Android 自定义视图中的 widthMeasureSpec 和 heightMeasureSpec 是什么

Android 自定义视图中的 widthMeasureSpec 和 heightMeasureSpec 是用于测量视图宽度和高度的重要参数。在 Android 中，视图的宽度和高度是通过 MeasureSpec 来确定的。MeasureSpec 是一个 32 位的 int 值，其中包含了两个主要信息：测量模式（MeasureSpec mode）和测量大小（MeasureSpec size）。

java
public class MyView extends View {
    
    public MyView(Context context) {
        super(context);
    }
    
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);
        
        int width, height;
        if (widthMode == MeasureSpec.EXACTLY) {
            width = widthSize;
        } else if (widthMode == MeasureSpec.AT_MOST) {
            width = Math.min(widthSize, 200); // 最大宽度为 200
        } else {
            width = 200; // 默认宽度为 200
        }
        
        if (heightMode == MeasureSpec.EXACTLY) {
            height = heightSize;
        } else if (heightMode == MeasureSpec.AT_MOST) {
            height = Math.min(heightSize, 200); // 最大高度为 200
        } else {
            height = 200; // 默认高度为 200
        }
        
        setMeasuredDimension(width, height);
    }
    
    @Override
    protected void onDraw(Canvas canvas) {
        // 绘制视图内容
    }
}

java
public static int resolveSize(int size, int measureSpec) {
    int result = size;
    int specMode = MeasureSpec.getMode(measureSpec);
    int specSize = MeasureSpec.getSize(measureSpec);
    
    switch (specMode) {
        case MeasureSpec.UNSPECIFIED:
            result = size;
            break;
        case MeasureSpec.AT_MOST:
            result = Math.min(size, specSize);
            break;
        case MeasureSpec.EXACTLY:
            result = specSize;
            break;
    }
    
    return result;
}

java
public class MyView extends View {
    
    public MyView(Context context) {
        super(context);
    }
    
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int widthSize = resolveSize(200, widthMeasureSpec);
        int heightSize = resolveSize(200, heightMeasureSpec);
        
        setMeasuredDimension(widthSize, heightSize);
    }
    
    @Override
    protected void onDraw(Canvas canvas) {
        // 绘制视图内容
    }
}