generateLayoutParams() 方法的作用

关于自定义ViewGroup，如何layout 子 View，在获取MarginLayoutParams方面一直不太懂如何操作，通过查阅资料知道获取MarginLayoutParams 的函数有三个：

public LayoutParams generateLayoutParams(AttributeSet attrs) {
    return new LayoutParams(getContext(), attrs);
}

protected LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) {
    return p;
}

protected LayoutParams generateDefaultLayoutParams() {
    return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
}

 我说下我自己的理解，

自定义ViewGroup会初始化Xml中的布局参数来获取LayoutParams，那么ViewGroup是如何获取的呢？

通过查阅源码得知，首先ViewGroup会通过调用 LayoutInflater 中 generateLayoutParams(attrs) 函数，生成ViewGroup.LayoutParams。

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
        final Resources res = getContext().getResources();
        if (DEBUG) {
            Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
                  + Integer.toHexString(resource) + ")");
        }

        View view = tryInflatePrecompiled(resource, res, root, attachToRoot);
        if (view != null) {
            return view;
        }
        XmlResourceParser parser = res.getLayout(resource);
        try {
            return inflate(parser, root, attachToRoot);
        } finally {
            parser.close();
        }
    }

    private @Nullable
    View tryInflatePrecompiled(@LayoutRes int resource, Resources res, @Nullable ViewGroup root,
        boolean attachToRoot) {
        if (!mUseCompiledView) {
            return null;
        }

        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate (precompiled)");

        // Try to inflate using a precompiled layout.
        String pkg = res.getResourcePackageName(resource);
        String layout = res.getResourceEntryName(resource);

        try {
            Class clazz = Class.forName("" + pkg + ".CompiledView", false, mPrecompiledClassLoader);
            Method inflater = clazz.getMethod(layout, Context.class, int.class);
            View view = (View) inflater.invoke(null, mContext, resource);

            if (view != null && root != null) {
                // We were able to use the precompiled inflater, but now we need to do some work to
                // attach the view to the root correctly.
                XmlResourceParser parser = res.getLayout(resource);
                try {
                    AttributeSet attrs = Xml.asAttributeSet(parser);
                    advanceToRootNode(parser);
                    ViewGroup.LayoutParams params = root.generateLayoutParams(attrs);

                    if (attachToRoot) {
                        root.addView(view, params);
                    } else {
                        view.setLayoutParams(params);
                    }
                } finally {
                    parser.close();
                }
            }

            return view;
        } catch (Throwable e) {
            if (DEBUG) {
                Log.e(TAG, "Failed to use precompiled view", e);
            }
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
        return null;
    }

查看上述代码得知：调用 inflate() 函数,执行 tryInflatePrecompiled(resource, res, root, attachToRoot) 函数。

在该方法中执行下面两句代码：

ViewGroup.LayoutParams params = root.generateLayoutParams(attrs);
root.addView(view, params);

其中重写 generateLayoutParams(attrs) 就可以获取 MarginLayoutParams。

@Override
    public LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new MarginLayoutParams(getContext(), attrs);
    }

如此通过上述两句代码，就把该子View和params添加到ViewGroup布局中了。

接下来看 addView() 函数：

public void addView(View child) {
        addView(child, -1);
    }

 public void addView(View child, int index) {
        if (child == null) {
            throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
        }
        LayoutParams params = child.getLayoutParams();
        if (params == null) {
            params = generateDefaultLayoutParams();
            if (params == null) {
                throw new IllegalArgumentException("generateDefaultLayoutParams() cannot return null");
            }
        }
        addView(child, index, params);
    }

  public void addView(View child, int width, int height) {
        final LayoutParams params = generateDefaultLayoutParams();
        params.width = width;
        params.height = height;
        addView(child, -1, params);
    }

    public void addView(View child, LayoutParams params) {
        addView(child, -1, params);
    }

 public void addView(View child, int width, int height) {
        final LayoutParams params = generateDefaultLayoutParams();
        params.width = width;
        params.height = height;
        addView(child, -1, params);
    }

public void addView(View child, int index, LayoutParams params) {
        if (DBG) {
            System.out.println(this + " addView");
        }

        if (child == null) {
            throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
        }

        // addViewInner() will call child.requestLayout() when setting the new LayoutParams
        // therefore, we call requestLayout() on ourselves before, so that the child's request
        // will be blocked at our level
        requestLayout();
        invalidate(true);
        addViewInner(child, index, params, false);
    }

private void addViewInner(View child, int index, LayoutParams params,
            boolean preventRequestLayout) {

        if (mTransition != null) {
            // Don't prevent other add transitions from completing, but cancel remove
            // transitions to let them complete the process before we add to the container
            mTransition.cancel(LayoutTransition.DISAPPEARING);
        }

        if (child.getParent() != null) {
            throw new IllegalStateException("The specified child already has a parent. " +
                    "You must call removeView() on the child's parent first.");
        }

        if (mTransition != null) {
            mTransition.addChild(this, child);
        }

        if (!checkLayoutParams(params)) {
            params = generateLayoutParams(params);
        }

        if (preventRequestLayout) {
            child.mLayoutParams = params;
        } else {
            child.setLayoutParams(params);
        }

        if (index < 0) {
            index = mChildrenCount;
        }

        addInArray(child, index);
        //省略后续无关代码
    }

查看上述代码得知：调用 addView(view) 函数才会执行到下面这句代码

final LayoutParams params = generateDefaultLayoutParams();

系统会给 LayoutParams 一个默认的值。

protected LayoutParams generateDefaultLayoutParams() {
    return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
}

注意：通过 inflate() 方法并不会触发 generateDefaultLayoutParams() 方法。

之后在 addViewInner(child, index, params, false) 方法中会进行检查

if (!checkLayoutParams(params)) {
    params = generateLayoutParams(params);
}

如果 p != null 就需要 generateLayoutParams(params)  重新赋值。

通过上述代码我们得知，只要重写 generateLayoutParams(AttributeSet attrs) 就可以获取 MarginLayoutParams。

	@Override
	public LayoutParams generateLayoutParams(AttributeSet attrs) {
		return new MarginLayoutParams(getContext(), attrs);
	}

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LayoutParams 相关博客：https://blog.csdn.net/yisizhu/article/details/51582622

                                         https://blog.csdn.net/CodingEnding/article/details/82717715