Android View measure （五） 支持margin属性，从一个异常说起

一、查看夏目

public class CustomViewGroup extends ViewGroup {
    
	......


    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        
		// 遍历所有子视图，进行measure操作
        for (int i = 0; i < getChildCount(); i++) {
            View child = getChildAt(i);
            if (child != null && child.getVisibility() != View.GONE) {
                measureChild(child, widthMeasureSpec, heightMeasureSpec);
                
				// 支持子视图设置的android:layout_margin属性
                MarginLayoutParams layoutParams = (MarginLayoutParams) child.getLayoutParams();
                int marginLeft = layoutParams.leftMargin;
            }
        }
        
        setMeasuredDimension(widthMeasureSpec, heightMeasureSpec);
    }


    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        for (int i = 0; i < getChildCount(); i++) {
            View childView = getChildAt(i);
			// 只为最简单代码复现BUG，所有子视图都随便放
            childView.layout(left, top, left + childView.getMeasuredWidth(), top + childView.getMeasuredHeight());
        }
    }
}

        <com.example.android.apis.CustomViewGroup
            android:id="@+id/custom_view_group"
            android:layout_width="match_parent"
            android:layout_height="wrap_content" >


            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
				android:layout_margin="10dip"
                android:text="love_world_" />
        </com.example.android.apis.CustomViewGroup>

java.lang.ClassCastException: android.view.ViewGroup$LayoutParams cannot be cast to android.view.ViewGroup$MarginLayoutParams
	at com.example.android.apis.CustomViewGroup.onMeasure(CustomViewGroup.java:50)
	at android.view.View.measure(View.java:16831)
	at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)
	at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1410)
	at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1052)
	at android.widget.LinearLayout.onMeasure(LinearLayout.java:590)
	at android.view.View.measure(View.java:16831)
	at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)
	at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1410)
	at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
	at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
	at android.view.View.measure(View.java:16831)
	at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)
	at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
	at android.view.View.measure(View.java:16831)
	at android.widget.LinearLayout.measureVertical(LinearLayout.java:847)
	at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
	at android.view.View.measure(View.java:16831)
	at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)
	at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
	at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2586)
	at android.view.View.measure(View.java:16831)
	at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2189)
	at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1352)
	at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1535)
	at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1249)
	at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6364)
	at android.view.Choreographer$CallbackRecord.run(Choreographer.java:791)
	at android.view.Choreographer.doCallbacks(Choreographer.java:591)
	at android.view.Choreographer.doFrame(Choreographer.java:561)
	at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:777)
	at android.os.Handler.handleCallback(Handler.java:730)
	at android.os.Handler.dispatchMessage(Handler.java:92)
	at android.os.Looper.loop(Looper.java:176)
	at android.app.ActivityThread.main(ActivityThread.java:5419)
	at java.lang.reflect.Method.invokeNative(Native Method)
	at java.lang.reflect.Method.invoke(Method.java:525)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1209)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1025)
	at dalvik.system.NativeStart.main(Native Method)

public class MainActivity extends Activity {


    @Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		CustomViewGroup customViewGroup = (CustomViewGroup) findViewById(R.id.custom_view_group);
		
		customViewGroup.addView(createTextView("Love_world_"));
		
	}


    private View createTextView(String value) {
        TextView textView = new TextView(this);  
        textView.setText("a child view");
        textView.setText(value);
        textView.setLayoutParams(new ViewGroup.MarginLayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT));
        return textView;
    }


}	

2. ViewGroup.addView源码，查找何处添加LayoutParams

public abstract class ViewGroup extends View implements ViewParent, ViewManager {


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


    public void addView(View child, int index) {
        LayoutParams params = child.getLayoutParams();
		// 子视图LayoutParams为为空是处理方式
        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 index, LayoutParams params) {
        if (DBG) {
            System.out.println(this + " addView");
        }


        // 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();
        addViewInner(child, index, params, false);
    }


	// 子视图默认LayoutParams实例
	protected LayoutParams generateDefaultLayoutParams() {
        return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
    }


	// 定义LayoutParams类
	public static class LayoutParams {
		public int width;
		public int height;
				
		public LayoutParams(int width, int height) {
            this.width = width;
            this.height = height;
        }
	}
	
}

public abstract class ViewGroup extends View implements ViewParent, ViewManager {
    private void addViewInner(View child, int index, LayoutParams params,
            boolean preventRequestLayout) {


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


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


        ......
    }


	protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {
        return  p != null;
    }
}

public class CustomViewGroup extends ViewGroup {


	@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
		......
	}
	
	@Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
		......	
	}


    @Override
    protected LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) {
        return new LayoutParams(p);
    }
    
    @Override
    protected LayoutParams generateDefaultLayoutParams() {
        return new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT);
    }    
    
    @Override
    public LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new LayoutParams(getContext(), attrs);
    }    


    // 继承自margin，支持子视图android:layout_margin属性
    public static class LayoutParams extends MarginLayoutParams {


        public LayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);
        }


        public LayoutParams(int width, int height) {
            super(width, height);
        }


        public LayoutParams(ViewGroup.LayoutParams source) {
            super(source);
        }


        public LayoutParams(ViewGroup.MarginLayoutParams source) {
            super(source);
        }
    }
}	

Android View measure （五） 支持margin属性，从一个异常说起

原文：http://blog.csdn.net/androiddevelop/article/details/44348753