What is the drawing process of View in Android

The View drawing process is the core of Android UI system. Understanding the drawing mechanism is crucial for custom Views and performance optimization.

Three Stages of View Drawing

View drawing follows Measure → Layout → Draw three stages:

shell
View drawing flow:
measure() → onMeasure() → Measure View width/height
    ↓
layout() → onLayout() → Determine View position
    ↓
draw() → onDraw() → Draw View content

Stage 1: Measure

Purpose

Determine View's width and height (measuredWidth/measuredHeight).

MeasureSpec

MeasureSpec is the parent View's size requirement for child View, composed of mode and size:

Mode	Value	Meaning
EXACTLY	0x40000000	Exact value, like match_parent or specific value
AT_MOST	0x80000000	Maximum value, like wrap_content
UNSPECIFIED	0x00000000	No limit, like child in ScrollView

Measurement Flow

java
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int widthMode = MeasureSpec.getMode(widthMeasureSpec);
    int widthSize = MeasureSpec.getSize(widthMeasureSpec);
    
    // Calculate actual size based on mode
    int width = calculateWidth(widthMode, widthSize);
    int height = calculateHeight(heightMeasureSpec);
    
    setMeasuredDimension(width, height);
}

Stage 2: Layout

Purpose

Determine View's position in parent container (left, top, right, bottom).

Layout Flow

java
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    // Iterate child Views, determine each child's position
    for (int i = 0; i < getChildCount(); i++) {
        View child = getChildAt(i);
        child.layout(childLeft, childTop, childRight, childBottom);
    }
}

Stage 3: Draw

Drawing Steps

draw() method contains 6 steps:

java
public void draw(Canvas canvas) {
    // 1. Draw background
    drawBackground(canvas);
    
    // 2. Save canvas state
    saveCount = canvas.getSaveCount();
    
    // 3. Draw content (implemented by subclass)
    onDraw(canvas);
    
    // 4. Draw child Views
    dispatchDraw(canvas);
    
    // 5. Draw decorations (like scrollbars)
    onDrawForeground(canvas);
    
    // 6. Restore canvas state
    canvas.restoreToCount(saveCount);
}

ViewGroup Drawing Characteristics

ViewGroup extends View but adds child View management:

1. Measure Stage: Iterate and measure all child Views
2. Layout Stage: Determine child View positions
3. Draw Stage: dispatchDraw() draws all child Views

Drawing Optimization Tips

1. Reduce Redraws

java
// Use invalidate(Rect) for partial redraw
invalidate(0, 0, 100, 100);

// Use ViewStub for lazy loading
<ViewStub android:id="@+id/stub" android:layout="@layout/view" />

2. Avoid Overdraw

• Remove unnecessary backgrounds
• Use clipRect to reduce drawing area
• Use GPU Overdraw debug tool

3. Use Hardware Acceleration

xml
<application android:hardwareAccelerated="true">

Custom View Essentials

java
public class CustomView extends View {
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        // Handle wrap_content
        if (MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.AT_MOST) {
            setMeasuredDimension(defaultWidth, defaultHeight);
        }
    }
    
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        // Draw custom content
        canvas.drawCircle(cx, cy, radius, paint);
    }
}

Key Points

• Understand three modes of MeasureSpec
• Master difference between requestLayout() and invalidate()
• Understand trigger timing of drawing process
• Master basic steps of custom View
• Understand difference between hardware and software rendering