LayoutParams是什么？

LayoutParams是View用来告诉它的父控件如何放置自己的，LayoutParams是ViewGroup类里的一个静态内部类。

ViewGroup.LayoutParams仅仅描述了这个View想要的宽度和高度。可以理解成LayoutParams是子View告诉父View自身宽高的桥梁。

可取的值：

• fill_parent
  强制性使子视图的大小扩展至与父视图大小相等（不含 padding )
• match_parent
  与fill_parent相同，用于Android 2.3 & 之后版本
• wrap_content
  自适应大小，强制性地使视图扩展以便显示其全部内容(含 padding )

不同ViewGroup的继承类对应着不同的ViewGroup.LayoutParams的子类，例如：ViewGroup.MarginLayoutParams, WindowManager.LayoutParams等。

实际使用的一个例子

Activity:

public class MoveActivity extends AppCompatActivity {
    private ImageView mIv;
    private TextView mTvBlue;
    private TextView mTvGreen;
    private ViewGroup.LayoutParams mVgLp;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_move);
        mIv = (ImageView) findViewById(R.id.mIv);
        mTvBlue = (TextView) findViewById(R.id.mTvBlue);
        mTvGreen = (TextView) findViewById(R.id.mTvGreen);

        mVgLp = mIv.getLayoutParams();

        findViewById(R.id.mTvMoveRight).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ViewGroup.LayoutParams vg_lp = mIv.getLayoutParams();
                // RelativeLayout.LayoutParams 不可抽取为成员变量
                RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(vg_lp);
                params.addRule(RelativeLayout.ALIGN_PARENT_RIGHT, R.id.mIv);
                mIv.setLayoutParams(params); //使layout更新
            }
        });

        findViewById(R.id.mTvMoveLeft).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                // 左 （默认处于顶部，所以看起来是左上）
                RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(mVgLp);
                params.addRule(RelativeLayout.ALIGN_PARENT_LEFT);
                mIv.setLayoutParams(params);
            }
        });

        findViewById(R.id.mTvCenter).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                // 居中
                RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(mVgLp);
                params.addRule(RelativeLayout.CENTER_IN_PARENT);
                mIv.setLayoutParams(params);
            }
        });

        findViewById(R.id.mTvLeftVer).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                // 左部垂直居中
                RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(mVgLp);
                params.addRule(RelativeLayout.ALIGN_PARENT_LEFT|RelativeLayout.CENTER_VERTICAL);
                mIv.setLayoutParams(params);
            }
        });

        findViewById(R.id.mTvRightBot).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                // 右下
                RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(mVgLp);
                params.addRule(RelativeLayout.ALIGN_PARENT_BOTTOM);
                params.addRule(RelativeLayout.ALIGN_PARENT_RIGHT);
                mIv.setLayoutParams(params);
            }
        });
        //  绿在蓝右边
        findViewById(R.id.mTvaGRightB).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ViewGroup.LayoutParams vgLp = mTvGreen.getLayoutParams();
                RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(vgLp);
                params.addRule(RelativeLayout.RIGHT_OF,R.id.mTvBlue);
                mTvGreen.setLayoutParams(params);
            }
        });

        // 绿在蓝下边
        findViewById(R.id.mTvGElowB).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ViewGroup.LayoutParams vgLp = mTvGreen.getLayoutParams();
                RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(vgLp);
                params.addRule(RelativeLayout.BELOW,R.id.mTvBlue);
                mTvGreen.setLayoutParams(params);
            }
        });
    }
}

布局xml：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    android:id="@+id/activity_main"
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:orientation="vertical"
    tools:context="com.amqr.movetest.MoveActivity">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        >
        <TextView
            android:id="@+id/mTvMoveLeft"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:background="#6600ff00"
            android:padding="10dp"
            android:layout_margin="10dp"
            android:text="黑色最左！！"/>
        <TextView
            android:id="@+id/mTvMoveRight"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:padding="10dp"
            android:background="#6600ff00"
            android:layout_margin="10dp"
            android:text="黑色最右！！"/>
        <TextView
            android:id="@+id/mTvCenter"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:background="#6600ff00"
            android:padding="10dp"
            android:layout_margin="10dp"
            android:text="黑色居中"/>
    </LinearLayout>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        >
        <TextView
            android:id="@+id/mTvLeftVer"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:padding="10dp"
            android:background="#6600ff00"
            android:layout_margin="10dp"
            android:text="黑色靠左垂直居中"/>
        <TextView
            android:id="@+id/mTvRightBot"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:background="#6600ff00"
            android:padding="10dp"
            android:layout_margin="10dp"
            android:text="黑色右下角"/>
    </LinearLayout>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        >
        <TextView
            android:id="@+id/mTvaGRightB"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:padding="10dp"
            android:background="#6600ff00"
            android:layout_margin="10dp"
            android:text="绿在蓝右边"/>
        <TextView
            android:id="@+id/mTvGElowB"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:background="#6600ff00"
            android:padding="10dp"
            android:layout_margin="10dp"
            android:text="绿在蓝下边"/>
    </LinearLayout>
    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="200px"
        android:background="#66ff0000"
        >
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="RelativeLayout"
            android:layout_centerInParent="true"

            />
        <ImageView
            android:id="@+id/mIv"
            android:layout_width="30dp"
            android:layout_height="40px"
            android:background="#000"
            android:layout_centerHorizontal="true"
            />
        <TextView
            android:id="@+id/mTvBlue"
            android:layout_width="20dp"
            android:layout_height="20dp"
            android:text="蓝"
            android:gravity="center"
            android:background="#660000ff"
            />
        <TextView
            android:id="@+id/mTvGreen"
            android:layout_width="20dp"
            android:layout_height="20dp"
            android:text="绿"
            android:gravity="center"
            android:background="#6600ff00"
            android:layout_below="@id/mTvBlue"
            />
    </RelativeLayout>
</LinearLayout>