Android LayoutParams source code analysis

LayoutParams source code analysis

Overview

• LayoutParams means layout parameters. The attributes such as layout_xxx in the XML layout file are all descriptions of LayoutParams.
• LayoutParams does not belong to the View, it is where the ViewGroup controls the specific display of the View.

Basic usage of LayoutParams

TextView textView1 = new TextView(this);
textView1.setText("TextView1");
linearLayout.addView(textView1);

TextView textView2 = new TextView(this);
textView2.setText("TextView2");
textView2.setLayoutParams(new LinearLayout.LayoutParams(LinearLayout.LayoutParams.WRAP_CONTENT, LinearLayout.LayoutParams.WRAP_CONTENT));
linearLayout.addView(textView2);

TextView textView3 = new TextView(this);
textView3.setText("TextView3");
textView3.setLayoutParams(new LinearLayout.LayoutParams(100, 100));
linearLayout.addView(textView3);

LayoutParams source code analysis

ViewGroup.LayoutParams

public LayoutParams(Context c, AttributeSet attrs) {
    
    
    TypedArray a = c.obtainStyledAttributes(attrs, R.styleable.ViewGroup_Layout);
    setBaseAttributes(a,
                      R.styleable.ViewGroup_Layout_layout_width,
                      R.styleable.ViewGroup_Layout_layout_height);
    a.recycle();
}

public LayoutParams(int width, int height) {
    
    
    this.width = width;
    this.height = height;
}

public LayoutParams(LayoutParams source) {
    
    
    this.width = source.width;
    this.height = source.height;
}

LayoutParams() {
    
    
}

ViewGroup.MarginLayoutParams

public static class MarginLayoutParams extends ViewGroup.LayoutParams {
    
    
    public int leftMargin;

    public int topMargin;

    public int rightMargin;

    public int bottomMargin;

    private int startMargin = DEFAULT_MARGIN_RELATIVE;

    private int endMargin = DEFAULT_MARGIN_RELATIVE;

    public static final int DEFAULT_MARGIN_RELATIVE = Integer.MIN_VALUE;

    public MarginLayoutParams(Context c, AttributeSet attrs) {
    
    
        super();

        TypedArray a = c.obtainStyledAttributes(attrs, R.styleable.ViewGroup_MarginLayout);
        setBaseAttributes(a,
                          R.styleable.ViewGroup_MarginLayout_layout_width,
                          R.styleable.ViewGroup_MarginLayout_layout_height);

        int margin = a.getDimensionPixelSize(
            com.android.internal.R.styleable.ViewGroup_MarginLayout_layout_margin, -1);
        if (margin >= 0) {
    
    
            leftMargin = margin;
            topMargin = margin;
            rightMargin= margin;
            bottomMargin = margin;
        } else {
    
    
            int horizontalMargin = a.getDimensionPixelSize(
                R.styleable.ViewGroup_MarginLayout_layout_marginHorizontal, -1);
            int verticalMargin = a.getDimensionPixelSize(
                R.styleable.ViewGroup_MarginLayout_layout_marginVertical, -1);
            ...
        }
        ...
    }
}

addView process

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();

    //若子View的LayoutParams为null，则使用默认LayoutParams
    if (params == null) {
    
    
        params = generateDefaultLayoutParams();
        if (params == null) {
    
    
            throw new IllegalArgumentException(
                "generateDefaultLayoutParams() cannot return null");
        }
    }
    addView(child, index, params);
}

//生成默认LayoutParams
@Override
protected LayoutParams generateDefaultLayoutParams() {
    
    
    if (mOrientation == HORIZONTAL) {
    
    
        return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
    } else if (mOrientation == VERTICAL) {
    
    
        return new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT);
    }
    return null;
}

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");
    }

    requestLayout();
    invalidate(true);
    addViewInner(child, index, params, false);
}

private void addViewInner(View child, int index, LayoutParams params,
                          boolean preventRequestLayout) {
    
    
    ...
        //检查LayoutParams是否合法，若不合法则使用默认值
        if (!checkLayoutParams(params)) {
    
    
            params = generateLayoutParams(params);
        }
    ...
        //子View添加到mChildren数组
        addInArray(child, index);
    ...
}

//检查LayoutParams是否合法
@Override
protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {
    
    
    return p instanceof LinearLayout.LayoutParams;
}

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