ViewRootImpl测量流程解析-优快云博客

本文详细解析了ViewRootImpl中的performTraversals方法，包括如何通过getRootMeasureSpec确定MeasureSpec，进而进行视图的测量、布局和绘制过程。

ViewRootImpl类的performTraversals()中组装了初始的MeasureSpec

int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
……
performMeasure();
……
performLayout();
……
performDraw();

getRootMeasureSpec()：

/**
 * Figures out the measure spec for the root view in a window based on it's
 * layout params.
 *
 * @param windowSize The available width or height of the window
 *
 * @param rootDimension The layout params for one dimension (width or height) of the window.
 *
 * @return The measure spec to use to measure the root view.
 */
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
    int measureSpec;
    switch (rootDimension) {

    case ViewGroup.LayoutParams.MATCH_PARENT:
        // Window can't resize. Force root view to be windowSize.
        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
        break;
    case ViewGroup.LayoutParams.WRAP_CONTENT:
        // Window can resize. Set max size for root view.
        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
        break;
    default:
        // Window wants to be an exact size. Force root view to be that size.
        measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
        break;
    }
    return measureSpec;
}

performMeasure()会调用View的measure()：

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
    try {
        mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

performLayout()会调用View的layout()，performDraw()会调用drawSoftware()，drawSoftware()中调用到mView.draw(canvas);
draw()中调用到onDraw()