How to draw several lines slowly in constant velocity on canvas by Android? - android-animation

Question

I need capture the mark to draw a figure on canvas in Android, and the effect just like the follow gif:
Well, as far, I can draw a side with constant velocity by ValueAnimator. However, I just only can draw one side at one time, because I can't save the last side when drawing the next side. So, is there a good way to solve the problem?
Code for draw a line slowly by ValueAnimator：
GraphicsView.java
public class GraphicsView extends View {
private int stepX, stepY = 0;
private int startX, startY, stopX, stopY = 0;
private Paint paint = null;
public GraphicsView(Context context) {
super(context);
// Paint
paint = new Paint();
paint.setAntiAlias(true);
paint.setColor(Color.RED);
paint.setStyle(Paint.Style.STROKE);
startX = 40;
startY = 397;
stopX = 1040;
stopY = 397;
Init();
}
public void Init(){
ValueAnimator animatorX = ValueAnimator.ofFloat(startX, stopX);
ValueAnimator animatorY = ValueAnimator.ofFloat(startY, stopY);
animatorX.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
#Override
public void onAnimationUpdate(ValueAnimator valueAnimator) {
stepX = Math.round((Float)valueAnimator.getAnimatedValue()); invalidate();
}
});
animatorY.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
#Override
public void onAnimationUpdate(ValueAnimator valueAnimator) {
stepY = Math.round((Float)valueAnimator.getAnimatedValue()); invalidate();
}
});
AnimatorSet set = new AnimatorSet();
LinearInterpolator l = new LinearInterpolator();
set.setInterpolator(l);
set.setDuration(3000);
set.playTogether(animatorX, animatorY);
set.start();
}
#Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
canvas.drawLine(startX, startY, stepX, stepY, paint);
}
}
MainActivity.java
public class MainActivity extends AppCompatActivity {
private Display display = null;
private GraphicsView view = null;
private ConstraintLayout layout = null;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
display = getWindowManager().getDefaultDisplay();
layout = (ConstraintLayout)findViewById(R.id.main_layout);
view = new GraphicsView(this);
view.setMinimumWidth(display.getWidth());
view.setMinimumHeight(display.getHeight());
layout.addView(view);
}
}

Answer 1

you can use the ObjectAnimator class to callback
to one of your class methods every time you'd like to draw a bit more of the path.
import android.animation.ObjectAnimator;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.DashPathEffect;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PathEffect;
import android.graphics.PathMeasure;
import android.util.AttributeSet;
import android.view.View;
import android.util.Log;
public class PathView extends View
{
Path path;
Paint paint;
float length;
public PathView(Context context)
{
super(context);
}
public PathView(Context context, AttributeSet attrs)
{
super(context, attrs);
}
public PathView(Context context, AttributeSet attrs, int defStyleAttr)
{
super(context, attrs, defStyleAttr);
}
public void init()
{
paint = new Paint();
paint.setColor(Color.BLUE);
paint.setStrokeWidth(10);
paint.setStyle(Paint.Style.STROKE);
path = new Path();
path.moveTo(50, 50);
path.lineTo(50, 500);
path.lineTo(200, 500);
path.lineTo(200, 300);
path.lineTo(350, 300);
// Measure the path
PathMeasure measure = new PathMeasure(path, false);
length = measure.getLength();
float[] intervals = new float[]{length, length};
ObjectAnimator animator = ObjectAnimator.ofFloat(PathView.this, "phase", 1.0f, 0.0f);
animator.setDuration(3000);
animator.start();
}
//is called by animtor object
public void setPhase(float phase)
{
Log.d("pathview","setPhase called with:" + String.valueOf(phase));
paint.setPathEffect(createPathEffect(length, phase, 0.0f));
invalidate();//will calll onDraw
}
private static PathEffect createPathEffect(float pathLength, float phase, float offset)
{
return new DashPathEffect(new float[] { pathLength, pathLength },
Math.max(phase * pathLength, offset));
}
#Override
public void onDraw(Canvas c)
{
super.onDraw(c);
c.drawPath(path, paint);
}
}
Then, just call init() to begin the animation, like this (or if you'd like it to start as soon as the view is inflated, put the init() call inside the constructors):
PathView path_view = (PathView) root_view.findViewById(R.id.path);
path_view.init();
Also see this question here, and
Using Value Animator Example
Reference 1
Reference 2
Reference 3