boolean undo() {
if (!undoStack.empty()) {
int undoSize = undoStack.size();
int redoSize = redoStack.size();
Pair<Float, Float> pair = undoStack.pop();
// Redraw a single part of the original bitmap
//unmaskPoint(maskedCanvas, pair.first, pair.second);
// Redraw the original bitmap, along with all the points in the undo stack
remaskCanvas(maskedCanvas);
redoStack.push(pair); // Do not need to check for > 50 here, since redoStack can only contain what was in undoStack
invalidate();
if (listener != null) {
if (undoSize != undoStack.size()) {
listener.onUndoStackChanged(undoSize, undoStack.size());
}
if (redoSize != redoStack.size()) {
listener.onRedoStackChanged(redoSize, redoStack.size());
}
}
return true;
}
return false;
}
public boolean redo() {
if (!redoStack.empty()) {
int undoSize = undoStack.size();
int redoSize = redoStack.size();
Pair<Float, Float> pair = redoStack.pop();
maskPoint(maskedCanvas, pair.first, pair.second);
undoStack.push(pair); // Do not need to check for > 50 here, since redoStack can only contain what was in undoStack
invalidate();
if (listener != null) {
if (undoSize != undoStack.size()) {
listener.onUndoStackChanged(undoSize, undoStack.size());
}
if (redoSize != redoStack.size()) {
listener.onRedoStackChanged(redoSize, redoStack.size());
}
}
return true;
}
return false;
}
private void maskPoint(Canvas canvas, float x, float y) {
if (canvas != null) {
canvas.drawCircle(x, y, drawRadius, paint);
}
}
private void unmaskPoint(Canvas canvas, float x, float y) {
if (canvas != null) {
Path path = new Path();
path.addCircle(x, y, drawRadius, Path.Direction.CW);
canvas.save();
canvas.clipPath(path);
canvas.drawBitmap(originalBitmap, 0, 0, new Paint());
canvas.restore();
}
}
private void remaskCanvas(Canvas canvas) {
if (canvas != null) {
canvas.drawBitmap(originalBitmap, 0, 0, new Paint());
for (int i = 0; i < undoStack.size(); i++) {
Pair<Float, Float> pair = undoStack.get(i);
maskPoint(canvas, pair.first, pair.second);
}
}
}
public interface StackListener {
void onUndoStackChanged(int previousSize, int newSize);
void onRedoStackChanged(int previousSize, int newSize);
}
}
您可能希望限制这些堆栈的大小，这样它们就不会在用户拖过屏幕时溢出。你可以玩弄这个数字，但 50 对我来说似乎是一个好的开始。
编辑
作为旁注，一次重做/撤消多个条目可能会很好。因为onTouchEvent 会触发非常精细的动作。按下撤消/重做时用户不会注意到的动作。
编辑 2
我已经添加到上面的实现中，也可以处理撤消。我发现仅在特定点重绘的策略是不够的，因为重叠点不正确。 （A 点和 B 点重叠，移除 B 会导致 A 的一部分被清除）。
因此，我在撤消操作中重新屏蔽了整个位图，这意味着撤消需要中间位图。如果没有中间位图，撤消操作将导致不再在堆栈中的点（最多 50 个）也被删除。由于我们不支持通过该点撤消，因此使用原始位图作为中间位图就足够了。
这两种方法都在代码中，因此您可以同时测试它们。
最后，我添加了一个监听器，让 Activity 知道堆栈的状态。启用/禁用按钮。
MainActivity
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final UndoCanvas canvas = (UndoCanvas) findViewById(R.id.undoCanvas);
final Button undoButton = (Button) findViewById(R.id.buttonUndo);
final Button redoButton = (Button) findViewById(R.id.buttonRedo);
undoButton.setEnabled(false);
redoButton.setEnabled(false);
canvas.setListener(new UndoCanvas.StackListener() {
@Override
public void onUndoStackChanged(int previousSize, int newSize) {
undoButton.setEnabled(newSize > 0);
}
@Override
public void onRedoStackChanged(int previousSize, int newSize) {
redoButton.setEnabled(newSize > 0);
}
});
undoButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
canvas.undo();
}
});
redoButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
canvas.redo();
}
});
canvas.setBitmap(BitmapFactory.decodeResource(getResources(), R.drawable.image));
}
}
截图
撤消之前
撤消后