Custom Views

...and how to make them

Alex Fu | @alexfu914

Why bother?

  • Uniqueness
    • Facebook Chat Heads / Messenger
  • Efficiency / Performance (layout)
    • Control where child elements are placed
    • Avoids nested layouts

Basics first

  • Measure
    • Measure the view and it's content.
    • Must report the size of the view before end of measure
      • ​We do this by calling setMeasuredDimensions
      • Or if subclassing, delegate to superclass
  • Layout
    • Assigns a position to each child
  • Draw
    • Draw content
    • Framework provides a Canvas to draw on

DIY

Before

After

DIY

public class FlaggedImageView extends ImageView {
  /* Code goes here */
}
public class FlaggedImageView extends ImageView {
  public FlaggedImageView(Context context, AttributeSet attrs) {
    super(context, attrs);
  }

  @Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
  }

  @Override
  protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    super.onLayout(changed, left, top, right, bottom);
  }

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

DIY

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  super.onMeasure(widthMeasureSpec, heightMeasureSpec);
  
  // Our code goes here
}
int flagWidth = 0, flagHeight = 0;
            
// Calculated from dividing width by height from designs
float aspectRatio = 4.25f;

flagWidth = (int) (getMeasuredWidth() * 0.30);
flagHeight = (int) (flagWidth / aspectRatio);

DIY

@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
  super.onLayout(changed, left, top, right, bottom);
  
  // Our code goes here
}

Path flagPath = new Path();

// We layout the path moving from right to left and top to bottom
// We start with the origin which will be the top-right corner of the flag
int currentX = getWidth();
int currentY = (int) dpToPx(20);
flagPath.moveTo(currentX, currentY);
Path flagPath = new Path();

// We layout the path moving from right to left and top to bottom
// We start with the origin which will be the top-right corner of the flag
int currentX = getWidth();
int currentY = (int) dpToPx(20);
flagPath.moveTo(currentX, currentY);

// Furthest top-left corner
currentX -= flagWidth;
flagPath.lineTo(currentX, currentY);
Path flagPath = new Path();

// We layout the path moving from right to left and top to bottom
// We start with the origin which will be the top-right corner of the flag
int currentX = getWidth();
int currentY = (int) dpToPx(20);
flagPath.moveTo(currentX, currentY);

// Furthest top-left corner
currentX -= flagWidth;
flagPath.lineTo(currentX, currentY);

// Layout the top half of the triangular indentation
currentX += (flagWidth * .10);
currentY += (flagHeight * 0.5);
flagPath.lineTo(currentX, currentY);
Path flagPath = new Path();

// We layout the path moving from right to left and top to bottom
// We start with the origin which will be the top-right corner of the flag
int currentX = getWidth();
int currentY = (int) dpToPx(20);
flagPath.moveTo(currentX, currentY);

// Furthest top-left corner
currentX -= flagWidth;
flagPath.lineTo(currentX, currentY);

// Layout the top half of the triangular indentation
currentX += (flagWidth * .10);
currentY += (flagHeight * 0.5);
flagPath.lineTo(currentX, currentY);

// ...and now the bottom half of the triangular indentation
currentX -= (flagWidth * .10);
currentY += (flagHeight * 0.5);
flagPath.lineTo(currentX, currentY);
// ...and now the bottom half of the triangular indentation
currentX -= (flagWidth * .10);
currentY += (flagHeight * 0.5);
flagPath.lineTo(currentX, currentY);

// Bottom right corner
currentX = getRight();
flagPath.lineTo(currentX, currentY);
// ...and now the bottom half of the triangular indentation
currentX -= (flagWidth * .10);
currentY += (flagHeight * 0.5);
flagPath.lineTo(currentX, currentY);

// Bottom right corner
currentX = getRight();
flagPath.lineTo(currentX, currentY);

// Back to origin
currentY = getTop();
flagPath.lineTo(currentX, currentY);

DIY

@Override protected void onDraw(Canvas canvas) {
  super.onDraw(canvas);
  
  // Our code goes here
}
Paint flagPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
flagPaint.setColor(Color.RED);
flagPaint.setStyle(Paint.Style.FILL);
Paint flagPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
flagPaint.setColor(Color.RED);
flagPaint.setStyle(Paint.Style.FILL);

// Draw path
canvas.drawPath(flagPath, flagPaint);

Results

Summary

  • Custom views can be more efficient/performant than using framework views
  • Three main phases of a View
    • (on)Measure
    • (on)Layout
    • (on)Draw
  • Must call setMeasuredDimensions before exiting onMeasure

Tips

  • Avoid object allocations during measure, layout, and draw
  • Assume onMeasure, onLayout, and onDraw gets called multiple times

Links

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