View的绘制原理

1. 基础知识

1.1 View绘制流程的三个阶段

1. 测量（Measure）

   • 确定View的大小
   • 自上而下遍历View树
   • 父View向子View传递测量要求

2. 布局（Layout）

   • 确定View的位置
   • 计算View的left、top、right、bottom
   • 根据测量结果和布局参数进行定位

3. 绘制（Draw）

   • 将View绘制到屏幕上
   • 绘制背景、内容、子View、前景等
   • 处理透明度、动画等效果

2. 核心问题

2.1 View的测量过程是怎样的？

答案：

1. MeasureSpec的生成

   public static class MeasureSpec {
       private static final int MODE_SHIFT = 30;
       private static final int MODE_MASK = 0x3 << MODE_SHIFT;
       
       // 精确模式，指定具体大小
       public static final int EXACTLY = 1 << MODE_SHIFT;
       
       // 最大模式，View的大小不能超过指定大小
       public static final int AT_MOST = 2 << MODE_SHIFT;
       
       // 未指定模式，View按需求取值
       public static final int UNSPECIFIED = 0;
   }

2. measure过程

   protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
       setMeasuredDimension(
           getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
           getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)
       );
   }

2.2 自定义View需要注意什么？

1. 正确重写测量方法

   @Override
   protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
       // 获取测量模式和大小
       int widthMode = MeasureSpec.getMode(widthMeasureSpec);
       int widthSize = MeasureSpec.getSize(widthMeasureSpec);
       
       int heightMode = MeasureSpec.getMode(heightMeasureSpec);
       int heightSize = MeasureSpec.getSize(heightMeasureSpec);
       
       // 计算实际大小
       int width = calculateWidth(widthMode, widthSize);
       int height = calculateHeight(heightMode, heightSize);
       
       // 保存测量结果
       setMeasuredDimension(width, height);
   }

2. 处理wrap_content

   • 指定默认大小
   • 避免与match_parent效果相同

3. 考虑padding和margin

   @Override
   protected void onDraw(Canvas canvas) {
       super.onDraw(canvas);
       // 考虑padding
       int paddingLeft = getPaddingLeft();
       int paddingTop = getPaddingTop();
       int width = getWidth() - getPaddingRight() - paddingLeft;
       int height = getHeight() - getPaddingBottom() - paddingTop;
   }

2.3 View的绘制流程是从哪里开始的？

答案：

1. ViewRootImpl的performTraversals方法

   • 开始View树的遍历
   • 触发measure、layout、draw三大流程

2. 绘制流程的传递

   private void performTraversals() {
       // 执行测量
       performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
       
       // 执行布局
       performLayout(lp, mWidth, mHeight);
       
       // 执行绘制
       performDraw();
   }

3. 性能优化

3.1 如何优化View的绘制性能？

1. 避免过度绘制

   • 移除不必要的背景
   • 使用clipRect裁剪画布
   • 使用ViewStub延迟加载

2. 扁平化View层级

   <!-- 优化前 -->
   <LinearLayout>
       <LinearLayout>
           <LinearLayout>
               <TextView/>
           </LinearLayout>
       </LinearLayout>
   </LinearLayout>
    
   <!-- 优化后 -->
   <ConstraintLayout>
       <TextView/>
   </ConstraintLayout>

3. 使用硬件加速

   <application android:hardwareAccelerated="true">

4. 实战示例

4.1 自定义圆形进度条

public class CircleProgressView extends View {
    private Paint mPaint;
    private int mProgress;
    private int mMax = 100;
 
    public CircleProgressView(Context context) {
        this(context, null);
    }
 
    public CircleProgressView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }
 
    private void init() {
        mPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeWidth(10);
    }
 
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int width = MeasureSpec.getSize(widthMeasureSpec);
        int height = MeasureSpec.getSize(heightMeasureSpec);
        int size = Math.min(width, height);
        setMeasuredDimension(size, size);
    }
 
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
 
        int centerX = getWidth() / 2;
        int centerY = getHeight() / 2;
        int radius = Math.min(centerX, centerY) - 10;
 
        // 绘制背景圆
        mPaint.setColor(Color.GRAY);
        canvas.drawCircle(centerX, centerY, radius, mPaint);
 
        // 绘制进度
        mPaint.setColor(Color.BLUE);
        float sweepAngle = 360f * mProgress / mMax;
        RectF rectF = new RectF(centerX - radius, centerY - radius,
                centerX + radius, centerY + radius);
        canvas.drawArc(rectF, -90, sweepAngle, false, mPaint);
    }
 
    public void setProgress(int progress) {
        mProgress = progress;
        invalidate();
    }
}

4.2 使用示例

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
 
        CircleProgressView progressView = findViewById(R.id.progressView);
        // 设置进度
        progressView.setProgress(75);
    }
}

<com.example.CircleProgressView
    android:id="@+id/progressView"
    android:layout_width="200dp"
    android:layout_height="200dp"
    android:layout_gravity="center"/>