Android自定义View实现LayoutParams的方法详解
目录
- 第一层理解
- 第二层理解
- 第三层理解
这一期我们来讲一讲LayoutParams这个玩意儿。Android入门的第一行代码就牵扯到这个东西,然而,你真的理解够了吗?
第一层理解
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" tools:context=".MainActivity"> </RelativeLayout>
layout_width和layout_height这个是不是最开始学的时候,就要搞清楚的基础知识,match_parent代表填充屏幕,wrap_content代表包裹内容。这些其实是系统控件定义的属性,通过TypedArray进行解析。
第二层理解
val lp = RelativeLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT) lp.addRule(RelativeLayout.ALIGN_PARENT_RIGHT) lp.addRule(RelativeLayout.CENTER_VERTICAL) lp.addRule(RelativeLayout.BELOW, viewId) lp.setMargins(10, 20, 10, 20)
使用代码动态布局的时候设置LayoutParams。
第三层理解
好了,知识是在不断打破旧的认识中进步的,第一层实际还没到LayoutParams,还只是AttributeSet。系统何时将布局中的AttributeSet解析成LayoutParams的呢?
@Override public LayoutParams generateLayoutParams(AttributeSet attrs) { return new RelativeLayout.LayoutParams(getContext(), attrs); } protected LayoutParams generateDefaultLayoutParams() { return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT); }
ViewGroup有个关键的方法,generateLayoutParams()。
public LayoutParams(Context c, AttributeSet attrs) { super(c, attrs); TypedArray a = c.obtainStyledAttributes(attrs, com.android.internal.R.styleable.RelativeLayout_Layout); final int targetSdkVersion = c.getApplicationInfo().targetSdkVersion; mIsRtlCompatibilityMode = (targetSdkVersion < JELLY_BEAN_MR1 || !c.getApplicationInfo().hasRtlSupport()); final int[] rules = mRules; //noinspection MismatchedReadAndWriteOfArray final int[] initialRules = mInitialRules; final int N = a.getIndexCount(); for (int i = 0; i < N; i++) { int attr = a.getIndex(i); switch (attr) { case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignWithParentIfMissing: alignWithParent = a.getBoolean(attr, false); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_toLeftOf: rules[LEFT_OF] = a.getResourceId(attr, 0); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_toRightOf: rules[RIGHT_OF] = a.getResourceId(attr, 0); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_above: rules[ABOVE] = a.getResourceId(attr, 0); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_below: rules[BELOW] = a.getResourceId(attr, 0); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignBaseline: rules[ALIGN_BASELINE] = a.getResourceId(attr, 0); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignLeft: rules[ALIGN_LEFT] = a.getResourceId(attr, 0); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignTop: rules[ALIGN_TOP] = a.getResourceId(attr, 0); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignRight: rules[ALIGN_RIGHT] = a.getResourceId(attr, 0); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignBottom: rules[ALIGN_BOTTOM] = a.getResourceId(attr, 0); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignParentLeft: rules[ALIGN_PARENT_LEFT] = a.getBoolean(attr, false) ? TRUE : 0; break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignParentTop: rules[ALIGN_PARENT_TOP] = a.getBoolean(attr, false) ? TRUE : 0; break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignParentRight: rules[ALIGN_PARENT_RIGHT] = a.getBoolean(attr, false) ? TRUE : 0; break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignParentBottom: rules[ALIGN_PARENT_BOTTOM] = a.getBoolean(attr, false) ? TRUE : 0; break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_centerInParent: rules[CENTER_IN_PARENT] = a.getBoolean(attr, false) ? TRUE : 0; break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_centerHorizontal: rules[CENTER_HORIZONTAL] = a.getBoolean(attr, false) ? TRUE : 0; break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_centerVertical: rules[CENTER_VERTICAL] = a.getBoolean(attr, false) ? TRUE : 0; break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_toStartOf: rules[START_OF] = a.getResourceId(attr, 0); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_toEndOf: rules[END_OF] = a.getResourceId(attr, 0); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignStart: rules[ALIGN_START] = a.getResourceId(attr, 0); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignEnd: rules[ALIGN_END] = a.getResourceId(attr, 0); break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignParentStart: rules[ALIGN_PARENT_START] = a.getBoolean(attr, false) ? TRUE : 0; break; case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignParentEnd: rules[ALIGN_PARENT_END] = a.getBoolean(attr, false) ? TRUE : 0; break; } } mRulesChanged = true; System.arraycopy(rules, LEFT_OF, initialRules, LEFT_OF, VERB_COUNT); a.recycle(); }
这个代码熟悉吧,这就是我们之前讲过的自定义属性啊!没错,xml布局中的属性会先被解析成LayoutParams。那么我问你个问题,你觉得generateLayoutParams()和generateDefaultLayoutParams()的这个LayoutParams是给自己用的呢?还是给它的子控件用的呢?它是给子控件用的。自己的那个直接在构造方法中就从AttributeSet解析出来了。这样你就理解了,为什么RelativeLayout的那些个
android:layout_centerVertical="true" android:layout_alignParentEnd="true"
怎么全部定义在子控件里面了。然后ViewGroup的addView()方法中就可以带上这个LayoutParams了。
/** * Adds a child view. If no layout parameters are already set on the child, the * default parameters for this ViewGroup are set on the child. * * <p><strong>Note:</strong> do not invoke this method from * {@link #draw(android.graphics.Canvas)}, {@link #onDraw(android.graphics.Canvas)}, * {@link #dispatchDraw(android.graphics.Canvas)} or any related method.</p> * * @param child the child view to add * @param index the position at which to add the child * * @see #generateDefaultLayoutParams() */ 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); }
你不重写generateLayoutParams()方法,怎么在添加子控件的时候,让子控件用你的LayoutParams呢?
public static class LayoutParams extends ViewGroup.MarginLayoutParams { }
以上是LinearLayout.LayoutParams的摘要,我们自定义ViewGroup的时候,是不是也可以继承个ViewGroup的LayoutParams玩一玩呢?然后重写generateLayoutParams()和generateDefaultLayoutParams()方法。
@Override public LayoutParams generateLayoutParams(AttributeSet attrs) { return new LinearLayout.LayoutParams(getContext(), attrs); }
这里return你的ViewGroup的LayoutParams,然后在你的ViewGroup的LayoutParams的构造方法中就可以解析自定义属性attrs了。如果忘记了解析方式,我给你个提示,使用context的obtainStyledAttributes()方法。
大部分停留在第二层理解,你如果学会了第三层,那么你自定义View又可以玩出新的高度了。
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