Android绘制一条线来跟随你的手指

根据这个问题的答案： Android绘制一条线来跟随你的手指

我尝试了我的解决方案（参见代码），但我得到的形状像意大利面。如何摆脱以前的线？ 我想在Microsoft Paint中绘制一条直线。

这是我的代码：

@Override public boolean onTouchEvent(MotionEvent event) { switch (event.getAction()) { case MotionEvent.ACTION_DOWN: downx = event.getX(); downy = event.getY(); invalidate(); break; case MotionEvent.ACTION_MOVE: upx = event.getX(); upy = event.getY(); mCanvas.drawLine(downx, downy, upx, upy, mPaint); invalidate(); break; case MotionEvent.ACTION_UP: upx = event.getX(); upy = event.getY(); mCanvas.drawLine(downx, downy, upx, upy, mPaint); invalidate(); break; } return true; } 

谢谢你们！

PS：这不是重复的问题，我在上面解释过！

Answer 1

您不应该在onTouchEvent绘制，不要存储对canvas的引用，将drawLine移动到

 @Override protected void onDraw(Canvas canvas) { ... } 

invalidate()将为您调用draw() ，然后调用onDraw() 。

通常：使用onTouchEvent和onClick等UI事件来修改视图的状态，然后从这些事件中调用invalidate()并实现onDraw以便它始终呈现对象的状态。

FingerLine.java

 import android.content.Context; import android.graphics.*; import android.graphics.Paint.Style; import android.support.annotation.NonNull; import android.util.AttributeSet; import android.view.*; public class FingerLine extends View { private final Paint mPaint; private float startX; private float startY; private float endX; private float endY; public FingerLine(Context context) { this(context, null); } public FingerLine(Context context, AttributeSet attrs) { super(context, attrs); mPaint = new Paint(Paint.ANTI_ALIAS_FLAG); mPaint.setStyle(Style.STROKE); mPaint.setColor(Color.RED); } @Override protected void onDraw(Canvas canvas) { super.onDraw(canvas); canvas.drawLine(startX, startY, endX, endY, mPaint); } @Override public boolean onTouchEvent(@NonNull MotionEvent event) { switch (event.getAction()) { case MotionEvent.ACTION_DOWN: startX = event.getX(); startY = event.getY(); // Set the end to prevent initial jump (like on the demo recording) endX = event.getX(); endY = event.getY(); invalidate(); break; case MotionEvent.ACTION_MOVE: endX = event.getX(); endY = event.getY(); invalidate(); break; case MotionEvent.ACTION_UP: endX = event.getX(); endY = event.getY(); invalidate(); break; } return true; } } 

activity_main.xml中

MainActivity.java

 import android.app.Activity; import android.os.Bundle; public class MainActivity extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); } } 

结果

注意 ：我正在使用 ，但我认为这不会有所作为。

Answer 2

您需要在线条绘制代码调用Canvas.drawColor(Color.WHITE)之前在onDraw（）函数中重置View的背景颜色，将WHITE替换为您的背景颜色。 或者你可以在任何其他代码之前调用onDraw（）中的super.onDraw(canvas)函数。

很抱歉缺少示例，我是通过手机写的。

Answer 3

这是使线条跟随手指的解决方案。 （之后不会消失）

 import android.content.Context; import android.graphics.*; import android.graphics.Paint.Style; import android.support.annotation.NonNull; import android.util.AttributeSet; import android.view.*; public class FingerLine extends View { private final Paint mPaint; private float startX; private float startY; private float endX; private float endY; public Canvas mCanvas; public FingerLine(Context context) { this(context, null); mBitmap = Bitmap.createBitmap(width, width, Bitmap.Config.ARGB_8888); }else { mBitmap = Bitmap.createBitmap(height, height, Bitmap.Config.ARGB_8888); } mCanvas = new Canvas(mBitmap); } public FingerLine(Context context, AttributeSet attrs) { super(context, attrs); mPaint = new Paint(Paint.ANTI_ALIAS_FLAG); mPaint.setStyle(Style.STROKE); mPaint.setColor(Color.RED); } @Override protected void onDraw(Canvas canvas) { super.onDraw(canvas); canvas.drawLine(startX, startY, endX, endY, mPaint); } @Override public boolean onTouchEvent(@NonNull MotionEvent event) { switch (event.getAction()) { case MotionEvent.ACTION_DOWN: startX = event.getX(); startY = event.getY(); invalidate(); break; case MotionEvent.ACTION_MOVE: endX = event.getX(); endY = event.getY(); invalidate(); break; case MotionEvent.ACTION_UP: endX = event.getX(); endY = event.getY(); mcanvas.drawLine(startX, startY, endX, endY, mPaint); invalidate(); break; } return true; } }