Progammatically設置TextView爲RelativeLayout.CENTER_OF_PARENT

Question

嘿，我只是學習Java和XML，並試圖設置一個TextView在其父母的RelativeLayout的中心。當我註釋掉前setContentView(homeScreen)

這裏的最後3行我的應用程序只加載是我的XML：

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout 
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" 
tools:context=".MainActivity"> 

</RelativeLayout> 

這裏是我的Java：

package com.example.android.testerapp1; 

import android.graphics.Color; 
import android.os.Bundle; 
import android.support.v7.app.AppCompatActivity; 
import android.view.Gravity; 
import android.view.ViewGroup; 
import android.widget.RelativeLayout; 
import android.widget.TextView; 

public class MainActivity extends AppCompatActivity { 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 


    TextView homeScreen = new TextView(this); 
    homeScreen.setText("Welcome to Test App 001" + "\nThis TextView was created dynamically in Java!"); 
    homeScreen.setTextSize(24); 
    homeScreen.setTextColor(Color.CYAN); 
    homeScreen.setCursorVisible(true); 
    homeScreen.setPadding(16,56,16,56); 
    homeScreen.setBackgroundColor(Color.BLACK); 
    homeScreen.setGravity(Gravity.CENTER); 

    //dynamically set width to dp (converted to pixels ~600) and height to 'wrap content' 
    // convert dp amount to pixels for size 
    final float scale = getResources().getDisplayMetrics().density; 
    int pixelWidth = (int) (2000/scale + 0.5f); 

    homeScreen.setLayoutParams(new ViewGroup.LayoutParams(pixelWidth , ViewGroup.LayoutParams.WRAP_CONTENT)); 

    RelativeLayout.LayoutParams params = (RelativeLayout.LayoutParams)homeScreen.getLayoutParams(); 
    params.addRule(RelativeLayout.CENTER_IN_PARENT); 
    homeScreen.setLayoutParams(params); 


    setContentView(homeScreen); 
    } 
} 

我已經看到這種帖子大約10倍，他們都有相同的解決方案，我似乎無法正確執行，它可能是我的代碼的另一部分？可能在哪裏我使用setLayoutParams也設置寬度和高度？

任何幫助將是偉大的！

Answer 1

您可以設置寬度和高度上的構造，然後用它

Relative.LayoutParams(int width, int height) 

，所以你需要做的，如：

homeScreen.setLayoutParams(width , height); 

Answer 2

的setContentView()調用應該被用來設置佈局的全屏。您在Activity代碼中正在進行的操作是將TextView設置爲屏幕的完整視圖，因此Activity沒有對您創建的XML佈局的引用。這就是爲什麼你的3行代碼最終失敗，因爲TextView試圖設置其父母應該如何放置和測量它的LayoutParams，但它在這種情況下沒有父母。我會建議做的是在XML給人一種id屬性爲RelativeLayout得到它的引用在Activity代碼如下所示：

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout 
xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:tools="http://schemas.android.com/tools" 
android:id="home_screen_layout" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
tools:context=".MainActivity"/> 

然後在你的Activity代碼，讓你與資源調用進行調整您的XML文件的ID。如果我們在資源目錄的佈局文件夾中（如src/main/resources/layout/act_main.xml）假設它被稱爲act_main.xml，那麼在調用super()之後，您會撥打setContentView(R.layout.act_main)作爲onCreate()的第一行，以便框架有機會解析您的XML並使其膨脹（即實例化，對尺寸進行計算並且確定其組件的位置等等）。之後，請使用findViewById(R.id.home_screen_layout)獲取對RelativeLayout的引用，以便您可以創建一個新的TextView並將其添加到已經充滿膨脹的佈局中。

package com.example.android.testerapp1; 

import android.graphics.Color; 
import android.os.Bundle; 
import android.support.v7.app.AppCompatActivity; 
import android.view.Gravity; 
import android.view.ViewGroup; 
import android.widget.RelativeLayout; 
import android.widget.TextView; 

public class MainActivity extends AppCompatActivity { 

    // make your view components private members as findViewById calls are expensive for the framework 
    private RelativeLayout homeScreenLayout; 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     // Have the activity inflate the XML file with your RelativeLayout 
     setContentView(R.layout.act_main); 

     // Now that it is inflated, get a reference to that parent 
     homeScreenLayout = (RelativeLayout) findViewById(R.id.home_screen_layout); 

     // Dynamically create a TextView associated with this Activity's context 
     TextView homeScreen = new TextView(this); 
     homeScreen.setText("Welcome to Test App 001" + "\nThis TextView was created dynamically in Java!"); 
     homeScreen.setTextSize(24); 
     homeScreen.setTextColor(Color.CYAN); 
     homeScreen.setCursorVisible(true); 
     homeScreen.setPadding(16,56,16,56); 
     homeScreen.setBackgroundColor(Color.BLACK); 
     homeScreen.setGravity(Gravity.CENTER); 

     //dynamically set width to dp (converted to pixels ~600) and height to 'wrap content' 
     // convert dp amount to pixels for size 
     final float scale = getResources().getDisplayMetrics().density; 
     int pixelWidth = (int) (2000/scale + 0.5f); 

     // Adjust the placement in the parent 
     RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(pixelWidth , RelativeLayout.LayoutParams.WRAP_CONTENT)  
     params.addRule(RelativeLayout.CENTER_IN_PARENT, RelativeLayout.TRUE); // make sure to use the function which takes a boolean value for rules like CENTER_IN_PARENT 
     homeScreen.setLayoutParams(params); // Add these parameters to the textview 

     // Let the layout know about your newly created textview so that it can re-draw its canvas 
     homeScreenLayout.addView(homeScreen); 

    } 
} 

作爲一個說明，我將添加一個你在做什麼可以在相對輕鬆的XML來完成，但既然你問它設置專門編程，我不會詳細討論那方面。但是，如果你有興趣在一些結構化的資源，我會建議您檢查出Android Developer Guide，特別是部分上XML layouts and how they interact with Activities

編輯：注意我對代碼所做的活動的變化。主要部分首先使用setContentView(int id)將空RelativeLayout xml充氣，然後將其他TextView添加到給定佈局。我所提供的關於CENTER_IN_PARENT系列的代碼存在一個小錯誤。根據[文檔]（https://developer.android.com/reference/android/widget/RelativeLayout.LayoutParams.html#addRule(int，int）），添加使用布爾值的規則時，必須使用該函數的addRule(int, int)版本。